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NAME | DESCRIPTION | SEE ALSO | COLOPHON |
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proc_sys_fs(5) File Formats Manual proc_sys_fs(5)
/proc/sys/fs/ - kernel variables related to filesystems
/proc/sys/fs/
This directory contains the files and subdirectories for
kernel variables related to filesystems.
/proc/sys/fs/aio-max-nr
/proc/sys/fs/aio-nr (both since Linux 2.6.4)
aio-nr is the running total of the number of events
specified by io_setup(2) calls for all currently active AIO
contexts. If aio-nr reaches aio-max-nr, then io_setup(2)
will fail with the error EAGAIN. Raising aio-max-nr does
not result in the preallocation or resizing of any kernel
data structures.
/proc/sys/fs/binfmt_misc
Documentation for files in this directory can be found in
the Linux kernel source in the file
Documentation/admin-guide/binfmt-misc.rst (or in
Documentation/binfmt_misc.txt on older kernels).
/proc/sys/fs/dentry-state (since Linux 2.2)
This file contains information about the status of the
directory cache (dcache). The file contains six numbers,
nr_dentry, nr_unused, age_limit (age in seconds),
want_pages (pages requested by system) and two dummy
values.
• nr_dentry is the number of allocated dentries (dcache
entries). This field is unused in Linux 2.2.
• nr_unused is the number of unused dentries.
• age_limit is the age in seconds after which dcache
entries can be reclaimed when memory is short.
• want_pages is nonzero when the kernel has called
shrink_dcache_pages() and the dcache isn't pruned yet.
/proc/sys/fs/dir-notify-enable
This file can be used to disable or enable the dnotify
interface described in fcntl(2) on a system-wide basis. A
value of 0 in this file disables the interface, and a value
of 1 enables it.
/proc/sys/fs/dquot-max
This file shows the maximum number of cached disk quota
entries. On some (2.4) systems, it is not present. If the
number of free cached disk quota entries is very low and
you have some awesome number of simultaneous system users,
you might want to raise the limit.
/proc/sys/fs/dquot-nr
This file shows the number of allocated disk quota entries
and the number of free disk quota entries.
/proc/sys/fs/epoll/ (since Linux 2.6.28)
This directory contains the file max_user_watches, which
can be used to limit the amount of kernel memory consumed
by the epoll interface. For further details, see epoll(7).
/proc/sys/fs/file-max
This file defines a system-wide limit on the number of open
files for all processes. System calls that fail when
encountering this limit fail with the error ENFILE. (See
also setrlimit(2), which can be used by a process to set
the per-process limit, RLIMIT_NOFILE, on the number of
files it may open.) If you get lots of error messages in
the kernel log about running out of file handles (open file
descriptions) (look for "VFS: file-max limit <number>
reached"), try increasing this value:
echo 100000 > /proc/sys/fs/file-max
Privileged processes (CAP_SYS_ADMIN) can override the
file-max limit.
/proc/sys/fs/file-nr
This (read-only) file contains three numbers: the number of
allocated file handles (i.e., the number of open file
descriptions; see open(2)); the number of free file
handles; and the maximum number of file handles (i.e., the
same value as /proc/sys/fs/file-max). If the number of
allocated file handles is close to the maximum, you should
consider increasing the maximum. Before Linux 2.6, the
kernel allocated file handles dynamically, but it didn't
free them again. Instead the free file handles were kept
in a list for reallocation; the "free file handles" value
indicates the size of that list. A large number of free
file handles indicates that there was a past peak in the
usage of open file handles. Since Linux 2.6, the kernel
does deallocate freed file handles, and the "free file
handles" value is always zero.
/proc/sys/fs/inode-max (only present until Linux 2.2)
This file contains the maximum number of in-memory inodes.
This value should be 3–4 times larger than the value in
file-max, since stdin, stdout and network sockets also need
an inode to handle them. When you regularly run out of
inodes, you need to increase this value.
Starting with Linux 2.4, there is no longer a static limit
on the number of inodes, and this file is removed.
