code:
===============================
Documentation for /proc/sys/fs/
===============================

Copyright (c) 1998, 1999,  Rik van Riel <riel@nl.linux.org>

Copyright (c) 2009,        Shen Feng<shen@cn.fujitsu.com>

For general info and legal blurb, please look in intro.rst.

------------------------------------------------------------------------------

This file contains documentation for the sysctl files and directories
in ``/proc/sys/fs/``.

The files in this directory can be used to tune and monitor
miscellaneous and general things in the operation of the Linux
kernel. Since some of the files *can* be used to screw up your
system, it is advisable to read both documentation and source
before actually making adjustments.

1. /proc/sys/fs
===============

Currently, these files might (depending on your configuration)
show up in ``/proc/sys/fs``:

.. contents:: :local:

aio-nr & aio-max-nr
-------------------

``aio-nr`` shows the current system-wide number of asynchronous io
requests.  ``aio-max-nr`` allows you to change the maximum value
``aio-nr`` can grow to.  If ``aio-nr`` reaches ``aio-nr-max`` then
``io_setup`` will fail with ``EAGAIN``.  Note that raising
``aio-max-nr`` does not result in the
pre-allocation or re-sizing of any kernel data structures.

dentry-state
------------

This file shows the values in ``struct dentry_stat``, as defined in
``linux/include/linux/dcache.h``::

  struct dentry_stat_t dentry_stat {
        int nr_dentry;
        int nr_unused;
        int age_limit;         /* age in seconds */
        int want_pages;        /* pages requested by system */
        int nr_negative;       /* # of unused negative dentries */
        int dummy;             /* Reserved for future use */
  };

Dentries are dynamically allocated and deallocated.

``nr_dentry`` shows the total number of dentries allocated (active
+ unused). ``nr_unused shows`` the number of dentries that are not
actively used, but are saved in the LRU list for future reuse.

``age_limit`` is the age in seconds after which dcache entries
can be reclaimed when memory is short and ``want_pages`` is
nonzero when ``shrink_dcache_pages()`` has been called and the
dcache isn't pruned yet.

``nr_negative`` shows the number of unused dentries that are also
negative dentries which do not map to any files. Instead,
they help speeding up rejection of non-existing files provided
by the users.

file-max & file-nr
------------------

The value in ``file-max`` denotes the maximum number of file-
handles that the Linux kernel will allocate. When you get lots
of error messages about running out of file handles, you might
want to increase this limit.

Historically,the kernel was able to allocate file handles
dynamically, but not to free them again. The three values in
``file-nr`` denote the number of allocated file handles, the number
of allocated but unused file handles, and the maximum number of
file handles. Linux 2.6 and later always reports 0 as the number of free
file handles -- this is not an error, it just means that the
number of allocated file handles exactly matches the number of
used file handles.

Attempts to allocate more file descriptors than ``file-max`` are
reported with ``printk``, look for::

  VFS: file-max limit <number> reached

in the kernel logs.

inode-nr & inode-state
----------------------

As with file handles, the kernel allocates the inode structures
dynamically, but can't free them yet.

The file ``inode-nr`` contains the first two items from
``inode-state``, so we'll skip to that file...

``inode-state`` contains three actual numbers and four dummies.
The actual numbers are, in order of appearance, ``nr_inodes``,
``nr_free_inodes`` and ``preshrink``.

``nr_inodes`` stands for the number of inodes the system has
allocated.

``nr_free_inodes`` represents the number of free inodes (?) and
preshrink is nonzero when the
system needs to prune the inode list instead of allocating
more.

mount-max
---------

This denotes the maximum number of mounts that may exist
in a mount namespace.

nr_open
-------

This denotes the maximum number of file-handles a process can
allocate. Default value is 1024*1024 (1048576) which should be
enough for most machines. Actual limit depends on ``RLIMIT_NOFILE``
resource limit.

overflowgid & overflowuid
-------------------------

Some filesystems only support 16-bit UIDs and GIDs, although in Linux
UIDs and GIDs are 32 bits. When one of these filesystems is mounted
with writes enabled, any UID or GID that would exceed 65535 is translated
to a fixed value before being written to disk.

These sysctls allow you to change the value of the fixed UID and GID.
The default is 65534.

pipe-user-pages-hard
--------------------

Maximum total number of pages a non-privileged user may allocate for pipes.
Once this limit is reached, no new pipes may be allocated until usage goes
below the limit again. When set to 0, no limit is applied, which is the default
setting.

pipe-user-pages-soft
--------------------

Maximum total number of pages a non-privileged user may allocate for pipes
before the pipe size gets limited to a single page. Once this limit is reached,
new pipes will be limited to a single page in size for this user in order to
limit total memory usage, and trying to increase them using ``fcntl()`` will be
denied until usage goes below the limit again. The default value allows to
allocate up to 1024 pipes at their default size. When set to 0, no limit is
applied.

protected_fifos
---------------

The intent of this protection is to avoid unintentional writes to
an attacker-controlled FIFO, where a program expected to create a regular
file.

When set to "0", writing to FIFOs is unrestricted.

When set to "1" don't allow ``O_CREAT`` open on FIFOs that we don't own
in world writable sticky directories, unless they are owned by the
owner of the directory.

When set to "2" it also applies to group writable sticky directories.

This protection is based on the restrictions in Openwall.

protected_hardlinks
--------------------

A long-standing class of security issues is the hardlink-based
time-of-check-time-of-use race, most commonly seen in world-writable
directories like ``/tmp``. The common method of exploitation of this flaw
is to cross privilege boundaries when following a given hardlink (i.e. a
root process follows a hardlink created by another user). Additionally,
on systems without separated partitions, this stops unauthorized users
from "pinning" vulnerable setuid/setgid files against being upgraded by
the administrator, or linking to special files.

When set to "0", hardlink creation behavior is unrestricted.

When set to "1" hardlinks cannot be created by users if they do not
already own the source file, or do not have read/write access to it.

This protection is based on the restrictions in Openwall and grsecurity.

protected_regular
-----------------

This protection is similar to `protected_fifos`_, but it
avoids writes to an attacker-controlled regular file, where a program
expected to create one.

When set to "0", writing to regular files is unrestricted.

When set to "1" don't allow ``O_CREAT`` open on regular files that we
don't own in world writable sticky directories, unless they are
owned by the owner of the directory.

When set to "2" it also applies to group writable sticky directories.

protected_symlinks
------------------

A long-standing class of security issues is the symlink-based
time-of-check-time-of-use race, most commonly seen in world-writable
directories like ``/tmp``. The common method of exploitation of this flaw
is to cross privilege boundaries when following a given symlink (i.e. a
root process follows a symlink belonging to another user). For a likely
incomplete list of hundreds of examples across the years, please see:
https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp

When set to "0", symlink following behavior is unrestricted.

When set to "1" symlinks are permitted to be followed only when outside
a sticky world-writable directory, or when the uid of the symlink and
follower match, or when the directory owner matches the symlink's owner.

This protection is based on the restrictions in Openwall and grsecurity.

suid_dumpable
-------------

This value can be used to query and set the core dump mode for setuid
or otherwise protected/tainted binaries. The modes are

=   ==========  ===============================================================
0   (default)	Traditional behaviour. Any process which has changed
		privilege levels or is execute only will not be dumped.
1   (debug)	All processes dump core when possible. The core dump is
		owned by the current user and no security is applied. This is
		intended for system debugging situations only.
		Ptrace is unchecked.
		This is insecure as it allows regular users to examine the
		memory contents of privileged processes.
2   (suidsafe)	Any binary which normally would not be dumped is dumped
		anyway, but only if the ``core_pattern`` kernel sysctl (see
		:ref:`Documentation/admin-guide/sysctl/kernel.rst <core_pattern>`)
		is set to
		either a pipe handler or a fully qualified path. (For more
		details on this limitation, see CVE-2006-2451.) This mode is
		appropriate when administrators are attempting to debug
		problems in a normal environment, and either have a core dump
		pipe handler that knows to treat privileged core dumps with
		care, or specific directory defined for catching core dumps.
		If a core dump happens without a pipe handler or fully
		qualified path, a message will be emitted to syslog warning
		about the lack of a correct setting.
=   ==========  ===============================================================

2. /proc/sys/fs/binfmt_misc
===========================

Documentation for the files in ``/proc/sys/fs/binfmt_misc`` is
in Documentation/admin-guide/binfmt-misc.rst.

3. /proc/sys/fs/mqueue - POSIX message queues filesystem
========================================================

The "mqueue"  filesystem provides  the necessary kernel features to enable the
creation of a  user space  library that  implements  the  POSIX message queues
API (as noted by the  MSG tag in the  POSIX 1003.1-2001 version  of the System
Interfaces specification.)

The "mqueue" filesystem contains values for determining/setting the
amount of resources used by the file system.

``/proc/sys/fs/mqueue/queues_max`` is a read/write file for
setting/getting the maximum number of message queues allowed on the
system.

``/proc/sys/fs/mqueue/msg_max`` is a read/write file for
setting/getting the maximum number of messages in a queue value.  In
fact it is the limiting value for another (user) limit which is set in
``mq_open`` invocation.  This attribute of a queue must be less than
or equal to ``msg_max``.

``/proc/sys/fs/mqueue/msgsize_max`` is a read/write file for
setting/getting the maximum message size value (it is an attribute of
every message queue, set during its creation).

``/proc/sys/fs/mqueue/msg_default`` is a read/write file for
setting/getting the default number of messages in a queue value if the
``attr`` parameter of ``mq_open(2)`` is ``NULL``. If it exceeds
``msg_max``, the default value is initialized to ``msg_max``.

