linux/Documentation/sysctl/fs.txt
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   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
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