/proc/sys/fs/inode-nr
This file contains the first two values from inode-state.
/proc/sys/fs/inode-state
This file contains seven numbers: nr_inodes,
nr_free_inodes, preshrink, and four dummy values (always
zero).
nr_inodes is the number of inodes the system has allocated.
nr_free_inodes represents the number of free inodes.
preshrink is nonzero when the nr_inodes > inode-max and the
system needs to prune the inode list instead of allocating
more; since Linux 2.4, this field is a dummy value (always
zero).
/proc/sys/fs/inotify/ (since Linux 2.6.13)
This directory contains files max_queued_events,
max_user_instances, and max_user_watches, that can be used
to limit the amount of kernel memory consumed by the
inotify interface. For further details, see inotify(7).
/proc/sys/fs/lease-break-time
This file specifies the grace period that the kernel grants
to a process holding a file lease (fcntl(2)) after it has
sent a signal to that process notifying it that another
process is waiting to open the file. If the lease holder
does not remove or downgrade the lease within this grace
period, the kernel forcibly breaks the lease.
/proc/sys/fs/leases-enable
This file can be used to enable or disable file leases
(fcntl(2)) on a system-wide basis. If this file contains
the value 0, leases are disabled. A nonzero value enables
leases.
/proc/sys/fs/mount-max (since Linux 4.9)
The value in this file specifies the maximum number of
mounts that may exist in a mount namespace. The default
value in this file is 100,000.
/proc/sys/fs/mqueue/ (since Linux 2.6.6)
This directory contains files msg_max, msgsize_max, and
queues_max, controlling the resources used by POSIX message
queues. See mq_overview(7) for details.
/proc/sys/fs/nr_open (since Linux 2.6.25)
This file imposes a ceiling on the value to which the
RLIMIT_NOFILE resource limit can be raised (see
getrlimit(2)). This ceiling is enforced for both
unprivileged and privileged process. The default value in
this file is 1048576. (Before Linux 2.6.25, the ceiling
for RLIMIT_NOFILE was hard-coded to the same value.)
/proc/sys/fs/overflowgid
/proc/sys/fs/overflowuid
These files allow you to change the value of the fixed UID
and GID. The default is 65534. Some filesystems support
only 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 the overflow value before being written to
disk.
/proc/sys/fs/pipe-max-size (since Linux 2.6.35)
See pipe(7).
/proc/sys/fs/pipe-user-pages-hard (since Linux 4.5)
See pipe(7).
/proc/sys/fs/pipe-user-pages-soft (since Linux 4.5)
See pipe(7).
/proc/sys/fs/protected_fifos (since Linux 4.19)
The value in this file is/can be set to one of the
following:
0 Writing to FIFOs is unrestricted.
1 Don't allow O_CREAT open(2) on FIFOs that the caller
doesn't own in world-writable sticky directories,
unless the FIFO is owned by the owner of the directory.
2 As for the value 1, but the restriction also applies to
group-writable sticky directories.
The intent of the above protections is to avoid
unintentional writes to an attacker-controlled FIFO when a
program expected to create a regular file.
/proc/sys/fs/protected_hardlinks (since Linux 3.6)
When the value in this file is 0, no restrictions are
placed on the creation of hard links (i.e., this is the
historical behavior before Linux 3.6). When the value in
this file is 1, a hard link can be created to a target file
only if one of the following conditions is true:
• The calling process has the CAP_FOWNER capability in its
user namespace and the file UID has a mapping in the
namespace.
• The filesystem UID of the process creating the link
matches the owner (UID) of the target file (as described
in credentials(7), a process's filesystem UID is
normally the same as its effective UID).
• All of the following conditions are true:
• the target is a regular file;
• the target file does not have its set-user-ID mode
bit enabled;
• the target file does not have both its set-group-ID
and group-executable mode bits enabled; and
• the caller has permission to read and write the
target file (either via the file's permissions mask
or because it has suitable capabilities).
