1=============================== 2Documentation for /proc/sys/fs/ 3=============================== 4 5Copyright (c) 1998, 1999, Rik van Riel <riel@nl.linux.org> 6 7Copyright (c) 2009, Shen Feng<shen@cn.fujitsu.com> 8 9For general info and legal blurb, please look in intro.rst. 10 11------------------------------------------------------------------------------ 12 13This file contains documentation for the sysctl files and directories 14in ``/proc/sys/fs/``. 15 16The files in this directory can be used to tune and monitor 17miscellaneous and general things in the operation of the Linux 18kernel. Since some of the files *can* be used to screw up your 19system, it is advisable to read both documentation and source 20before actually making adjustments. 21 221. /proc/sys/fs 23=============== 24 25Currently, these files might (depending on your configuration) 26show up in ``/proc/sys/fs``: 27 28.. contents:: :local: 29 30 31aio-nr & aio-max-nr 32------------------- 33 34``aio-nr`` shows the current system-wide number of asynchronous io 35requests. ``aio-max-nr`` allows you to change the maximum value 36``aio-nr`` can grow to. If ``aio-nr`` reaches ``aio-nr-max`` then 37``io_setup`` will fail with ``EAGAIN``. Note that raising 38``aio-max-nr`` does not result in the 39pre-allocation or re-sizing of any kernel data structures. 40 41dentry-negative 42---------------------------- 43 44Policy for negative dentries. Set to 1 to to always delete the dentry when a 45file is removed, and 0 to disable it. By default, this behavior is disabled. 46 47dentry-state 48------------ 49 50This file shows the values in ``struct dentry_stat_t``, as defined in 51``fs/dcache.c``:: 52 53 struct dentry_stat_t dentry_stat { 54 long nr_dentry; 55 long nr_unused; 56 long age_limit; /* age in seconds */ 57 long want_pages; /* pages requested by system */ 58 long nr_negative; /* # of unused negative dentries */ 59 long dummy; /* Reserved for future use */ 60 }; 61 62Dentries are dynamically allocated and deallocated. 63 64``nr_dentry`` shows the total number of dentries allocated (active 65+ unused). ``nr_unused shows`` the number of dentries that are not 66actively used, but are saved in the LRU list for future reuse. 67 68``age_limit`` is the age in seconds after which dcache entries 69can be reclaimed when memory is short and ``want_pages`` is 70nonzero when ``shrink_dcache_pages()`` has been called and the 71dcache isn't pruned yet. 72 73``nr_negative`` shows the number of unused dentries that are also 74negative dentries which do not map to any files. Instead, 75they help speeding up rejection of non-existing files provided 76by the users. 77 78 79file-max & file-nr 80------------------ 81 82The value in ``file-max`` denotes the maximum number of file- 83handles that the Linux kernel will allocate. When you get lots 84of error messages about running out of file handles, you might 85want to increase this limit. 86 87Historically,the kernel was able to allocate file handles 88dynamically, but not to free them again. The three values in 89``file-nr`` denote the number of allocated file handles, the number 90of allocated but unused file handles, and the maximum number of 91file handles. Linux 2.6 and later always reports 0 as the number of free 92file handles -- this is not an error, it just means that the 93number of allocated file handles exactly matches the number of 94used file handles. 95 96Attempts to allocate more file descriptors than ``file-max`` are 97reported with ``printk``, look for:: 98 99 VFS: file-max limit <number> reached 100 101in the kernel logs. 102 103 104inode-nr & inode-state 105---------------------- 106 107As with file handles, the kernel allocates the inode structures 108dynamically, but can't free them yet. 109 110The file ``inode-nr`` contains the first two items from 111``inode-state``, so we'll skip to that file... 112 113``inode-state`` contains three actual numbers and four dummies. 114The actual numbers are, in order of appearance, ``nr_inodes``, 115``nr_free_inodes`` and ``preshrink``. 116 117``nr_inodes`` stands for the number of inodes the system has 118allocated. 119 120``nr_free_inodes`` represents the number of free inodes (?) and 121preshrink is nonzero when the 122system needs to prune the inode list instead of allocating 123more. 124 125 126mount-max 127--------- 128 129This denotes the maximum number of mounts that may exist 130in a mount namespace. 