``/proc/sys/fs/mqueue/msgsize_default`` is a read/write file for
setting/getting the default message size value if the ``attr``
parameter of ``mq_open(2)`` is ``NULL``. If it exceeds
``msgsize_max``, the default value is initialized to ``msgsize_max``.

4. /proc/sys/fs/epoll - Configuration options for the epoll interface
=====================================================================

This directory contains configuration options for the epoll(7) interface.

max_user_watches
----------------

Every epoll file descriptor can store a number of files to be monitored
for event readiness. Each one of these monitored files constitutes a "watch".
This configuration option sets the maximum number of "watches" that are
allowed for each user.
Each "watch" costs roughly 90 bytes on a 32-bit kernel, and roughly 160 bytes
on a 64-bit one.
The current default value for ``max_user_watches`` is 4% of the
available low memory, divided by the "watch" cost in bytes.


code:
===========================
Documentation for /proc/sys
===========================

Copyright (c) 1998, 1999,  Rik van Riel <riel@nl.linux.org>

------------------------------------------------------------------------------

'Why', I hear you ask, 'would anyone even _want_ documentation
for them sysctl files? If anybody really needs it, it's all in
the source...'

Well, this documentation is written because some people either
don't know they need to tweak something, or because they don't
have the time or knowledge to read the source code.

Furthermore, the programmers who built sysctl have built it to
be actually used, not just for the fun of programming it :-)

------------------------------------------------------------------------------

Legal blurb:

As usual, there are two main things to consider:

1. you get what you pay for
2. it's free

The consequences are that I won't guarantee the correctness of
this document, and if you come to me complaining about how you
screwed up your system because of wrong documentation, I won't
feel sorry for you. I might even laugh at you...

But of course, if you _do_ manage to screw up your system using
only the sysctl options used in this file, I'd like to hear of
it. Not only to have a great laugh, but also to make sure that
you're the last RTFMing person to screw up.

In short, e-mail your suggestions, corrections and / or horror
stories to: <riel@nl.linux.org>

Rik van Riel.

--------------------------------------------------------------

Introduction
============

Sysctl is a means of configuring certain aspects of the kernel
at run-time, and the /proc/sys/ directory is there so that you
don't even need special tools to do it!
In fact, there are only four things needed to use these config
facilities:

- a running Linux system
- root access
- common sense (this is especially hard to come by these days)
- knowledge of what all those values mean

As a quick 'ls /proc/sys' will show, the directory consists of
several (arch-dependent?) subdirs. Each subdir is mainly about
one part of the kernel, so you can do configuration on a piece
by piece basis, or just some 'thematic frobbing'.

This documentation is about:

=============== ===============================================================
abi/		execution domains & personalities
debug/		<empty>
dev/		device specific information (eg dev/cdrom/info)
fs/		specific filesystems
		filehandle, inode, dentry and quota tuning
		binfmt_misc <Documentation/admin-guide/binfmt-misc.rst>
kernel/		global kernel info / tuning
		miscellaneous stuff
net/		networking stuff, for documentation look in:
		<Documentation/networking/>
proc/		<empty>
sunrpc/		SUN Remote Procedure Call (NFS)
vm/		memory management tuning
		buffer and cache management
user/		Per user per user namespace limits
=============== ===============================================================

These are the subdirs I have on my system. There might be more
or other subdirs in another setup. If you see another dir, I'd
really like to hear about it :-)

.. toctree::
   :maxdepth: 1

   abi
   fs
   kernel
   net
   sunrpc
   user
   vm

repo_name: angery/linux


code:
===================================
Documentation for /proc/sys/kernel/
===================================

.. See scripts/check-sysctl-docs to keep this up to date

Copyright (c) 1998, 1999,  Rik van Riel <riel@nl.linux.org>

Copyright (c) 2009,        Shen Feng<shen@cn.fujitsu.com>

For general info and legal blurb, please look in
Documentation/admin-guide/sysctl/index.rst.

------------------------------------------------------------------------------

This file contains documentation for the sysctl files in
``/proc/sys/kernel/``.

The files in this directory can be used to tune and monitor
miscellaneous and general things in the operation of the Linux
kernel. Since some of the files *can* be used to screw up your
system, it is advisable to read both documentation and source
before actually making adjustments.

Currently, these files might (depending on your configuration)
show up in ``/proc/sys/kernel``:

.. contents:: :local:

acct
====

::

    highwater lowwater frequency

If BSD-style process accounting is enabled these values control
its behaviour. If free space on filesystem where the log lives
goes below ``lowwater``\ % accounting suspends. If free space gets
above ``highwater``\ % accounting resumes. ``frequency`` determines
how often do we check the amount of free space (value is in
seconds). Default:

::

    4 2 30

That is, suspend accounting if free space drops below 2%; resume it
if it increases to at least 4%; consider information about amount of
free space valid for 30 seconds.

acpi_video_flags
================

See Documentation/power/video.rst. This allows the video resume mode to be set,
in a similar fashion to the ``acpi_sleep`` kernel parameter, by
combining the following values:

= =======
1 s3_bios
2 s3_mode
4 s3_beep
= =======

arch
====

The machine hardware name, the same output as ``uname -m``
(e.g. ``x86_64`` or ``aarch64``).

auto_msgmni
===========

This variable has no effect and may be removed in future kernel
releases. Reading it always returns 0.
Up to Linux 3.17, it enabled/disabled automatic recomputing of
`msgmni`_
upon memory add/remove or upon IPC namespace creation/removal.
Echoing "1" into this file enabled msgmni automatic recomputing.
Echoing "0" turned it off. The default value was 1.

bootloader_type (x86 only)
==========================

This gives the bootloader type number as indicated by the bootloader,
shifted left by 4, and OR'd with the low four bits of the bootloader
version.  The reason for this encoding is that this used to match the
``type_of_loader`` field in the kernel header; the encoding is kept for
backwards compatibility.  That is, if the full bootloader type number
is 0x15 and the full version number is 0x234, this file will contain
the value 340 = 0x154.

See the ``type_of_loader`` and ``ext_loader_type`` fields in
Documentation/x86/boot.rst for additional information.

bootloader_version (x86 only)
=============================

The complete bootloader version number.  In the example above, this
file will contain the value 564 = 0x234.

See the ``type_of_loader`` and ``ext_loader_ver`` fields in
Documentation/x86/boot.rst for additional information.

bpf_stats_enabled
=================

Controls whether the kernel should collect statistics on BPF programs
(total time spent running, number of times run...). Enabling
statistics causes a slight reduction in performance on each program
run. The statistics can be seen using ``bpftool``.

= ===================================
0 Don't collect statistics (default).
1 Collect statistics.
= ===================================

cad_pid
=======

This is the pid which will be signalled on reboot (notably, by
Ctrl-Alt-Delete). Writing a value to this file which doesn't
correspond to a running process will result in ``-ESRCH``.

See also `ctrl-alt-del`_.

cap_last_cap
============

Highest valid capability of the running kernel.  Exports
``CAP_LAST_CAP`` from the kernel.

.. _core_pattern:

core_pattern
============

``core_pattern`` is used to specify a core dumpfile pattern name.

* max length 127 characters; default value is "core"
* ``core_pattern`` is used as a pattern template for the output
  filename; certain string patterns (beginning with '%') are
  substituted with their actual values.
* backward compatibility with ``core_uses_pid``:

	If ``core_pattern`` does not include "%p" (default does not)
	and ``core_uses_pid`` is set, then .PID will be appended to
	the filename.

* corename format specifiers

	========	==========================================
	%<NUL>		'%' is dropped
	%%		output one '%'
	%p		pid
	%P		global pid (init PID namespace)
	%i		tid
	%I		global tid (init PID namespace)
	%u		uid (in initial user namespace)
	%g		gid (in initial user namespace)
	%d		dump mode, matches ``PR_SET_DUMPABLE`` and
			``/proc/sys/fs/suid_dumpable``
	%s		signal number
	%t		UNIX time of dump
	%h		hostname
	%e		executable filename (may be shortened, could be changed by prctl etc)
	%f      	executable filename
	%E		executable path
	%c		maximum size of core file by resource limit RLIMIT_CORE
	%C		CPU the task ran on
	%<OTHER>	both are dropped
	========	==========================================

* If the first character of the pattern is a '|', the kernel will treat
  the rest of the pattern as a command to run.  The core dump will be
  written to the standard input of that program instead of to a file.

core_pipe_limit
===============

This sysctl is only applicable when `core_pattern`_ is configured to
pipe core files to a user space helper (when the first character of
``core_pattern`` is a '|', see above).
When collecting cores via a pipe to an application, it is occasionally
useful for the collecting application to gather data about the
crashing process from its ``/proc/pid`` directory.
In order to do this safely, the kernel must wait for the collecting
process to exit, so as not to remove the crashing processes proc files
prematurely.
This in turn creates the possibility that a misbehaving userspace
collecting process can block the reaping of a crashed process simply
by never exiting.
This sysctl defends against that.
It defines how many concurrent crashing processes may be piped to user
space applications in parallel.
If this value is exceeded, then those crashing processes above that
value are noted via the kernel log and their cores are skipped.
0 is a special value, indicating that unlimited processes may be
captured in parallel, but that no waiting will take place (i.e. the
collecting process is not guaranteed access to ``/proc/<crashing
pid>/``).
This value defaults to 0.

core_uses_pid
=============

The default coredump filename is "core".  By setting
``core_uses_pid`` to 1, the coredump filename becomes core.PID.
If `core_pattern`_ does not include "%p" (default does not)
and ``core_uses_pid`` is set, then .PID will be appended to
the filename.

ctrl-alt-del
============

When the value in this file is 0, ctrl-alt-del is trapped and
sent to the ``init(1)`` program to handle a graceful restart.
When, however, the value is > 0, Linux's reaction to a Vulcan
Nerve Pinch (tm) will be an immediate reboot, without even
syncing its dirty buffers.