The default value in this file is 0. Setting the value to
1 prevents a longstanding class of security issues caused
by hard-link-based time-of-check, time-of-use races, most
commonly seen in world-writable directories such as /tmp.
The common method of exploiting this flaw is to cross
privilege boundaries when following a given hard link
(i.e., a root process follows a hard link created by
another user). Additionally, on systems without separated
partitions, this stops unauthorized users from "pinning"
vulnerable set-user-ID and set-group-ID files against being
upgraded by the administrator, or linking to special files.
/proc/sys/fs/protected_regular (since Linux 4.19)
The value in this file is/can be set to one of the
following:
0 Writing to regular files is unrestricted.
1 Don't allow O_CREAT open(2) on regular files that the
caller doesn't own in world-writable sticky
directories, unless the regular file is owned by the
owner of the directory.
2 As for the value 1, but the restriction also applies to
group-writable sticky directories.
The intent of the above protections is similar to
protected_fifos, but allows an application to avoid writes
to an attacker-controlled regular file, where the
application expected to create one.
/proc/sys/fs/protected_symlinks (since Linux 3.6)
When the value in this file is 0, no restrictions are
placed on following symbolic links (i.e., this is the
historical behavior before Linux 3.6). When the value in
this file is 1, symbolic links are followed only in the
following circumstances:
• the filesystem UID of the process following the link
matches the owner (UID) of the symbolic link (as
described in credentials(7), a process's filesystem UID
is normally the same as its effective UID);
• the link is not in a sticky world-writable directory; or
• the symbolic link and its parent directory have the same
owner (UID)
A system call that fails to follow a symbolic link because
of the above restrictions returns the error EACCES in
errno.
The default value in this file is 0. Setting the value to
1 avoids a longstanding class of security issues based on
time-of-check, time-of-use races when accessing symbolic
links.
/proc/sys/fs/suid_dumpable (since Linux 2.6.13)
The value in this file is assigned to a process's
"dumpable" flag in the circumstances described in prctl(2).
In effect, the value in this file determines whether core
dump files are produced for set-user-ID or otherwise
protected/tainted binaries. The "dumpable" setting also
affects the ownership of files in a process's /proc/pid
directory, as described above.
Three different integer values can be specified:
0 (default)
This provides the traditional (pre-Linux 2.6.13)
behavior. A core dump will not be produced for a
process which has changed credentials (by calling
seteuid(2), setgid(2), or similar, or by executing a
set-user-ID or set-group-ID program) or whose binary
does not have read permission enabled.
1 ("debug")
All processes dump core when possible. (Reasons why
a process might nevertheless not dump core are
described in core(5).) The core dump is owned by
the filesystem user ID of the dumping process and no
security is applied. This is intended for system
debugging situations only: this mode is insecure
because it allows unprivileged users to examine the
memory contents of privileged processes.
2 ("suidsafe")
Any binary which normally would not be dumped (see
"0" above) is dumped readable by root only. This
allows the user to remove the core dump file but not
to read it. For security reasons core dumps in this
mode will not overwrite one another or other files.
This mode is appropriate when administrators are
attempting to debug problems in a normal
environment.
Additionally, since Linux 3.6,
/proc/sys/kernel/core_pattern must either be an
absolute pathname or a pipe command, as detailed in
core(5). Warnings will be written to the kernel log
if core_pattern does not follow these rules, and no
core dump will be produced.
For details of the effect of a process's "dumpable" setting
on ptrace access mode checking, see ptrace(2).
/proc/sys/fs/super-max
This file controls the maximum number of superblocks, and
thus the maximum number of mounted filesystems the kernel
can have. You need increase only super-max if you need to
mount more filesystems than the current value in super-max
allows you to.
/proc/sys/fs/super-nr
This file contains the number of filesystems currently
mounted.
proc(5), proc_sys(5)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 proc_sys_fs(5)
Pages that refer to this page: F_GETPIPE_SZ(2const), link(2), open(2), PR_SET_DUMPABLE(2const)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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