NAME¶
/proc/sys/fs/ - kernel variables related to filesystems
DESCRIPTION¶
- /proc/sys/fs/
- This directory contains the files and subdirectories for kernel variables
related to filesystems.
- /proc/sys/fs/aio-max-nr and /proc/sys/fs/aio-nr (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 and /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.