131 132 133nr_open 134------- 135 136This denotes the maximum number of file-handles a process can 137allocate. Default value is 1024*1024 (1048576) which should be 138enough for most machines. Actual limit depends on ``RLIMIT_NOFILE`` 139resource limit. 140 141 142overflowgid & overflowuid 143------------------------- 144 145Some filesystems only support 16-bit UIDs and GIDs, although in Linux 146UIDs and GIDs are 32 bits. When one of these filesystems is mounted 147with writes enabled, any UID or GID that would exceed 65535 is translated 148to a fixed value before being written to disk. 149 150These sysctls allow you to change the value of the fixed UID and GID. 151The default is 65534. 152 153 154pipe-user-pages-hard 155-------------------- 156 157Maximum total number of pages a non-privileged user may allocate for pipes. 158Once this limit is reached, no new pipes may be allocated until usage goes 159below the limit again. When set to 0, no limit is applied, which is the default 160setting. 161 162 163pipe-user-pages-soft 164-------------------- 165 166Maximum total number of pages a non-privileged user may allocate for pipes 167before the pipe size gets limited to a single page. Once this limit is reached, 168new pipes will be limited to a single page in size for this user in order to 169limit total memory usage, and trying to increase them using ``fcntl()`` will be 170denied until usage goes below the limit again. The default value allows to 171allocate up to 1024 pipes at their default size. When set to 0, no limit is 172applied. 173 174 175protected_fifos 176--------------- 177 178The intent of this protection is to avoid unintentional writes to 179an attacker-controlled FIFO, where a program expected to create a regular 180file. 181 182When set to "0", writing to FIFOs is unrestricted. 183 184When set to "1" don't allow ``O_CREAT`` open on FIFOs that we don't own 185in world writable sticky directories, unless they are owned by the 186owner of the directory. 187 188When set to "2" it also applies to group writable sticky directories. 189 190This protection is based on the restrictions in Openwall. 191 192 193protected_hardlinks 194-------------------- 195 196A long-standing class of security issues is the hardlink-based 197time-of-check-time-of-use race, most commonly seen in world-writable 198directories like ``/tmp``. The common method of exploitation of this flaw 199is to cross privilege boundaries when following a given hardlink (i.e. a 200root process follows a hardlink created by another user). Additionally, 201on systems without separated partitions, this stops unauthorized users 202from "pinning" vulnerable setuid/setgid files against being upgraded by 203the administrator, or linking to special files. 204 205When set to "0", hardlink creation behavior is unrestricted. 206 207When set to "1" hardlinks cannot be created by users if they do not 208already own the source file, or do not have read/write access to it. 209 210This protection is based on the restrictions in Openwall and grsecurity. 211 212 213protected_regular 214----------------- 215 216This protection is similar to `protected_fifos`_, but it 217avoids writes to an attacker-controlled regular file, where a program 218expected to create one. 219 220When set to "0", writing to regular files is unrestricted. 221 222When set to "1" don't allow ``O_CREAT`` open on regular files that we 223don't own in world writable sticky directories, unless they are 224owned by the owner of the directory. 225 226When set to "2" it also applies to group writable sticky directories. 227 228 229protected_symlinks 230------------------ 231 232A long-standing class of security issues is the symlink-based 233time-of-check-time-of-use race, most commonly seen in world-writable 234directories like ``/tmp``. The common method of exploitation of this flaw 235is to cross privilege boundaries when following a given symlink (i.e. a 236root process follows a symlink belonging to another user). For a likely 237incomplete list of hundreds of examples across the years, please see: 238https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp 239 240When set to "0", symlink following behavior is unrestricted. 241 242When set to "1" symlinks are permitted to be followed only when outside 243a sticky world-writable directory, or when the uid of the symlink and 244follower match, or when the directory owner matches the symlink's owner. 245 246This protection is based on the restrictions in Openwall and grsecurity. 