1=============================== 2Documentation for /proc/sys/fs/ 3=============================== 4 5kernel version 2.2.10 6 7Copyright (c) 1998, 1999, Rik van Riel <riel@nl.linux.org> 8 9Copyright (c) 2009, Shen Feng<shen@cn.fujitsu.com> 10 11For general info and legal blurb, please look in intro.rst. 12 13------------------------------------------------------------------------------ 14 15This file contains documentation for the sysctl files in 16/proc/sys/fs/ and is valid for Linux kernel version 2.2. 17 18The files in this directory can be used to tune and monitor 19miscellaneous and general things in the operation of the Linux 20kernel. Since some of the files _can_ be used to screw up your 21system, it is advisable to read both documentation and source 22before actually making adjustments. 23 241. /proc/sys/fs 25=============== 26 27Currently, these files are in /proc/sys/fs: 28 29- aio-max-nr 30- aio-nr 31- dentry-state 32- dquot-max 33- dquot-nr 34- file-max 35- file-nr 36- inode-max 37- inode-nr 38- inode-state 39- nr_open 40- overflowuid 41- overflowgid 42- pipe-user-pages-hard 43- pipe-user-pages-soft 44- protected_fifos 45- protected_hardlinks 46- protected_regular 47- protected_symlinks 48- suid_dumpable 49- super-max 50- super-nr 51 52 53aio-nr & aio-max-nr 54------------------- 55 56aio-nr is the running total of the number of events specified on the 57io_setup system call for all currently active aio contexts. If aio-nr 58reaches aio-max-nr then io_setup will fail with EAGAIN. Note that 59raising aio-max-nr does not result in the pre-allocation or re-sizing 60of any kernel data structures. 61 62 63dentry-state 64------------ 65 66From linux/include/linux/dcache.h:: 67 68 struct dentry_stat_t dentry_stat { 69 int nr_dentry; 70 int nr_unused; 71 int age_limit; /* age in seconds */ 72 int want_pages; /* pages requested by system */ 73 int nr_negative; /* # of unused negative dentries */ 74 int dummy; /* Reserved for future use */ 75 }; 76 77Dentries are dynamically allocated and deallocated. 78 79nr_dentry shows the total number of dentries allocated (active 80+ unused). nr_unused shows the number of dentries that are not 81actively used, but are saved in the LRU list for future reuse. 82 83Age_limit is the age in seconds after which dcache entries 84can be reclaimed when memory is short and want_pages is 85nonzero when shrink_dcache_pages() has been called and the 86dcache isn't pruned yet. 87 88nr_negative shows the number of unused dentries that are also 89negative dentries which do not map to any files. Instead, 90they help speeding up rejection of non-existing files provided 91by the users. 92 93 94dquot-max & dquot-nr 95-------------------- 96 97The file dquot-max shows the maximum number of cached disk 98quota entries. 99 100The file dquot-nr shows the number of allocated disk quota 101entries and the number of free disk quota entries. 102 103If the number of free cached disk quotas is very low and 104you have some awesome number of simultaneous system users, 105you might want to raise the limit. 106 107 108file-max & file-nr 109------------------ 110 111The value in file-max denotes the maximum number of file- 112handles that the Linux kernel will allocate. When you get lots 113of error messages about running out of file handles, you might 114want to increase this limit. 115 116Historically,the kernel was able to allocate file handles 117dynamically, but not to free them again. The three values in 118file-nr denote the number of allocated file handles, the number 119of allocated but unused file handles, and the maximum number of 120file handles. Linux 2.6 always reports 0 as the number of free 121file handles -- this is not an error, it just means that the 122number of allocated file handles exactly matches the number of 123used file handles. 124 125Attempts to allocate more file descriptors than file-max are 126reported with printk, look for "VFS: file-max limit <number> 127reached". 128 129 130nr_open 131------- 132 133This denotes the maximum number of file-handles a process can 134allocate. Default value is 1024*1024 (1048576) which should be 135enough for most machines. Actual limit depends on RLIMIT_NOFILE 136resource limit. 137 138 139inode-max, inode-nr & inode-state 140--------------------------------- 141 142As with file handles, the kernel allocates the inode structures 143dynamically, but can't free them yet. 144 145The value in inode-max denotes the maximum number of inode 146handlers. This value should be 3-4 times larger than the value 147in file-max, since stdin, stdout and network sockets also 148need an inode struct to handle them. When you regularly run 149out of inodes, you need to increase this value. 