1Documentation for /proc/sys/fs/* kernel version 2.2.10 2 (c) 1998, 1999, Rik van Riel <riel@nl.linux.org> 3 (c) 2009, Shen Feng<shen@cn.fujitsu.com> 4 5For general info and legal blurb, please look in README. 6 7============================================================== 8 9This file contains documentation for the sysctl files in 10/proc/sys/fs/ and is valid for Linux kernel version 2.2. 11 12The files in this directory can be used to tune and monitor 13miscellaneous and general things in the operation of the Linux 14kernel. Since some of the files _can_ be used to screw up your 15system, it is advisable to read both documentation and source 16before actually making adjustments. 17 181. /proc/sys/fs 19---------------------------------------------------------- 20 21Currently, these files are in /proc/sys/fs: 22- aio-max-nr 23- aio-nr 24- dentry-state 25- dquot-max 26- dquot-nr 27- file-max 28- file-nr 29- inode-max 30- inode-nr 31- inode-state 32- nr_open 33- overflowuid 34- overflowgid 35- protected_hardlinks 36- protected_symlinks 37- suid_dumpable 38- super-max 39- super-nr 40 41============================================================== 42 43aio-nr & aio-max-nr: 44 45aio-nr is the running total of the number of events specified on the 46io_setup system call for all currently active aio contexts. If aio-nr 47reaches aio-max-nr then io_setup will fail with EAGAIN. Note that 48raising aio-max-nr does not result in the pre-allocation or re-sizing 49of any kernel data structures. 50 51============================================================== 52 53dentry-state: 54 55From linux/fs/dentry.c: 56-------------------------------------------------------------- 57struct { 58 int nr_dentry; 59 int nr_unused; 60 int age_limit; /* age in seconds */ 61 int want_pages; /* pages requested by system */ 62 int dummy[2]; 63} dentry_stat = {0, 0, 45, 0,}; 64-------------------------------------------------------------- 65 66Dentries are dynamically allocated and deallocated, and 67nr_dentry seems to be 0 all the time. Hence it's safe to 68assume that only nr_unused, age_limit and want_pages are 69used. Nr_unused seems to be exactly what its name says. 70Age_limit is the age in seconds after which dcache entries 71can be reclaimed when memory is short and want_pages is 72nonzero when shrink_dcache_pages() has been called and the 73dcache isn't pruned yet. 74 75============================================================== 76 77dquot-max & dquot-nr: 78 79The file dquot-max shows the maximum number of cached disk 80quota entries. 81 82The file dquot-nr shows the number of allocated disk quota 83entries and the number of free disk quota entries. 84 85If the number of free cached disk quotas is very low and 86you have some awesome number of simultaneous system users, 87you might want to raise the limit. 88 89============================================================== 90 91file-max & file-nr: 92 93The value in file-max denotes the maximum number of file- 94handles that the Linux kernel will allocate. When you get lots 95of error messages about running out of file handles, you might 96want to increase this limit. 97 98Historically,the kernel was able to allocate file handles 99dynamically, but not to free them again. The three values in 100file-nr denote the number of allocated file handles, the number 101of allocated but unused file handles, and the maximum number of 102file handles. Linux 2.6 always reports 0 as the number of free 103file handles -- this is not an error, it just means that the 104number of allocated file handles exactly matches the number of 105used file handles. 106 107Attempts to allocate more file descriptors than file-max are 108reported with printk, look for "VFS: file-max limit <number> 109reached". 110============================================================== 111 112nr_open: 113 114This denotes the maximum number of file-handles a process can 115allocate. Default value is 1024*1024 (1048576) which should be 116enough for most machines. Actual limit depends on RLIMIT_NOFILE 117resource limit. 118 119============================================================== 120 121inode-max, inode-nr & inode-state: 122 123As with file handles, the kernel allocates the inode structures 124dynamically, but can't free them yet. 125 126The value in inode-max denotes the maximum number of inode 127handlers. This value should be 3-4 times larger than the value 128in file-max, since stdin, stdout and network sockets also 129need an inode struct to handle them. When you regularly run 130out of inodes, you need to increase this value. 131 132The file inode-nr contains the first two items from 133inode-state, so we'll skip to that file... 134 135Inode-state contains three actual numbers and four dummies. 136The actual numbers are, in order of appearance, nr_inodes, 137nr_free_inodes and preshrink. 138 139Nr_inodes stands for the number of inodes the system has 140allocated, this can be slightly more than inode-max because 141Linux allocates them one pageful at a time. 142 143Nr_free_inodes represents the number of free inodes (?) and 144preshrink is nonzero when the nr_inodes > inode-max and the 145system needs to prune the inode list instead of allocating 146more. 147 148============================================================== 149 150overflowgid & overflowuid: 151 152Some filesystems only support 16-bit UIDs and GIDs, although in Linux 153UIDs and GIDs are 32 bits. When one of these filesystems is mounted 154with writes enabled, any UID or GID that would exceed 65535 is translated 155to a fixed value before being written to disk. 156 157These sysctls allow you to change the value of the fixed UID and GID. 158The default is 65534. 159 160============================================================== 161 162protected_hardlinks: 163 164A long-standing class of security issues is the hardlink-based 165time-of-check-time-of-use race, most commonly seen in world-writable 166directories like /tmp. The common method of exploitation of this flaw 167is to cross privilege boundaries when following a given hardlink (i.e. a 168root process follows a hardlink created by another user). Additionally, 169on systems without separated partitions, this stops unauthorized users 170from "pinning" vulnerable setuid/setgid files against being upgraded by 171the administrator, or linking to special files. 