Note:
  when a program (like dosemu) has the keyboard in 'raw'
  mode, the ctrl-alt-del is intercepted by the program before it
  ever reaches the kernel tty layer, and it's up to the program
  to decide what to do with it.

dmesg_restrict
==============

This toggle indicates whether unprivileged users are prevented
from using ``dmesg(8)`` to view messages from the kernel's log
buffer.
When ``dmesg_restrict`` is set to 0 there are no restrictions.
When ``dmesg_restrict`` is set to 1, users must have
``CAP_SYSLOG`` to use ``dmesg(8)``.

The kernel config option ``CONFIG_SECURITY_DMESG_RESTRICT`` sets the
default value of ``dmesg_restrict``.

domainname & hostname
=====================

These files can be used to set the NIS/YP domainname and the
hostname of your box in exactly the same way as the commands
domainname and hostname, i.e.::

	# echo "darkstar" > /proc/sys/kernel/hostname
	# echo "mydomain" > /proc/sys/kernel/domainname

has the same effect as::

	# hostname "darkstar"
	# domainname "mydomain"

Note, however, that the classic darkstar.frop.org has the
hostname "darkstar" and DNS (Internet Domain Name Server)
domainname "frop.org", not to be confused with the NIS (Network
Information Service) or YP (Yellow Pages) domainname. These two
domain names are in general different. For a detailed discussion
see the ``hostname(1)`` man page.

firmware_config
===============

See Documentation/driver-api/firmware/fallback-mechanisms.rst.

The entries in this directory allow the firmware loader helper
fallback to be controlled:

* ``force_sysfs_fallback``, when set to 1, forces the use of the
  fallback;
* ``ignore_sysfs_fallback``, when set to 1, ignores any fallback.

ftrace_dump_on_oops
===================

Determines whether ``ftrace_dump()`` should be called on an oops (or
kernel panic). This will output the contents of the ftrace buffers to
the console.  This is very useful for capturing traces that lead to
crashes and outputting them to a serial console.

= ===================================================
0 Disabled (default).
1 Dump buffers of all CPUs.
2 Dump the buffer of the CPU that triggered the oops.
= ===================================================

ftrace_enabled, stack_tracer_enabled
====================================

See Documentation/trace/ftrace.rst.

hardlockup_all_cpu_backtrace
============================

This value controls the hard lockup detector behavior when a hard
lockup condition is detected as to whether or not to gather further
debug information. If enabled, arch-specific all-CPU stack dumping
will be initiated.

= ============================================
0 Do nothing. This is the default behavior.
1 On detection capture more debug information.
= ============================================

hardlockup_panic
================

This parameter can be used to control whether the kernel panics
when a hard lockup is detected.

= ===========================
0 Don't panic on hard lockup.
1 Panic on hard lockup.
= ===========================

See Documentation/admin-guide/lockup-watchdogs.rst for more information.
This can also be set using the nmi_watchdog kernel parameter.

hotplug
=======

Path for the hotplug policy agent.
Default value is ``CONFIG_UEVENT_HELPER_PATH``, which in turn defaults
to the empty string.

This file only exists when ``CONFIG_UEVENT_HELPER`` is enabled. Most
modern systems rely exclusively on the netlink-based uevent source and
don't need this.

hung_task_all_cpu_backtrace
===========================

If this option is set, the kernel will send an NMI to all CPUs to dump
their backtraces when a hung task is detected. This file shows up if
CONFIG_DETECT_HUNG_TASK and CONFIG_SMP are enabled.

0: Won't show all CPUs backtraces when a hung task is detected.
This is the default behavior.

1: Will non-maskably interrupt all CPUs and dump their backtraces when
a hung task is detected.

hung_task_panic
===============

Controls the kernel's behavior when a hung task is detected.
This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.

= =================================================
0 Continue operation. This is the default behavior.
1 Panic immediately.
= =================================================

hung_task_check_count
=====================

The upper bound on the number of tasks that are checked.
This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.

hung_task_timeout_secs
======================

When a task in D state did not get scheduled
for more than this value report a warning.
This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.

0 means infinite timeout, no checking is done.

Possible values to set are in range {0:``LONG_MAX``/``HZ``}.

hung_task_check_interval_secs
=============================

Hung task check interval. If hung task checking is enabled
(see `hung_task_timeout_secs`_), the check is done every
``hung_task_check_interval_secs`` seconds.
This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.

0 (default) means use ``hung_task_timeout_secs`` as checking
interval.

Possible values to set are in range {0:``LONG_MAX``/``HZ``}.

hung_task_warnings
==================

The maximum number of warnings to report. During a check interval
if a hung task is detected, this value is decreased by 1.
When this value reaches 0, no more warnings will be reported.
This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.

-1: report an infinite number of warnings.

hyperv_record_panic_msg
=======================

Controls whether the panic kmsg data should be reported to Hyper-V.

= =========================================================
0 Do not report panic kmsg data.
1 Report the panic kmsg data. This is the default behavior.
= =========================================================

ignore-unaligned-usertrap
=========================

On architectures where unaligned accesses cause traps, and where this
feature is supported (``CONFIG_SYSCTL_ARCH_UNALIGN_NO_WARN``;
currently, ``arc`` and ``ia64``), controls whether all unaligned traps
are logged.

= =============================================================
0 Log all unaligned accesses.
1 Only warn the first time a process traps. This is the default
  setting.
= =============================================================

See also `unaligned-trap`_ and `unaligned-dump-stack`_. On ``ia64``,
this allows system administrators to override the
``IA64_THREAD_UAC_NOPRINT`` ``prctl`` and avoid logs being flooded.

kexec_load_disabled
===================

A toggle indicating if the ``kexec_load`` syscall has been disabled.
This value defaults to 0 (false: ``kexec_load`` enabled), but can be
set to 1 (true: ``kexec_load`` disabled).
Once true, kexec can no longer be used, and the toggle cannot be set
back to false.
This allows a kexec image to be loaded before disabling the syscall,
allowing a system to set up (and later use) an image without it being
altered.
Generally used together with the `modules_disabled`_ sysctl.

kptr_restrict
=============

This toggle indicates whether restrictions are placed on
exposing kernel addresses via ``/proc`` and other interfaces.

When ``kptr_restrict`` is set to 0 (the default) the address is hashed
before printing.
(This is the equivalent to %p.)

When ``kptr_restrict`` is set to 1, kernel pointers printed using the
%pK format specifier will be replaced with 0s unless the user has
``CAP_SYSLOG`` and effective user and group ids are equal to the real
ids.
This is because %pK checks are done at read() time rather than open()
time, so if permissions are elevated between the open() and the read()
(e.g via a setuid binary) then %pK will not leak kernel pointers to
unprivileged users.
Note, this is a temporary solution only.
The correct long-term solution is to do the permission checks at
open() time.
Consider removing world read permissions from files that use %pK, and
using `dmesg_restrict`_ to protect against uses of %pK in ``dmesg(8)``
if leaking kernel pointer values to unprivileged users is a concern.

When ``kptr_restrict`` is set to 2, kernel pointers printed using
%pK will be replaced with 0s regardless of privileges.

modprobe
========

The full path to the usermode helper for autoloading kernel modules,
by default ``CONFIG_MODPROBE_PATH``, which in turn defaults to
"/sbin/modprobe".  This binary is executed when the kernel requests a
module.  For example, if userspace passes an unknown filesystem type
to mount(), then the kernel will automatically request the
corresponding filesystem module by executing this usermode helper.
This usermode helper should insert the needed module into the kernel.

This sysctl only affects module autoloading.  It has no effect on the
ability to explicitly insert modules.

This sysctl can be used to debug module loading requests::

    echo '#! /bin/sh' > /tmp/modprobe
    echo 'echo "$@" >> /tmp/modprobe.log' >> /tmp/modprobe
    echo 'exec /sbin/modprobe "$@"' >> /tmp/modprobe
    chmod a+x /tmp/modprobe
    echo /tmp/modprobe > /proc/sys/kernel/modprobe

Alternatively, if this sysctl is set to the empty string, then module
autoloading is completely disabled.  The kernel will not try to
execute a usermode helper at all, nor will it call the
kernel_module_request LSM hook.

If CONFIG_STATIC_USERMODEHELPER=y is set in the kernel configuration,
then the configured static usermode helper overrides this sysctl,
except that the empty string is still accepted to completely disable
module autoloading as described above.

modules_disabled
================

A toggle value indicating if modules are allowed to be loaded
in an otherwise modular kernel.  This toggle defaults to off
(0), but can be set true (1).  Once true, modules can be
neither loaded nor unloaded, and the toggle cannot be set back
to false.  Generally used with the `kexec_load_disabled`_ toggle.

.. _msgmni:

msgmax, msgmnb, and msgmni
==========================

``msgmax`` is the maximum size of an IPC message, in bytes. 8192 by
default (``MSGMAX``).

``msgmnb`` is the maximum size of an IPC queue, in bytes. 16384 by
default (``MSGMNB``).

``msgmni`` is the maximum number of IPC queues. 32000 by default
(``MSGMNI``).

msg_next_id, sem_next_id, and shm_next_id (System V IPC)
========================================================

These three toggles allows to specify desired id for next allocated IPC
object: message, semaphore or shared memory respectively.

By default they are equal to -1, which means generic allocation logic.
Possible values to set are in range {0:``INT_MAX``}.