150 151The file inode-nr contains the first two items from 152inode-state, so we'll skip to that file... 153 154Inode-state contains three actual numbers and four dummies. 155The actual numbers are, in order of appearance, nr_inodes, 156nr_free_inodes and preshrink. 157 158Nr_inodes stands for the number of inodes the system has 159allocated, this can be slightly more than inode-max because 160Linux allocates them one pageful at a time. 161 162Nr_free_inodes represents the number of free inodes (?) and 163preshrink is nonzero when the nr_inodes > inode-max and the 164system needs to prune the inode list instead of allocating 165more. 166 167 168overflowgid & overflowuid 169------------------------- 170 171Some filesystems only support 16-bit UIDs and GIDs, although in Linux 172UIDs and GIDs are 32 bits. When one of these filesystems is mounted 173with writes enabled, any UID or GID that would exceed 65535 is translated 174to a fixed value before being written to disk. 175 176These sysctls allow you to change the value of the fixed UID and GID. 177The default is 65534. 178 179 180pipe-user-pages-hard 181-------------------- 182 183Maximum total number of pages a non-privileged user may allocate for pipes. 184Once this limit is reached, no new pipes may be allocated until usage goes 185below the limit again. When set to 0, no limit is applied, which is the default 186setting. 187 188 189pipe-user-pages-soft 190-------------------- 191 192Maximum total number of pages a non-privileged user may allocate for pipes 193before the pipe size gets limited to a single page. Once this limit is reached, 194new pipes will be limited to a single page in size for this user in order to 195limit total memory usage, and trying to increase them using fcntl() will be 196denied until usage goes below the limit again. The default value allows to 197allocate up to 1024 pipes at their default size. When set to 0, no limit is 198applied. 199 200 201protected_fifos 202--------------- 203 204The intent of this protection is to avoid unintentional writes to 205an attacker-controlled FIFO, where a program expected to create a regular 206file. 207 208When set to "0", writing to FIFOs is unrestricted. 209 210When set to "1" don't allow O_CREAT open on FIFOs that we don't own 211in world writable sticky directories, unless they are owned by the 212owner of the directory. 213 214When set to "2" it also applies to group writable sticky directories. 215 216This protection is based on the restrictions in Openwall. 217 218 219protected_hardlinks 220-------------------- 221 222A long-standing class of security issues is the hardlink-based 223time-of-check-time-of-use race, most commonly seen in world-writable 224directories like /tmp. The common method of exploitation of this flaw 225is to cross privilege boundaries when following a given hardlink (i.e. a 226root process follows a hardlink created by another user). Additionally, 227on systems without separated partitions, this stops unauthorized users 228from "pinning" vulnerable setuid/setgid files against being upgraded by 229the administrator, or linking to special files. 230 231When set to "0", hardlink creation behavior is unrestricted. 232 233When set to "1" hardlinks cannot be created by users if they do not 234already own the source file, or do not have read/write access to it. 235 236This protection is based on the restrictions in Openwall and grsecurity. 237 238 239protected_regular 240----------------- 241 242This protection is similar to protected_fifos, but it 243avoids writes to an attacker-controlled regular file, where a program 244expected to create one. 245 246When set to "0", writing to regular files is unrestricted. 247 248When set to "1" don't allow O_CREAT open on regular files that we 249don't own in world writable sticky directories, unless they are 250owned by the owner of the directory. 251 252When set to "2" it also applies to group writable sticky directories. 253 254 255protected_symlinks 256------------------ 257 258A long-standing class of security issues is the symlink-based 259time-of-check-time-of-use race, most commonly seen in world-writable 260directories like /tmp. The common method of exploitation of this flaw 261is to cross privilege boundaries when following a given symlink (i.e. a 262root process follows a symlink belonging to another user). For a likely 263incomplete list of hundreds of examples across the years, please see: 264https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp 265 266When set to "0", symlink following behavior is unrestricted. 267 268When set to "1" symlinks are permitted to be followed only when outside 269a sticky world-writable directory, or when the uid of the symlink and 270follower match, or when the directory owner matches the symlink's owner. 