proc_sys_fs(5) File Formats Manual proc_sys_fs(5)
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 preal-
location 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/ad-
min-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 re-
quested by system) and two dummy values.
• nr_dentry is the number of allocated dentries (dcache en-
tries). 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 encounter-
ing this limit fail with the error ENFILE. (See also setr-
limit(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 al-
located file handles (i.e., the number of open file descrip-
tions; 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 max-
imum. Before Linux 2.6, the kernel allocated file handles dy-
namically, 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 sys-
tem 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_in-
stances, 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 sig-
nal to that process notifying it that another process is waiting
to open the file. If the lease holder does not remove or down-
grade 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 (fc-
ntl(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 en-
abled, 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 behav-
ior 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 follow-
ing 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 creden-
tials(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 "pin-
ning" vulnerable set-user-ID and set-group-ID files against be-
ing 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_fi-
fos, but allows an application to avoid writes to an attacker-
controlled regular file, where the application expected to cre-
ate 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 creden-
tials(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 pro-
duced 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) behav-
ior. 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 ap-
propriate when administrators are attempting to debug
problems in a normal environment.
Additionally, since Linux 3.6, /proc/sys/kernel/core_pat-
tern must either be an absolute pathname or a pipe com-
mand, 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.
SEE ALSO
proc(5), proc_sys(5)
Linux man-pages 6.7 2023-09-30 proc_sys_fs(5)
Generated by dwww version 1.16 on Tue Dec 16 17:15:22 CET 2025.