Notes:
  1) kernel doesn't guarantee, that new object will have desired id. So,
     it's up to userspace, how to handle an object with "wrong" id.
  2) Toggle with non-default value will be set back to -1 by kernel after
     successful IPC object allocation. If an IPC object allocation syscall
     fails, it is undefined if the value remains unmodified or is reset to -1.

ngroups_max
===========

Maximum number of supplementary groups, _i.e._ the maximum size which
``setgroups`` will accept. Exports ``NGROUPS_MAX`` from the kernel.

nmi_watchdog
============

This parameter can be used to control the NMI watchdog
(i.e. the hard lockup detector) on x86 systems.

= =================================
0 Disable the hard lockup detector.
1 Enable the hard lockup detector.
= =================================

The hard lockup detector monitors each CPU for its ability to respond to
timer interrupts. The mechanism utilizes CPU performance counter registers
that are programmed to generate Non-Maskable Interrupts (NMIs) periodically
while a CPU is busy. Hence, the alternative name 'NMI watchdog'.

The NMI watchdog is disabled by default if the kernel is running as a guest
in a KVM virtual machine. This default can be overridden by adding::

   nmi_watchdog=1

to the guest kernel command line (see
Documentation/admin-guide/kernel-parameters.rst).

nmi_wd_lpm_factor (PPC only)
============================

Factor to apply to the NMI watchdog timeout (only when ``nmi_watchdog`` is
set to 1). This factor represents the percentage added to
``watchdog_thresh`` when calculating the NMI watchdog timeout during an
LPM. The soft lockup timeout is not impacted.

A value of 0 means no change. The default value is 200 meaning the NMI
watchdog is set to 30s (based on ``watchdog_thresh`` equal to 10).

numa_balancing
==============

Enables/disables and configures automatic page fault based NUMA memory
balancing.  Memory is moved automatically to nodes that access it often.
The value to set can be the result of ORing the following:

= =================================
0 NUMA_BALANCING_DISABLED
1 NUMA_BALANCING_NORMAL
2 NUMA_BALANCING_MEMORY_TIERING
= =================================

Or NUMA_BALANCING_NORMAL to optimize page placement among different
NUMA nodes to reduce remote accessing.  On NUMA machines, there is a
performance penalty if remote memory is accessed by a CPU. When this
feature is enabled the kernel samples what task thread is accessing
memory by periodically unmapping pages and later trapping a page
fault. At the time of the page fault, it is determined if the data
being accessed should be migrated to a local memory node.

The unmapping of pages and trapping faults incur additional overhead that
ideally is offset by improved memory locality but there is no universal
guarantee. If the target workload is already bound to NUMA nodes then this
feature should be disabled.

Or NUMA_BALANCING_MEMORY_TIERING to optimize page placement among
different types of memory (represented as different NUMA nodes) to
place the hot pages in the fast memory.  This is implemented based on
unmapping and page fault too.

numa_balancing_promote_rate_limit_MBps
======================================

Too high promotion/demotion throughput between different memory types
may hurt application latency.  This can be used to rate limit the
promotion throughput.  The per-node max promotion throughput in MB/s
will be limited to be no more than the set value.

A rule of thumb is to set this to less than 1/10 of the PMEM node
write bandwidth.

oops_all_cpu_backtrace
======================

If this option is set, the kernel will send an NMI to all CPUs to dump
their backtraces when an oops event occurs. It should be used as a last
resort in case a panic cannot be triggered (to protect VMs running, for
example) or kdump can't be collected. This file shows up if CONFIG_SMP
is enabled.

0: Won't show all CPUs backtraces when an oops is detected.
This is the default behavior.

1: Will non-maskably interrupt all CPUs and dump their backtraces when
an oops event is detected.

oops_limit
==========

Number of kernel oopses after which the kernel should panic when
``panic_on_oops`` is not set. Setting this to 0 disables checking
the count. Setting this to  1 has the same effect as setting
``panic_on_oops=1``. The default value is 10000.

osrelease, ostype & version
===========================

::

  # cat osrelease
  2.1.88
  # cat ostype
  Linux
  # cat version
  #5 Wed Feb 25 21:49:24 MET 1998

The files ``osrelease`` and ``ostype`` should be clear enough.
``version``
needs a little more clarification however. The '#5' means that
this is the fifth kernel built from this source base and the
date behind it indicates the time the kernel was built.
The only way to tune these values is to rebuild the kernel :-)

overflowgid & overflowuid
=========================

if your architecture did not always support 32-bit UIDs (i.e. arm,
i386, m68k, sh, and sparc32), a fixed UID and GID will be returned to
applications that use the old 16-bit UID/GID system calls, if the
actual UID or GID would exceed 65535.

These sysctls allow you to change the value of the fixed UID and GID.
The default is 65534.

panic
=====

The value in this file determines the behaviour of the kernel on a
panic:

* if zero, the kernel will loop forever;
* if negative, the kernel will reboot immediately;
* if positive, the kernel will reboot after the corresponding number
  of seconds.

When you use the software watchdog, the recommended setting is 60.

panic_on_io_nmi
===============

Controls the kernel's behavior when a CPU receives an NMI caused by
an IO error.

= ==================================================================
0 Try to continue operation (default).
1 Panic immediately. The IO error triggered an NMI. This indicates a
  serious system condition which could result in IO data corruption.
  Rather than continuing, panicking might be a better choice. Some
  servers issue this sort of NMI when the dump button is pushed,
  and you can use this option to take a crash dump.
= ==================================================================

panic_on_oops
=============

Controls the kernel's behaviour when an oops or BUG is encountered.

= ===================================================================
0 Try to continue operation.
1 Panic immediately.  If the `panic` sysctl is also non-zero then the
  machine will be rebooted.
= ===================================================================

panic_on_stackoverflow
======================

Controls the kernel's behavior when detecting the overflows of
kernel, IRQ and exception stacks except a user stack.
This file shows up if ``CONFIG_DEBUG_STACKOVERFLOW`` is enabled.

= ==========================
0 Try to continue operation.
1 Panic immediately.
= ==========================

panic_on_unrecovered_nmi
========================

The default Linux behaviour on an NMI of either memory or unknown is
to continue operation. For many environments such as scientific
computing it is preferable that the box is taken out and the error
dealt with than an uncorrected parity/ECC error get propagated.

A small number of systems do generate NMIs for bizarre random reasons
such as power management so the default is off. That sysctl works like
the existing panic controls already in that directory.

panic_on_warn
=============

Calls panic() in the WARN() path when set to 1.  This is useful to avoid
a kernel rebuild when attempting to kdump at the location of a WARN().

= ================================================
0 Only WARN(), default behaviour.
1 Call panic() after printing out WARN() location.
= ================================================

panic_print
===========

Bitmask for printing system info when panic happens. User can chose
combination of the following bits:

=====  ============================================
bit 0  print all tasks info
bit 1  print system memory info
bit 2  print timer info
bit 3  print locks info if ``CONFIG_LOCKDEP`` is on
bit 4  print ftrace buffer
bit 5  print all printk messages in buffer
bit 6  print all CPUs backtrace (if available in the arch)
=====  ============================================

So for example to print tasks and memory info on panic, user can::

  echo 3 > /proc/sys/kernel/panic_print

panic_on_rcu_stall
==================

When set to 1, calls panic() after RCU stall detection messages. This
is useful to define the root cause of RCU stalls using a vmcore.

= ============================================================
0 Do not panic() when RCU stall takes place, default behavior.
1 panic() after printing RCU stall messages.
= ============================================================

max_rcu_stall_to_panic
======================

When ``panic_on_rcu_stall`` is set to 1, this value determines the
number of times that RCU can stall before panic() is called.

When ``panic_on_rcu_stall`` is set to 0, this value is has no effect.

perf_cpu_time_max_percent
=========================

Hints to the kernel how much CPU time it should be allowed to
use to handle perf sampling events.  If the perf subsystem
is informed that its samples are exceeding this limit, it
will drop its sampling frequency to attempt to reduce its CPU
usage.

Some perf sampling happens in NMIs.  If these samples
unexpectedly take too long to execute, the NMIs can become
stacked up next to each other so much that nothing else is
allowed to execute.

===== ========================================================
0     Disable the mechanism.  Do not monitor or correct perf's
      sampling rate no matter how CPU time it takes.

1-100 Attempt to throttle perf's sample rate to this
      percentage of CPU.  Note: the kernel calculates an
      "expected" length of each sample event.  100 here means
      100% of that expected length.  Even if this is set to
      100, you may still see sample throttling if this
      length is exceeded.  Set to 0 if you truly do not care
      how much CPU is consumed.
===== ========================================================

perf_event_paranoid
===================

Controls use of the performance events system by unprivileged
users (without CAP_PERFMON).  The default value is 2.

For backward compatibility reasons access to system performance
monitoring and observability remains open for CAP_SYS_ADMIN
privileged processes but CAP_SYS_ADMIN usage for secure system
performance monitoring and observability operations is discouraged
with respect to CAP_PERFMON use cases.

===  ==================================================================
 -1  Allow use of (almost) all events by all users.

     Ignore mlock limit after perf_event_mlock_kb without
     ``CAP_IPC_LOCK``.

>=0  Disallow ftrace function tracepoint by users without
     ``CAP_PERFMON``.

     Disallow raw tracepoint access by users without ``CAP_PERFMON``.

>=1  Disallow CPU event access by users without ``CAP_PERFMON``.

>=2  Disallow kernel profiling by users without ``CAP_PERFMON``.
===  ==================================================================

perf_event_max_stack
====================

Controls maximum number of stack frames to copy for (``attr.sample_type &
PERF_SAMPLE_CALLCHAIN``) configured events, for instance, when using
'``perf record -g``' or '``perf trace --call-graph fp``'.