271 272This protection is based on the restrictions in Openwall and grsecurity. 273 274 275suid_dumpable: 276-------------- 277 278This value can be used to query and set the core dump mode for setuid 279or otherwise protected/tainted binaries. The modes are 280 281= ========== =============================================================== 2820 (default) traditional behaviour. Any process which has changed 283 privilege levels or is execute only will not be dumped. 2841 (debug) all processes dump core when possible. The core dump is 285 owned by the current user and no security is applied. This is 286 intended for system debugging situations only. 287 Ptrace is unchecked. 288 This is insecure as it allows regular users to examine the 289 memory contents of privileged processes. 2902 (suidsafe) any binary which normally would not be dumped is dumped 291 anyway, but only if the "core_pattern" kernel sysctl is set to 292 either a pipe handler or a fully qualified path. (For more 293 details on this limitation, see CVE-2006-2451.) This mode is 294 appropriate when administrators are attempting to debug 295 problems in a normal environment, and either have a core dump 296 pipe handler that knows to treat privileged core dumps with 297 care, or specific directory defined for catching core dumps. 298 If a core dump happens without a pipe handler or fully 299 qualified path, a message will be emitted to syslog warning 300 about the lack of a correct setting. 301= ========== =============================================================== 302 303 304super-max & super-nr 305-------------------- 306 307These numbers control the maximum number of superblocks, and 308thus the maximum number of mounted filesystems the kernel 309can have. You only need to increase super-max if you need to 310mount more filesystems than the current value in super-max 311allows you to. 312 313 314aio-nr & aio-max-nr 315------------------- 316 317aio-nr shows the current system-wide number of asynchronous io 318requests. aio-max-nr allows you to change the maximum value 319aio-nr can grow to. 320 321 322mount-max 323--------- 324 325This denotes the maximum number of mounts that may exist 326in a mount namespace. 327 328 329 3302. /proc/sys/fs/binfmt_misc 331=========================== 332 333Documentation for the files in /proc/sys/fs/binfmt_misc is 334in Documentation/admin-guide/binfmt-misc.rst. 335 336 3373. /proc/sys/fs/mqueue - POSIX message queues filesystem 338======================================================== 339 340 341The "mqueue" filesystem provides the necessary kernel features to enable the 342creation of a user space library that implements the POSIX message queues 343API (as noted by the MSG tag in the POSIX 1003.1-2001 version of the System 344Interfaces specification.) 345 346The "mqueue" filesystem contains values for determining/setting the amount of 347resources used by the file system. 348 349/proc/sys/fs/mqueue/queues_max is a read/write file for setting/getting the 350maximum number of message queues allowed on the system. 351 352/proc/sys/fs/mqueue/msg_max is a read/write file for setting/getting the 353maximum number of messages in a queue value. In fact it is the limiting value 354for another (user) limit which is set in mq_open invocation. This attribute of 355a queue must be less or equal then msg_max. 356 357/proc/sys/fs/mqueue/msgsize_max is a read/write file for setting/getting the 358maximum message size value (it is every message queue's attribute set during 359its creation). 360 361/proc/sys/fs/mqueue/msg_default is a read/write file for setting/getting the 362default number of messages in a queue value if attr parameter of mq_open(2) is 363NULL. If it exceed msg_max, the default value is initialized msg_max. 364 365/proc/sys/fs/mqueue/msgsize_default is a read/write file for setting/getting 366the default message size value if attr parameter of mq_open(2) is NULL. If it 367exceed msgsize_max, the default value is initialized msgsize_max. 368 3694. /proc/sys/fs/epoll - Configuration options for the epoll interface 370===================================================================== 371 372This directory contains configuration options for the epoll(7) interface. 373 374max_user_watches 375---------------- 376 377Every epoll file descriptor can store a number of files to be monitored 378for event readiness. Each one of these monitored files constitutes a "watch". 379This configuration option sets the maximum number of "watches" that are 380allowed for each user. 381Each "watch" costs roughly 90 bytes on a 32bit kernel, and roughly 160 bytes 382on a 64bit one. 383The current default value for max_user_watches is the 1/25 (4%) of the 384available low memory, divided for the "watch" cost in bytes. 385