247 248 249suid_dumpable 250------------- 251 252This value can be used to query and set the core dump mode for setuid 253or otherwise protected/tainted binaries. The modes are 254 255= ========== =============================================================== 2560 (default) Traditional behaviour. Any process which has changed 257 privilege levels or is execute only will not be dumped. 2581 (debug) All processes dump core when possible. The core dump is 259 owned by the current user and no security is applied. This is 260 intended for system debugging situations only. 261 Ptrace is unchecked. 262 This is insecure as it allows regular users to examine the 263 memory contents of privileged processes. 2642 (suidsafe) Any binary which normally would not be dumped is dumped 265 anyway, but only if the ``core_pattern`` kernel sysctl (see 266 :ref:`Documentation/admin-guide/sysctl/kernel.rst <core_pattern>`) 267 is set to 268 either a pipe handler or a fully qualified path. (For more 269 details on this limitation, see CVE-2006-2451.) This mode is 270 appropriate when administrators are attempting to debug 271 problems in a normal environment, and either have a core dump 272 pipe handler that knows to treat privileged core dumps with 273 care, or specific directory defined for catching core dumps. 274 If a core dump happens without a pipe handler or fully 275 qualified path, a message will be emitted to syslog warning 276 about the lack of a correct setting. 277= ========== =============================================================== 278 279 280 2812. /proc/sys/fs/binfmt_misc 282=========================== 283 284Documentation for the files in ``/proc/sys/fs/binfmt_misc`` is 285in Documentation/admin-guide/binfmt-misc.rst. 286 287 2883. /proc/sys/fs/mqueue - POSIX message queues filesystem 289======================================================== 290 291 292The "mqueue" filesystem provides the necessary kernel features to enable the 293creation of a user space library that implements the POSIX message queues 294API (as noted by the MSG tag in the POSIX 1003.1-2001 version of the System 295Interfaces specification.) 296 297The "mqueue" filesystem contains values for determining/setting the 298amount of resources used by the file system. 299 300``/proc/sys/fs/mqueue/queues_max`` is a read/write file for 301setting/getting the maximum number of message queues allowed on the 302system. 303 304``/proc/sys/fs/mqueue/msg_max`` is a read/write file for 305setting/getting the maximum number of messages in a queue value. In 306fact it is the limiting value for another (user) limit which is set in 307``mq_open`` invocation. This attribute of a queue must be less than 308or equal to ``msg_max``. 309 310``/proc/sys/fs/mqueue/msgsize_max`` is a read/write file for 311setting/getting the maximum message size value (it is an attribute of 312every message queue, set during its creation). 313 314``/proc/sys/fs/mqueue/msg_default`` is a read/write file for 315setting/getting the default number of messages in a queue value if the 316``attr`` parameter of ``mq_open(2)`` is ``NULL``. If it exceeds 317``msg_max``, the default value is initialized to ``msg_max``. 318 319``/proc/sys/fs/mqueue/msgsize_default`` is a read/write file for 320setting/getting the default message size value if the ``attr`` 321parameter of ``mq_open(2)`` is ``NULL``. If it exceeds 322``msgsize_max``, the default value is initialized to ``msgsize_max``. 323 3244. /proc/sys/fs/epoll - Configuration options for the epoll interface 325===================================================================== 326 327This directory contains configuration options for the epoll(7) interface. 328 329max_user_watches 330---------------- 331 332Every epoll file descriptor can store a number of files to be monitored 333for event readiness. Each one of these monitored files constitutes a "watch". 334This configuration option sets the maximum number of "watches" that are 335allowed for each user. 336Each "watch" costs roughly 90 bytes on a 32-bit kernel, and roughly 160 bytes 337on a 64-bit one. 338The current default value for ``max_user_watches`` is 4% of the 339available low memory, divided by the "watch" cost in bytes. 340 3415. /proc/sys/fs/fuse - Configuration options for FUSE filesystems 342===================================================================== 343 344This directory contains the following configuration options for FUSE 345filesystems: 346 347``/proc/sys/fs/fuse/max_pages_limit`` is a read/write file for 348setting/getting the maximum number of pages that can be used for servicing 349requests in FUSE. 350