===============================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 directoriesin ``/proc/sys/fs/``. The files in this directory can be used to tune and monitormiscellaneous and general things in the operation of the Linuxkernel. Since some of the files *can* be used to screw up yoursystem, it is advisable to read both documentation and sourcebefore 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 iorequests.  ``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 thepre-allocation or re-sizing of any kernel data structures. dentry-negative---------------------------- Policy for negative dentries. Set to 1 to always delete the dentry when afile is removed, and 0 to disable it. By default, this behavior is disabled. dentry-state------------ This file shows the values in ``struct dentry_stat_t``, as defined in``fs/dcache.c``::   struct dentry_stat_t dentry_stat {        long nr_dentry;        long nr_unused;        long age_limit;         /* age in seconds */        long want_pages;        /* pages requested by system */        long nr_negative;       /* # of unused negative dentries */        long 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 notactively used, but are saved in the LRU list for future reuse. ``age_limit`` is the age in seconds after which dcache entriescan be reclaimed when memory is short and ``want_pages`` isnonzero when ``shrink_dcache_pages()`` has been called and thedcache isn't pruned yet. ``nr_negative`` shows the number of unused dentries that are alsonegative dentries which do not map to any files. Instead,they help speeding up rejection of non-existing files providedby 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 lotsof error messages about running out of file handles, you mightwant to increase this limit. Historically,the kernel was able to allocate file handlesdynamically, but not to free them again. The three values in``file-nr`` denote the number of allocated file handles, the numberof allocated but unused file handles, and the maximum number offile handles. Linux 2.6 and later always reports 0 as the number of freefile handles -- this is not an error, it just means that thenumber of allocated file handles exactly matches the number ofused file handles. Attempts to allocate more file descriptors than ``file-max`` arereported 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 structuresdynamically, 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 hasallocated. ``nr_free_inodes`` represents the number of free inodes (?) andpreshrink is nonzero when thesystem needs to prune the inode list instead of allocatingmore.  mount-max--------- This denotes the maximum number of mounts that may existin a mount namespace.  nr_open------- This denotes the maximum number of file-handles a process canallocate. Default value is 1024*1024 (1048576) which should beenough for most machines. Actual limit depends on ``RLIMIT_NOFILE``resource limit.  overflowgid & overflowuid------------------------- Some filesystems only support 16-bit UIDs and GIDs, although in LinuxUIDs and GIDs are 32 bits. When one of these filesystems is mountedwith writes enabled, any UID or GID that would exceed 65535 is translatedto 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 goesbelow the limit again. When set to 0, no limit is applied, which is the defaultsetting.  pipe-user-pages-soft-------------------- Maximum total number of pages a non-privileged user may allocate for pipesbefore the pipe size gets limited to two pages. Once this limit is reached,new pipes will be limited to two pages in size for this user in order tolimit total memory usage, and trying to increase them using ``fcntl()`` will bedenied until usage goes below the limit again. The default value allows toallocate up to 1024 pipes at their default size. When set to 0, no limit isapplied.  protected_fifos--------------- The intent of this protection is to avoid unintentional writes toan attacker-controlled FIFO, where a program expected to create a regularfile. 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 ownin world writable sticky directories, unless they are owned by theowner 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-basedtime-of-check-time-of-use race, most commonly seen in world-writabledirectories like ``/tmp``. The common method of exploitation of this flawis to cross privilege boundaries when following a given hardlink (i.e. aroot process follows a hardlink created by another user). Additionally,on systems without separated partitions, this stops unauthorized usersfrom "pinning" vulnerable setuid/setgid files against being upgraded bythe 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 notalready 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 itavoids writes to an attacker-controlled regular file, where a programexpected 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 wedon't own in world writable sticky directories, unless they areowned 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-basedtime-of-check-time-of-use race, most commonly seen in world-writabledirectories like ``/tmp``. The common method of exploitation of this flawis to cross privilege boundaries when following a given symlink (i.e. aroot process follows a symlink belonging to another user). For a likelyincomplete 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 outsidea sticky world-writable directory, or when the uid of the symlink andfollower 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 setuidor 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`` isin 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 thecreation of a  user space  library that  implements  the  POSIX message queuesAPI (as noted by the  MSG tag in the  POSIX 1003.1-2001 version  of the SystemInterfaces specification.) The "mqueue" filesystem contains values for determining/setting theamount of resources used by the file system. ``/proc/sys/fs/mqueue/queues_max`` is a read/write file forsetting/getting the maximum number of message queues allowed on thesystem. ``/proc/sys/fs/mqueue/msg_max`` is a read/write file forsetting/getting the maximum number of messages in a queue value.  Infact it is the limiting value for another (user) limit which is set in``mq_open`` invocation.  This attribute of a queue must be less thanor equal to ``msg_max``. ``/proc/sys/fs/mqueue/msgsize_max`` is a read/write file forsetting/getting the maximum message size value (it is an attribute ofevery message queue, set during its creation). ``/proc/sys/fs/mqueue/msg_default`` is a read/write file forsetting/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 forsetting/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 monitoredfor event readiness. Each one of these monitored files constitutes a "watch".This configuration option sets the maximum number of "watches" that areallowed for each user.Each "watch" costs roughly 90 bytes on a 32-bit kernel, and roughly 160 byteson a 64-bit one.The current default value for ``max_user_watches`` is 4% of theavailable low memory, divided by the "watch" cost in bytes. 5. /proc/sys/fs/fuse - Configuration options for FUSE filesystems===================================================================== This directory contains the following configuration options for FUSEfilesystems: ``/proc/sys/fs/fuse/max_pages_limit`` is a read/write file forsetting/getting the maximum number of pages that can be used for servicingrequests in FUSE. ``/proc/sys/fs/fuse/default_request_timeout`` is a read/write file forsetting/getting the default timeout (in seconds) for a fuse server toreply to a kernel-issued request in the event where the server did notspecify a timeout at mount. If the server set a timeout,then default_request_timeout will be ignored.  The default"default_request_timeout" is set to 0. 0 indicates no default timeout.The maximum value that can be set is 65535. ``/proc/sys/fs/fuse/max_request_timeout`` is a read/write file forsetting/getting the maximum timeout (in seconds) for a fuse server toreply to a kernel-issued request. A value greater than 0 automatically optsthe server into a timeout that will be set to at most "max_request_timeout",even if the server did not specify a timeout and default_request_timeout isset to 0. If max_request_timeout is greater than 0 and the server set a timeoutgreater than max_request_timeout or default_request_timeout is set to a valuegreater than max_request_timeout, the system will use max_request_timeout as thetimeout. 0 indicates no max request timeout. The maximum value that can be setis 65535. For timeouts, if the server does not respond to the request by the timethe set timeout elapses, then the connection to the fuse server will be aborted.Please note that the timeouts are not 100% precise (eg you may set 60 seconds butthe timeout may kick in after 70 seconds). The upper margin of error for thetimeout is roughly FUSE_TIMEOUT_TIMER_FREQ seconds.