This can only be done when no events are in use that have callchains
enabled, otherwise writing to this file will return ``-EBUSY``.

The default value is 127.

perf_event_mlock_kb
===================

Control size of per-cpu ring buffer not counted against mlock limit.

The default value is 512 + 1 page

perf_event_max_contexts_per_stack
=================================

Controls maximum number of stack frame context entries for
(``attr.sample_type & PERF_SAMPLE_CALLCHAIN``) configured events, for
instance, when using '``perf record -g``' or '``perf trace --call-graph fp``'.

This can only be done when no events are in use that have callchains
enabled, otherwise writing to this file will return ``-EBUSY``.

The default value is 8.

perf_user_access (arm64 only)
=================================

Controls user space access for reading perf event counters. When set to 1,
user space can read performance monitor counter registers directly.

The default value is 0 (access disabled).

See Documentation/arm64/perf.rst for more information.

pid_max
=======

PID allocation wrap value.  When the kernel's next PID value
reaches this value, it wraps back to a minimum PID value.
PIDs of value ``pid_max`` or larger are not allocated.

ns_last_pid
===========

The last pid allocated in the current (the one task using this sysctl
lives in) pid namespace. When selecting a pid for a next task on fork
kernel tries to allocate a number starting from this one.

powersave-nap (PPC only)
========================

If set, Linux-PPC will use the 'nap' mode of powersaving,
otherwise the 'doze' mode will be used.

==============================================================

printk
======

The four values in printk denote: ``console_loglevel``,
``default_message_loglevel``, ``minimum_console_loglevel`` and
``default_console_loglevel`` respectively.

These values influence printk() behavior when printing or
logging error messages. See '``man 2 syslog``' for more info on
the different loglevels.

======================== =====================================
console_loglevel         messages with a higher priority than
                         this will be printed to the console
default_message_loglevel messages without an explicit priority
                         will be printed with this priority
minimum_console_loglevel minimum (highest) value to which
                         console_loglevel can be set
default_console_loglevel default value for console_loglevel
======================== =====================================

printk_delay
============

Delay each printk message in ``printk_delay`` milliseconds

Value from 0 - 10000 is allowed.

printk_ratelimit
================

Some warning messages are rate limited. ``printk_ratelimit`` specifies
the minimum length of time between these messages (in seconds).
The default value is 5 seconds.

A value of 0 will disable rate limiting.

printk_ratelimit_burst
======================

While long term we enforce one message per `printk_ratelimit`_
seconds, we do allow a burst of messages to pass through.
``printk_ratelimit_burst`` specifies the number of messages we can
send before ratelimiting kicks in.

The default value is 10 messages.

printk_devkmsg
==============

Control the logging to ``/dev/kmsg`` from userspace:

========= =============================================
ratelimit default, ratelimited
on        unlimited logging to /dev/kmsg from userspace
off       logging to /dev/kmsg disabled
========= =============================================

The kernel command line parameter ``printk.devkmsg=`` overrides this and is
a one-time setting until next reboot: once set, it cannot be changed by
this sysctl interface anymore.

==============================================================

pty
===

See Documentation/filesystems/devpts.rst.

random
======

This is a directory, with the following entries:

* ``boot_id``: a UUID generated the first time this is retrieved, and
  unvarying after that;

* ``uuid``: a UUID generated every time this is retrieved (this can
  thus be used to generate UUIDs at will);

* ``entropy_avail``: the pool's entropy count, in bits;

* ``poolsize``: the entropy pool size, in bits;

* ``urandom_min_reseed_secs``: obsolete (used to determine the minimum
  number of seconds between urandom pool reseeding). This file is
  writable for compatibility purposes, but writing to it has no effect
  on any RNG behavior;

* ``write_wakeup_threshold``: when the entropy count drops below this
  (as a number of bits), processes waiting to write to ``/dev/random``
  are woken up. This file is writable for compatibility purposes, but
  writing to it has no effect on any RNG behavior.

randomize_va_space
==================

This option can be used to select the type of process address
space randomization that is used in the system, for architectures
that support this feature.

==  ===========================================================================
0   Turn the process address space randomization off.  This is the
    default for architectures that do not support this feature anyways,
    and kernels that are booted with the "norandmaps" parameter.

1   Make the addresses of mmap base, stack and VDSO page randomized.
    This, among other things, implies that shared libraries will be
    loaded to random addresses.  Also for PIE-linked binaries, the
    location of code start is randomized.  This is the default if the
    ``CONFIG_COMPAT_BRK`` option is enabled.

2   Additionally enable heap randomization.  This is the default if
    ``CONFIG_COMPAT_BRK`` is disabled.

    There are a few legacy applications out there (such as some ancient
    versions of libc.so.5 from 1996) that assume that brk area starts
    just after the end of the code+bss.  These applications break when
    start of the brk area is randomized.  There are however no known
    non-legacy applications that would be broken this way, so for most
    systems it is safe to choose full randomization.

    Systems with ancient and/or broken binaries should be configured
    with ``CONFIG_COMPAT_BRK`` enabled, which excludes the heap from process
    address space randomization.
==  ===========================================================================

real-root-dev
=============

See Documentation/admin-guide/initrd.rst.

reboot-cmd (SPARC only)
=======================

??? This seems to be a way to give an argument to the Sparc
ROM/Flash boot loader. Maybe to tell it what to do after
rebooting. ???

sched_energy_aware
==================

Enables/disables Energy Aware Scheduling (EAS). EAS starts
automatically on platforms where it can run (that is,
platforms with asymmetric CPU topologies and having an Energy
Model available). If your platform happens to meet the
requirements for EAS but you do not want to use it, change
this value to 0.

task_delayacct
===============

Enables/disables task delay accounting (see
Documentation/accounting/delay-accounting.rst. Enabling this feature incurs
a small amount of overhead in the scheduler but is useful for debugging
and performance tuning. It is required by some tools such as iotop.

sched_schedstats
================

Enables/disables scheduler statistics. Enabling this feature
incurs a small amount of overhead in the scheduler but is
useful for debugging and performance tuning.

sched_util_clamp_min
====================

Max allowed *minimum* utilization.

Default value is 1024, which is the maximum possible value.

It means that any requested uclamp.min value cannot be greater than
sched_util_clamp_min, i.e., it is restricted to the range
[0:sched_util_clamp_min].

sched_util_clamp_max
====================

Max allowed *maximum* utilization.

Default value is 1024, which is the maximum possible value.

It means that any requested uclamp.max value cannot be greater than
sched_util_clamp_max, i.e., it is restricted to the range
[0:sched_util_clamp_max].

sched_util_clamp_min_rt_default
===============================

By default Linux is tuned for performance. Which means that RT tasks always run
at the highest frequency and most capable (highest capacity) CPU (in
heterogeneous systems).

Uclamp achieves this by setting the requested uclamp.min of all RT tasks to
1024 by default, which effectively boosts the tasks to run at the highest
frequency and biases them to run on the biggest CPU.

This knob allows admins to change the default behavior when uclamp is being
used. In battery powered devices particularly, running at the maximum
capacity and frequency will increase energy consumption and shorten the battery
life.

This knob is only effective for RT tasks which the user hasn't modified their
requested uclamp.min value via sched_setattr() syscall.

This knob will not escape the range constraint imposed by sched_util_clamp_min
defined above.

For example if

	sched_util_clamp_min_rt_default = 800
	sched_util_clamp_min = 600

Then the boost will be clamped to 600 because 800 is outside of the permissible
range of [0:600]. This could happen for instance if a powersave mode will
restrict all boosts temporarily by modifying sched_util_clamp_min. As soon as
this restriction is lifted, the requested sched_util_clamp_min_rt_default
will take effect.

seccomp
=======

See Documentation/userspace-api/seccomp_filter.rst.

sg-big-buff
===========

This file shows the size of the generic SCSI (sg) buffer.
You can't tune it just yet, but you could change it on
compile time by editing ``include/scsi/sg.h`` and changing
the value of ``SG_BIG_BUFF``.

There shouldn't be any reason to change this value. If
you can come up with one, you probably know what you
are doing anyway :)

shmall
======

This parameter sets the total amount of shared memory pages that
can be used system wide. Hence, ``shmall`` should always be at least
``ceil(shmmax/PAGE_SIZE)``.

If you are not sure what the default ``PAGE_SIZE`` is on your Linux
system, you can run the following command::

	# getconf PAGE_SIZE

shmmax
======

This value can be used to query and set the run time limit
on the maximum shared memory segment size that can be created.
Shared memory segments up to 1Gb are now supported in the
kernel.  This value defaults to ``SHMMAX``.

shmmni
======

This value determines the maximum number of shared memory segments.
4096 by default (``SHMMNI``).

shm_rmid_forced
===============

Linux lets you set resource limits, including how much memory one
process can consume, via ``setrlimit(2)``.  Unfortunately, shared memory
segments are allowed to exist without association with any process, and
thus might not be counted against any resource limits.  If enabled,
shared memory segments are automatically destroyed when their attach
count becomes zero after a detach or a process termination.  It will
also destroy segments that were created, but never attached to, on exit
from the process.  The only use left for ``IPC_RMID`` is to immediately
destroy an unattached segment.  Of course, this breaks the way things are
defined, so some applications might stop working.  Note that this
feature will do you no good unless you also configure your resource
limits (in particular, ``RLIMIT_AS`` and ``RLIMIT_NPROC``).  Most systems don't
need this.

