kernel/Documentation/admin-guide/sysctl/fs.rst

===============================
Documentation for /proc/sys/fs/
===============================

kernel version 2.2.10

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 in
/proc/sys/fs/ and is valid for Linux kernel version 2.2.

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 are in /proc/sys/fs:

- aio-max-nr
- aio-nr
- dentry-state
- dquot-max
- dquot-nr
- file-max
- file-nr
- inode-max
- inode-nr
- inode-state
- nr_open
- overflowuid
- overflowgid
- pipe-user-pages-hard
- pipe-user-pages-soft
- protected_fifos
- protected_hardlinks
- protected_regular
- protected_symlinks
- suid_dumpable
- super-max
- super-nr


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

aio-nr is the running total of the number of events specified on the
io_setup system call for all currently active aio contexts.  If aio-nr
reaches aio-max-nr 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
------------

From 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.


dquot-max & dquot-nr
--------------------

The file dquot-max shows the maximum number of cached disk
quota entries.

The file dquot-nr shows the number of allocated disk quota
entries and the number of free disk quota entries.

If the number of free cached disk quotas is very low and
you have some awesome number of simultaneous system users,
you might want to raise the limit.


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 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".


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.


inode-max, inode-nr & inode-state
---------------------------------

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

The value in inode-max denotes the maximum number of inode
handlers. This value should be 3-4 times larger than the value
in file-max, since stdin, stdout and network sockets also
need an inode struct to handle them. When you regularly run
out of inodes, you need to increase this value.

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, this can be slightly more than inode-max because
Linux allocates them one pageful at a time.

Nr_free_inodes represents the number of free inodes (?) and
preshrink is nonzero when the nr_inodes > inode-max and the
system needs to prune the inode list instead of allocating
more.


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 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.
=   ==========  ===============================================================


super-max & super-nr
--------------------

These numbers control the maximum number of superblocks, and
thus the maximum number of mounted filesystems the kernel
can have. You only need to increase super-max if you need to
mount more filesystems than the current value in super-max
allows you to.


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.


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

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



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 or equal then msg_max.

/proc/sys/fs/mqueue/msgsize_max is  a read/write  file for setting/getting the
maximum  message size value (it is every  message queue's attribute 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 attr parameter of mq_open(2) is
NULL. If it exceed msg_max, the default value is initialized msg_max.

/proc/sys/fs/mqueue/msgsize_default is a read/write file for setting/getting
the default message size value if attr parameter of mq_open(2) is NULL. If it
exceed msgsize_max, the default value is initialized 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 32bit kernel, and roughly 160 bytes
on a 64bit one.
The current default value for  max_user_watches  is the 1/25 (4%) of the
available low memory, divided for the "watch" cost in bytes.