172 173When set to "0", hardlink creation behavior is unrestricted. 174 175When set to "1" hardlinks cannot be created by users if they do not 176already own the source file, or do not have read/write access to it. 177 178This protection is based on the restrictions in Openwall and grsecurity. 179 180============================================================== 181 182protected_symlinks: 183 184A long-standing class of security issues is the symlink-based 185time-of-check-time-of-use race, most commonly seen in world-writable 186directories like /tmp. The common method of exploitation of this flaw 187is to cross privilege boundaries when following a given symlink (i.e. a 188root process follows a symlink belonging to another user). For a likely 189incomplete list of hundreds of examples across the years, please see: 190http://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp 191 192When set to "0", symlink following behavior is unrestricted. 193 194When set to "1" symlinks are permitted to be followed only when outside 195a sticky world-writable directory, or when the uid of the symlink and 196follower match, or when the directory owner matches the symlink's owner. 197 198This protection is based on the restrictions in Openwall and grsecurity. 199 200============================================================== 201 202suid_dumpable: 203 204This value can be used to query and set the core dump mode for setuid 205or otherwise protected/tainted binaries. The modes are 206 2070 - (default) - traditional behaviour. Any process which has changed 208 privilege levels or is execute only will not be dumped. 2091 - (debug) - all processes dump core when possible. The core dump is 210 owned by the current user and no security is applied. This is 211 intended for system debugging situations only. Ptrace is unchecked. 212 This is insecure as it allows regular users to examine the memory 213 contents of privileged processes. 2142 - (suidsafe) - any binary which normally would not be dumped is dumped 215 anyway, but only if the "core_pattern" kernel sysctl is set to 216 either a pipe handler or a fully qualified path. (For more details 217 on this limitation, see CVE-2006-2451.) This mode is appropriate 218 when administrators are attempting to debug problems in a normal 219 environment, and either have a core dump pipe handler that knows 220 to treat privileged core dumps with care, or specific directory 221 defined for catching core dumps. If a core dump happens without 222 a pipe handler or fully qualifid path, a message will be emitted 223 to syslog warning about the lack of a correct setting. 224 225============================================================== 226 227super-max & super-nr: 228 229These numbers control the maximum number of superblocks, and 230thus the maximum number of mounted filesystems the kernel 231can have. You only need to increase super-max if you need to 232mount more filesystems than the current value in super-max 233allows you to. 234 235============================================================== 236 237aio-nr & aio-max-nr: 238 239aio-nr shows the current system-wide number of asynchronous io 240requests. aio-max-nr allows you to change the maximum value 241aio-nr can grow to. 242 243============================================================== 244 245 2462. /proc/sys/fs/binfmt_misc 247---------------------------------------------------------- 248 249Documentation for the files in /proc/sys/fs/binfmt_misc is 250in Documentation/binfmt_misc.txt. 251 252 2533. /proc/sys/fs/mqueue - POSIX message queues filesystem 254---------------------------------------------------------- 255 256The "mqueue" filesystem provides the necessary kernel features to enable the 257creation of a user space library that implements the POSIX message queues 258API (as noted by the MSG tag in the POSIX 1003.1-2001 version of the System 259Interfaces specification.) 260 261The "mqueue" filesystem contains values for determining/setting the amount of 262resources used by the file system. 263 264/proc/sys/fs/mqueue/queues_max is a read/write file for setting/getting the 265maximum number of message queues allowed on the system. 266 267/proc/sys/fs/mqueue/msg_max is a read/write file for setting/getting the 268maximum number of messages in a queue value. In fact it is the limiting value 269for another (user) limit which is set in mq_open invocation. This attribute of 270a queue must be less or equal then msg_max. 271 272/proc/sys/fs/mqueue/msgsize_max is a read/write file for setting/getting the 273maximum message size value (it is every message queue's attribute set during 274its creation). 275 276/proc/sys/fs/mqueue/msg_default is a read/write file for setting/getting the 277default number of messages in a queue value if attr parameter of mq_open(2) is 278NULL. If it exceed msg_max, the default value is initialized msg_max. 279 280/proc/sys/fs/mqueue/msgsize_default is a read/write file for setting/getting 281the default message size value if attr parameter of mq_open(2) is NULL. If it 282exceed msgsize_max, the default value is initialized msgsize_max. 283 2844. /proc/sys/fs/epoll - Configuration options for the epoll interface 285-------------------------------------------------------- 286 287This directory contains configuration options for the epoll(7) interface. 288 289max_user_watches 290---------------- 291 292Every epoll file descriptor can store a number of files to be monitored 293for event readiness. Each one of these monitored files constitutes a "watch". 294This configuration option sets the maximum number of "watches" that are 295allowed for each user. 296Each "watch" costs roughly 90 bytes on a 32bit kernel, and roughly 160 bytes 297on a 64bit one. 298The current default value for max_user_watches is the 1/32 of the available 299low memory, divided for the "watch" cost in bytes. 300 301