Note that if you change this from 0 to 1, already created segments
without users and with a dead originative process will be destroyed.

sysctl_writes_strict
====================

Control how file position affects the behavior of updating sysctl values
via the ``/proc/sys`` interface:

  ==   ======================================================================
  -1   Legacy per-write sysctl value handling, with no printk warnings.
       Each write syscall must fully contain the sysctl value to be
       written, and multiple writes on the same sysctl file descriptor
       will rewrite the sysctl value, regardless of file position.
   0   Same behavior as above, but warn about processes that perform writes
       to a sysctl file descriptor when the file position is not 0.
   1   (default) Respect file position when writing sysctl strings. Multiple
       writes will append to the sysctl value buffer. Anything past the max
       length of the sysctl value buffer will be ignored. Writes to numeric
       sysctl entries must always be at file position 0 and the value must
       be fully contained in the buffer sent in the write syscall.
  ==   ======================================================================

softlockup_all_cpu_backtrace
============================

This value controls the soft lockup detector thread's behavior
when a soft lockup condition is detected as to whether or not
to gather further debug information. If enabled, each cpu will
be issued an NMI and instructed to capture stack trace.

This feature is only applicable for architectures which support
NMI.

= ============================================
0 Do nothing. This is the default behavior.
1 On detection capture more debug information.
= ============================================

softlockup_panic
=================

This parameter can be used to control whether the kernel panics
when a soft lockup is detected.

= ============================================
0 Don't panic on soft lockup.
1 Panic on soft lockup.
= ============================================

This can also be set using the softlockup_panic kernel parameter.

soft_watchdog
=============

This parameter can be used to control the soft lockup detector.

= =================================
0 Disable the soft lockup detector.
1 Enable the soft lockup detector.
= =================================

The soft lockup detector monitors CPUs for threads that are hogging the CPUs
without rescheduling voluntarily, and thus prevent the 'migration/N' threads
from running, causing the watchdog work fail to execute. The mechanism depends
on the CPUs ability to respond to timer interrupts which are needed for the
watchdog work to be queued by the watchdog timer function, otherwise the NMI
watchdog — if enabled — can detect a hard lockup condition.

split_lock_mitigate (x86 only)
==============================

On x86, each "split lock" imposes a system-wide performance penalty. On larger
systems, large numbers of split locks from unprivileged users can result in
denials of service to well-behaved and potentially more important users.

The kernel mitigates these bad users by detecting split locks and imposing
penalties: forcing them to wait and only allowing one core to execute split
locks at a time.

These mitigations can make those bad applications unbearably slow. Setting
split_lock_mitigate=0 may restore some application performance, but will also
increase system exposure to denial of service attacks from split lock users.

= ===================================================================
0 Disable the mitigation mode - just warns the split lock on kernel log
  and exposes the system to denials of service from the split lockers.
1 Enable the mitigation mode (this is the default) - penalizes the split
  lockers with intentional performance degradation.
= ===================================================================

stack_erasing
=============

This parameter can be used to control kernel stack erasing at the end
of syscalls for kernels built with ``CONFIG_GCC_PLUGIN_STACKLEAK``.

That erasing reduces the information which kernel stack leak bugs
can reveal and blocks some uninitialized stack variable attacks.
The tradeoff is the performance impact: on a single CPU system kernel
compilation sees a 1% slowdown, other systems and workloads may vary.

= ====================================================================
0 Kernel stack erasing is disabled, STACKLEAK_METRICS are not updated.
1 Kernel stack erasing is enabled (default), it is performed before
  returning to the userspace at the end of syscalls.
= ====================================================================

stop-a (SPARC only)
===================

Controls Stop-A:

= ====================================
0 Stop-A has no effect.
1 Stop-A breaks to the PROM (default).
= ====================================

Stop-A is always enabled on a panic, so that the user can return to
the boot PROM.

sysrq
=====

See Documentation/admin-guide/sysrq.rst.

tainted
=======

Non-zero if the kernel has been tainted. Numeric values, which can be
ORed together. The letters are seen in "Tainted" line of Oops reports.

======  =====  ==============================================================
     1  `(P)`  proprietary module was loaded
     2  `(F)`  module was force loaded
     4  `(S)`  kernel running on an out of specification system
     8  `(R)`  module was force unloaded
    16  `(M)`  processor reported a Machine Check Exception (MCE)
    32  `(B)`  bad page referenced or some unexpected page flags
    64  `(U)`  taint requested by userspace application
   128  `(D)`  kernel died recently, i.e. there was an OOPS or BUG
   256  `(A)`  an ACPI table was overridden by user
   512  `(W)`  kernel issued warning
  1024  `(C)`  staging driver was loaded
  2048  `(I)`  workaround for bug in platform firmware applied
  4096  `(O)`  externally-built ("out-of-tree") module was loaded
  8192  `(E)`  unsigned module was loaded
 16384  `(L)`  soft lockup occurred
 32768  `(K)`  kernel has been live patched
 65536  `(X)`  Auxiliary taint, defined and used by for distros
131072  `(T)`  The kernel was built with the struct randomization plugin
======  =====  ==============================================================

See Documentation/admin-guide/tainted-kernels.rst for more information.

Note:
  writes to this sysctl interface will fail with ``EINVAL`` if the kernel is
  booted with the command line option ``panic_on_taint=<bitmask>,nousertaint``
  and any of the ORed together values being written to ``tainted`` match with
  the bitmask declared on panic_on_taint.
  See Documentation/admin-guide/kernel-parameters.rst for more details on
  that particular kernel command line option and its optional
  ``nousertaint`` switch.

threads-max
===========

This value controls the maximum number of threads that can be created
using ``fork()``.

During initialization the kernel sets this value such that even if the
maximum number of threads is created, the thread structures occupy only
a part (1/8th) of the available RAM pages.

The minimum value that can be written to ``threads-max`` is 1.

The maximum value that can be written to ``threads-max`` is given by the
constant ``FUTEX_TID_MASK`` (0x3fffffff).

If a value outside of this range is written to ``threads-max`` an
``EINVAL`` error occurs.

traceoff_on_warning
===================

When set, disables tracing (see Documentation/trace/ftrace.rst) when a
``WARN()`` is hit.

tracepoint_printk
=================

When tracepoints are sent to printk() (enabled by the ``tp_printk``
boot parameter), this entry provides runtime control::

    echo 0 > /proc/sys/kernel/tracepoint_printk

will stop tracepoints from being sent to printk(), and::

    echo 1 > /proc/sys/kernel/tracepoint_printk

will send them to printk() again.

This only works if the kernel was booted with ``tp_printk`` enabled.

See Documentation/admin-guide/kernel-parameters.rst and
Documentation/trace/boottime-trace.rst.

.. _unaligned-dump-stack:

unaligned-dump-stack (ia64)
===========================

When logging unaligned accesses, controls whether the stack is
dumped.

= ===================================================
0 Do not dump the stack. This is the default setting.
1 Dump the stack.
= ===================================================

See also `ignore-unaligned-usertrap`_.

unaligned-trap
==============

On architectures where unaligned accesses cause traps, and where this
feature is supported (``CONFIG_SYSCTL_ARCH_UNALIGN_ALLOW``; currently,
``arc`` and ``parisc``), controls whether unaligned traps are caught
and emulated (instead of failing).

= ========================================================
0 Do not emulate unaligned accesses.
1 Emulate unaligned accesses. This is the default setting.
= ========================================================

See also `ignore-unaligned-usertrap`_.

unknown_nmi_panic
=================

The value in this file affects behavior of handling NMI. When the
value is non-zero, unknown NMI is trapped and then panic occurs. At
that time, kernel debugging information is displayed on console.

NMI switch that most IA32 servers have fires unknown NMI up, for
example.  If a system hangs up, try pressing the NMI switch.

unprivileged_bpf_disabled
=========================

Writing 1 to this entry will disable unprivileged calls to ``bpf()``;
once disabled, calling ``bpf()`` without ``CAP_SYS_ADMIN`` or ``CAP_BPF``
will return ``-EPERM``. Once set to 1, this can't be cleared from the
running kernel anymore.

Writing 2 to this entry will also disable unprivileged calls to ``bpf()``,
however, an admin can still change this setting later on, if needed, by
writing 0 or 1 to this entry.

If ``BPF_UNPRIV_DEFAULT_OFF`` is enabled in the kernel config, then this
entry will default to 2 instead of 0.

= =============================================================
0 Unprivileged calls to ``bpf()`` are enabled
1 Unprivileged calls to ``bpf()`` are disabled without recovery
2 Unprivileged calls to ``bpf()`` are disabled
= =============================================================

warn_limit
==========

Number of kernel warnings after which the kernel should panic when
``panic_on_warn`` is not set. Setting this to 0 disables checking
the warning count. Setting this to 1 has the same effect as setting
``panic_on_warn=1``. The default value is 0.

watchdog
========

This parameter can be used to disable or enable the soft lockup detector
*and* the NMI watchdog (i.e. the hard lockup detector) at the same time.

= ==============================
0 Disable both lockup detectors.
1 Enable both lockup detectors.
= ==============================

The soft lockup detector and the NMI watchdog can also be disabled or
enabled individually, using the ``soft_watchdog`` and ``nmi_watchdog``
parameters.
If the ``watchdog`` parameter is read, for example by executing::

   cat /proc/sys/kernel/watchdog

the output of this command (0 or 1) shows the logical OR of
``soft_watchdog`` and ``nmi_watchdog``.

watchdog_cpumask
================

This value can be used to control on which cpus the watchdog may run.
The default cpumask is all possible cores, but if ``NO_HZ_FULL`` is
enabled in the kernel config, and cores are specified with the
``nohz_full=`` boot argument, those cores are excluded by default.
Offline cores can be included in this mask, and if the core is later
brought online, the watchdog will be started based on the mask value.

Typically this value would only be touched in the ``nohz_full`` case
to re-enable cores that by default were not running the watchdog,
if a kernel lockup was suspected on those cores.

The argument value is the standard cpulist format for cpumasks,
so for example to enable the watchdog on cores 0, 2, 3, and 4 you
might say::

  echo 0,2-4 > /proc/sys/kernel/watchdog_cpumask

watchdog_thresh
===============

This value can be used to control the frequency of hrtimer and NMI
events and the soft and hard lockup thresholds. The default threshold
is 10 seconds.

The softlockup threshold is (``2 * watchdog_thresh``). Setting this
tunable to zero will disable lockup detection altogether.


code:
================================
Documentation for /proc/sys/net/
================================

Copyright

Copyright (c) 1999

	- Terrehon Bowden <terrehon@pacbell.net>
	- Bodo Bauer <bb@ricochet.net>

Copyright (c) 2000

	- Jorge Nerin <comandante@zaralinux.com>

Copyright (c) 2009

	- Shen Feng <shen@cn.fujitsu.com>

For general info and legal blurb, please look in index.rst.

------------------------------------------------------------------------------

This file contains the documentation for the sysctl files in
/proc/sys/net

The interface  to  the  networking  parts  of  the  kernel  is  located  in
/proc/sys/net. The following table shows all possible subdirectories.  You may
see only some of them, depending on your kernel's configuration.

Table : Subdirectories in /proc/sys/net

 ========= =================== = ========== ===================
 Directory Content               Directory  Content
 ========= =================== = ========== ===================
 802       E802 protocol         mptcp      Multipath TCP
 appletalk Appletalk protocol    netfilter  Network Filter
 ax25      AX25                  netrom     NET/ROM
 bridge    Bridging              rose       X.25 PLP layer
 core      General parameter     tipc       TIPC
 ethernet  Ethernet protocol     unix       Unix domain sockets
 ipv4      IP version 4          x25        X.25 protocol
 ipv6      IP version 6
 ========= =================== = ========== ===================

1. /proc/sys/net/core - Network core options
============================================

bpf_jit_enable
--------------

This enables the BPF Just in Time (JIT) compiler. BPF is a flexible
and efficient infrastructure allowing to execute bytecode at various
hook points. It is used in a number of Linux kernel subsystems such
as networking (e.g. XDP, tc), tracing (e.g. kprobes, uprobes, tracepoints)
and security (e.g. seccomp). LLVM has a BPF back end that can compile
restricted C into a sequence of BPF instructions. After program load
through bpf(2) and passing a verifier in the kernel, a JIT will then
translate these BPF proglets into native CPU instructions. There are
two flavors of JITs, the newer eBPF JIT currently supported on:

  - x86_64
  - x86_32
  - arm64
  - arm32
  - ppc64
  - ppc32
  - sparc64
  - mips64
  - s390x
  - riscv64
  - riscv32

And the older cBPF JIT supported on the following archs:

  - mips
  - sparc

eBPF JITs are a superset of cBPF JITs, meaning the kernel will
migrate cBPF instructions into eBPF instructions and then JIT
compile them transparently. Older cBPF JITs can only translate
tcpdump filters, seccomp rules, etc, but not mentioned eBPF
programs loaded through bpf(2).

Values:

	- 0 - disable the JIT (default value)
	- 1 - enable the JIT
	- 2 - enable the JIT and ask the compiler to emit traces on kernel log.

bpf_jit_harden
--------------

This enables hardening for the BPF JIT compiler. Supported are eBPF
JIT backends. Enabling hardening trades off performance, but can
mitigate JIT spraying.

Values:

	- 0 - disable JIT hardening (default value)
	- 1 - enable JIT hardening for unprivileged users only
	- 2 - enable JIT hardening for all users

where "privileged user" in this context means a process having
CAP_BPF or CAP_SYS_ADMIN in the root user name space.

bpf_jit_kallsyms
----------------

When BPF JIT compiler is enabled, then compiled images are unknown
addresses to the kernel, meaning they neither show up in traces nor
in /proc/kallsyms. This enables export of these addresses, which can
be used for debugging/tracing. If bpf_jit_harden is enabled, this
feature is disabled.

Values :

	- 0 - disable JIT kallsyms export (default value)
	- 1 - enable JIT kallsyms export for privileged users only

bpf_jit_limit
-------------

This enforces a global limit for memory allocations to the BPF JIT
compiler in order to reject unprivileged JIT requests once it has
been surpassed. bpf_jit_limit contains the value of the global limit
in bytes.

dev_weight
----------

The maximum number of packets that kernel can handle on a NAPI interrupt,
it's a Per-CPU variable. For drivers that support LRO or GRO_HW, a hardware
aggregated packet is counted as one packet in this context.

Default: 64

dev_weight_rx_bias
------------------

RPS (e.g. RFS, aRFS) processing is competing with the registered NAPI poll function
of the driver for the per softirq cycle netdev_budget. This parameter influences
the proportion of the configured netdev_budget that is spent on RPS based packet
processing during RX softirq cycles. It is further meant for making current
dev_weight adaptable for asymmetric CPU needs on RX/TX side of the network stack.
(see dev_weight_tx_bias) It is effective on a per CPU basis. Determination is based
on dev_weight and is calculated multiplicative (dev_weight * dev_weight_rx_bias).

Default: 1

dev_weight_tx_bias
------------------

Scales the maximum number of packets that can be processed during a TX softirq cycle.
Effective on a per CPU basis. Allows scaling of current dev_weight for asymmetric
net stack processing needs. Be careful to avoid making TX softirq processing a CPU hog.

Calculation is based on dev_weight (dev_weight * dev_weight_tx_bias).

Default: 1

default_qdisc
-------------

The default queuing discipline to use for network devices. This allows
overriding the default of pfifo_fast with an alternative. Since the default
queuing discipline is created without additional parameters so is best suited
to queuing disciplines that work well without configuration like stochastic
fair queue (sfq), CoDel (codel) or fair queue CoDel (fq_codel). Don't use
queuing disciplines like Hierarchical Token Bucket or Deficit Round Robin
which require setting up classes and bandwidths. Note that physical multiqueue
interfaces still use mq as root qdisc, which in turn uses this default for its
leaves. Virtual devices (like e.g. lo or veth) ignore this setting and instead
default to noqueue.

Default: pfifo_fast

busy_read
---------

Low latency busy poll timeout for socket reads. (needs CONFIG_NET_RX_BUSY_POLL)
Approximate time in us to busy loop waiting for packets on the device queue.
This sets the default value of the SO_BUSY_POLL socket option.
Can be set or overridden per socket by setting socket option SO_BUSY_POLL,
which is the preferred method of enabling. If you need to enable the feature
globally via sysctl, a value of 50 is recommended.

Will increase power usage.

Default: 0 (off)

busy_poll
----------------
Low latency busy poll timeout for poll and select. (needs CONFIG_NET_RX_BUSY_POLL)
Approximate time in us to busy loop waiting for events.
Recommended value depends on the number of sockets you poll on.
For several sockets 50, for several hundreds 100.
For more than that you probably want to use epoll.
Note that only sockets with SO_BUSY_POLL set will be busy polled,
so you want to either selectively set SO_BUSY_POLL on those sockets or set
sysctl.net.busy_read globally.

Will increase power usage.

Default: 0 (off)

rmem_default
------------

The default setting of the socket receive buffer in bytes.

rmem_max
--------

The maximum receive socket buffer size in bytes.

tstamp_allow_data
-----------------
Allow processes to receive tx timestamps looped together with the original
packet contents. If disabled, transmit timestamp requests from unprivileged
processes are dropped unless socket option SOF_TIMESTAMPING_OPT_TSONLY is set.

Default: 1 (on)

wmem_default
------------

The default setting (in bytes) of the socket send buffer.

wmem_max
--------

The maximum send socket buffer size in bytes.

message_burst and message_cost
------------------------------

These parameters  are used to limit the warning messages written to the kernel
log from  the  networking  code.  They  enforce  a  rate  limit  to  make  a
denial-of-service attack  impossible. A higher message_cost factor, results in
fewer messages that will be written. Message_burst controls when messages will
be dropped.  The  default  settings  limit  warning messages to one every five
seconds.

warnings
--------

This sysctl is now unused.

This was used to control console messages from the networking stack that
occur because of problems on the network like duplicate address or bad
checksums.

These messages are now emitted at KERN_DEBUG and can generally be enabled
and controlled by the dynamic_debug facility.

netdev_budget
-------------

Maximum number of packets taken from all interfaces in one polling cycle (NAPI
poll). In one polling cycle interfaces which are registered to polling are
probed in a round-robin manner. Also, a polling cycle may not exceed
netdev_budget_usecs microseconds, even if netdev_budget has not been
exhausted.

netdev_budget_usecs
---------------------

Maximum number of microseconds in one NAPI polling cycle. Polling
will exit when either netdev_budget_usecs have elapsed during the
poll cycle or the number of packets processed reaches netdev_budget.

netdev_max_backlog
------------------

Maximum number of packets, queued on the INPUT side, when the interface
receives packets faster than kernel can process them.

netdev_rss_key
--------------

RSS (Receive Side Scaling) enabled drivers use a 40 bytes host key that is
randomly generated.
Some user space might need to gather its content even if drivers do not
provide ethtool -x support yet.

::

  myhost:~# cat /proc/sys/net/core/netdev_rss_key
  84:50:f4:00:a8:15:d1:a7:e9:7f:1d:60:35:c7:47:25:42:97:74:ca:56:bb:b6:a1:d8: ... (52 bytes total)

File contains nul bytes if no driver ever called netdev_rss_key_fill() function.

Note:
  /proc/sys/net/core/netdev_rss_key contains 52 bytes of key,
  but most drivers only use 40 bytes of it.

::

  myhost:~# ethtool -x eth0
  RX flow hash indirection table for eth0 with 8 RX ring(s):
      0:    0     1     2     3     4     5     6     7
  RSS hash key:
  84:50:f4:00:a8:15:d1:a7:e9:7f:1d:60:35:c7:47:25:42:97:74:ca:56:bb:b6:a1:d8:43:e3:c9:0c:fd:17:55:c2:3a:4d:69:ed:f1:42:89

netdev_tstamp_prequeue
----------------------

If set to 0, RX packet timestamps can be sampled after RPS processing, when
the target CPU processes packets. It might give some delay on timestamps, but
permit to distribute the load on several cpus.

If set to 1 (default), timestamps are sampled as soon as possible, before
queueing.

netdev_unregister_timeout_secs
------------------------------

Unregister network device timeout in seconds.
This option controls the timeout (in seconds) used to issue a warning while
waiting for a network device refcount to drop to 0 during device
unregistration. A lower value may be useful during bisection to detect
a leaked reference faster. A larger value may be useful to prevent false
warnings on slow/loaded systems.
Default value is 10, minimum 1, maximum 3600.

skb_defer_max
-------------

Max size (in skbs) of the per-cpu list of skbs being freed
by the cpu which allocated them. Used by TCP stack so far.

Default: 64

optmem_max
----------

Maximum ancillary buffer size allowed per socket. Ancillary data is a sequence
of struct cmsghdr structures with appended data.

fb_tunnels_only_for_init_net
----------------------------

Controls if fallback tunnels (like tunl0, gre0, gretap0, erspan0,
sit0, ip6tnl0, ip6gre0) are automatically created. There are 3 possibilities
(a) value = 0; respective fallback tunnels are created when module is
loaded in every net namespaces (backward compatible behavior).
(b) value = 1; [kcmd value: initns] respective fallback tunnels are
created only in init net namespace and every other net namespace will
not have them.
(c) value = 2; [kcmd value: none] fallback tunnels are not created
when a module is loaded in any of the net namespace. Setting value to
"2" is pointless after boot if these modules are built-in, so there is
a kernel command-line option that can change this default. Please refer to
Documentation/admin-guide/kernel-parameters.txt for additional details.

Not creating fallback tunnels gives control to userspace to create
whatever is needed only and avoid creating devices which are redundant.

Default : 0  (for compatibility reasons)

devconf_inherit_init_net
------------------------

Controls if a new network namespace should inherit all current
settings under /proc/sys/net/{ipv4,ipv6}/conf/{all,default}/. By
default, we keep the current behavior: for IPv4 we inherit all current
settings from init_net and for IPv6 we reset all settings to default.

If set to 1, both IPv4 and IPv6 settings are forced to inherit from
current ones in init_net. If set to 2, both IPv4 and IPv6 settings are
forced to reset to their default values. If set to 3, both IPv4 and IPv6
settings are forced to inherit from current ones in the netns where this
new netns has been created.

Default : 0  (for compatibility reasons)

txrehash
--------

Controls default hash rethink behaviour on listening socket when SO_TXREHASH
option is set to SOCK_TXREHASH_DEFAULT (i. e. not overridden by setsockopt).

If set to 1 (default), hash rethink is performed on listening socket.
If set to 0, hash rethink is not performed.

gro_normal_batch
----------------

Maximum number of the segments to batch up on output of GRO. When a packet
exits GRO, either as a coalesced superframe or as an original packet which
GRO has decided not to coalesce, it is placed on a per-NAPI list. This
list is then passed to the stack when the number of segments reaches the
gro_normal_batch limit.

high_order_alloc_disable
------------------------

By default the allocator for page frags tries to use high order pages (order-3
on x86). While the default behavior gives good results in most cases, some users
might have hit a contention in page allocations/freeing. This was especially
true on older kernels (< 5.14) when high-order pages were not stored on per-cpu
lists. This allows to opt-in for order-0 allocation instead but is now mostly of
historical importance.

Default: 0

2. /proc/sys/net/unix - Parameters for Unix domain sockets
----------------------------------------------------------

There is only one file in this directory.
unix_dgram_qlen limits the max number of datagrams queued in Unix domain
socket's buffer. It will not take effect unless PF_UNIX flag is specified.

3. /proc/sys/net/ipv4 - IPV4 settings
-------------------------------------
Please see: Documentation/networking/ip-sysctl.rst and
Documentation/admin-guide/sysctl/net.rst for descriptions of these entries.

4. Appletalk
------------

The /proc/sys/net/appletalk  directory  holds the Appletalk configuration data
when Appletalk is loaded. The configurable parameters are:

aarp-expiry-time
----------------

The amount  of  time  we keep an ARP entry before expiring it. Used to age out
old hosts.

aarp-resolve-time
-----------------

The amount of time we will spend trying to resolve an Appletalk address.

aarp-retransmit-limit
---------------------

The number of times we will retransmit a query before giving up.

aarp-tick-time
--------------

Controls the rate at which expires are checked.

The directory  /proc/net/appletalk  holds the list of active Appletalk sockets
on a machine.

The fields  indicate  the DDP type, the local address (in network:node format)
the remote  address,  the  size of the transmit pending queue, the size of the
received queue  (bytes waiting for applications to read) the state and the uid
owning the socket.

/proc/net/atalk_iface lists  all  the  interfaces  configured for appletalk.It
shows the  name  of the interface, its Appletalk address, the network range on
that address  (or  network number for phase 1 networks), and the status of the
interface.

/proc/net/atalk_route lists  each  known  network  route.  It lists the target
(network) that the route leads to, the router (may be directly connected), the
route flags, and the device the route is using.

5. TIPC
-------

tipc_rmem
---------

The TIPC protocol now has a tunable for the receive memory, similar to the
tcp_rmem - i.e. a vector of 3 INTEGERs: (min, default, max)

::

    # cat /proc/sys/net/tipc/tipc_rmem
    4252725 34021800        68043600
    #

The max value is set to CONN_OVERLOAD_LIMIT, and the default and min values
are scaled (shifted) versions of that same value.  Note that the min value
is not at this point in time used in any meaningful way, but the triplet is
preserved in order to be consistent with things like tcp_rmem.

named_timeout
-------------

TIPC name table updates are distributed asynchronously in a cluster, without
any form of transaction handling. This means that different race scenarios are
possible. One such is that a name withdrawal sent out by one node and received
by another node may arrive after a second, overlapping name publication already
has been accepted from a third node, although the conflicting updates
originally may have been issued in the correct sequential order.
If named_timeout is nonzero, failed topology updates will be placed on a defer
queue until another event arrives that clears the error, or until the timeout
expires. Value is in milliseconds.


code:
===================================
Documentation for /proc/sys/sunrpc/
===================================

kernel version 2.2.10

Copyright (c) 1998, 1999,  Rik van Riel <riel@nl.linux.org>

For general info and legal blurb, please look in index.rst.

------------------------------------------------------------------------------

This file contains the documentation for the sysctl files in
/proc/sys/sunrpc and is valid for Linux kernel version 2.2.

The files in this directory can be used to (re)set the debug
flags of the SUN Remote Procedure Call (RPC) subsystem in
the Linux kernel. This stuff is used for NFS, KNFSD and
maybe a few other things as well.

The files in there are used to control the debugging flags:
rpc_debug, nfs_debug, nfsd_debug and nlm_debug.

These flags are for kernel hackers only. You should read the
source code in net/sunrpc/ for more information.


code:
=================================
Documentation for /proc/sys/user/
=================================

kernel version 4.9.0

Copyright (c) 2016		Eric Biederman <ebiederm@xmission.com>

------------------------------------------------------------------------------

This file contains the documentation for the sysctl files in
/proc/sys/user.

The files in this directory can be used to override the default
limits on the number of namespaces and other objects that have
per user per user namespace limits.

The primary purpose of these limits is to stop programs that
malfunction and attempt to create a ridiculous number of objects,
before the malfunction becomes a system wide problem.  It is the
intention that the defaults of these limits are set high enough that
no program in normal operation should run into these limits.

The creation of per user per user namespace objects are charged to
the user in the user namespace who created the object and
verified to be below the per user limit in that user namespace.

The creation of objects is also charged to all of the users
who created user namespaces the creation of the object happens
in (user namespaces can be nested) and verified to be below the per user
limits in the user namespaces of those users.

This recursive counting of created objects ensures that creating a
user namespace does not allow a user to escape their current limits.

Currently, these files are in /proc/sys/user:

max_cgroup_namespaces
=====================

  The maximum number of cgroup namespaces that any user in the current
  user namespace may create.

max_ipc_namespaces
==================

  The maximum number of ipc namespaces that any user in the current
  user namespace may create.

max_mnt_namespaces
==================

  The maximum number of mount namespaces that any user in the current
  user namespace may create.

max_net_namespaces
==================

  The maximum number of network namespaces that any user in the
  current user namespace may create.

max_pid_namespaces
==================

  The maximum number of pid namespaces that any user in the current
  user namespace may create.

max_time_namespaces
===================

  The maximum number of time namespaces that any user in the current
  user namespace may create.

max_user_namespaces
===================

  The maximum number of user namespaces that any user in the current
  user namespace may create.

max_uts_namespaces
==================

  The maximum number of user namespaces that any user in the current
  user namespace may create.