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ReStructuredText
385 lines
13 KiB
ReStructuredText
===============================
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Documentation for /proc/sys/fs/
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===============================
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kernel version 2.2.10
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Copyright (c) 1998, 1999, Rik van Riel <riel@nl.linux.org>
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Copyright (c) 2009, Shen Feng<shen@cn.fujitsu.com>
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For general info and legal blurb, please look in intro.rst.
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------------------------------------------------------------------------------
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This file contains documentation for the sysctl files in
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/proc/sys/fs/ and is valid for Linux kernel version 2.2.
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The files in this directory can be used to tune and monitor
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miscellaneous and general things in the operation of the Linux
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kernel. Since some of the files _can_ be used to screw up your
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system, it is advisable to read both documentation and source
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before actually making adjustments.
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1. /proc/sys/fs
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===============
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Currently, these files are in /proc/sys/fs:
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- aio-max-nr
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- aio-nr
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- dentry-state
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- dquot-max
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- dquot-nr
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- file-max
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- file-nr
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- inode-max
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- inode-nr
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- inode-state
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- nr_open
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- overflowuid
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- overflowgid
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- pipe-user-pages-hard
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- pipe-user-pages-soft
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- protected_fifos
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- protected_hardlinks
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- protected_regular
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- protected_symlinks
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- suid_dumpable
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- super-max
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- super-nr
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aio-nr & aio-max-nr
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-------------------
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aio-nr is the running total of the number of events specified on the
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io_setup system call for all currently active aio contexts. If aio-nr
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reaches aio-max-nr then io_setup will fail with EAGAIN. Note that
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raising aio-max-nr does not result in the pre-allocation or re-sizing
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of any kernel data structures.
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dentry-state
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------------
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From linux/include/linux/dcache.h::
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struct dentry_stat_t dentry_stat {
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int nr_dentry;
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int nr_unused;
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int age_limit; /* age in seconds */
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int want_pages; /* pages requested by system */
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int nr_negative; /* # of unused negative dentries */
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int dummy; /* Reserved for future use */
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};
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Dentries are dynamically allocated and deallocated.
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nr_dentry shows the total number of dentries allocated (active
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+ unused). nr_unused shows the number of dentries that are not
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actively used, but are saved in the LRU list for future reuse.
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Age_limit is the age in seconds after which dcache entries
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can be reclaimed when memory is short and want_pages is
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nonzero when shrink_dcache_pages() has been called and the
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dcache isn't pruned yet.
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nr_negative shows the number of unused dentries that are also
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negative dentries which do not map to any files. Instead,
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they help speeding up rejection of non-existing files provided
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by the users.
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dquot-max & dquot-nr
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--------------------
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The file dquot-max shows the maximum number of cached disk
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quota entries.
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The file dquot-nr shows the number of allocated disk quota
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entries and the number of free disk quota entries.
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If the number of free cached disk quotas is very low and
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you have some awesome number of simultaneous system users,
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you might want to raise the limit.
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file-max & file-nr
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------------------
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The value in file-max denotes the maximum number of file-
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handles that the Linux kernel will allocate. When you get lots
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of error messages about running out of file handles, you might
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want to increase this limit.
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Historically,the kernel was able to allocate file handles
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dynamically, but not to free them again. The three values in
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file-nr denote the number of allocated file handles, the number
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of allocated but unused file handles, and the maximum number of
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file handles. Linux 2.6 always reports 0 as the number of free
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file handles -- this is not an error, it just means that the
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number of allocated file handles exactly matches the number of
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used file handles.
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Attempts to allocate more file descriptors than file-max are
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reported with printk, look for "VFS: file-max limit <number>
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reached".
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nr_open
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-------
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This denotes the maximum number of file-handles a process can
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allocate. Default value is 1024*1024 (1048576) which should be
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enough for most machines. Actual limit depends on RLIMIT_NOFILE
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resource limit.
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inode-max, inode-nr & inode-state
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---------------------------------
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As with file handles, the kernel allocates the inode structures
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dynamically, but can't free them yet.
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The value in inode-max denotes the maximum number of inode
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handlers. This value should be 3-4 times larger than the value
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in file-max, since stdin, stdout and network sockets also
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need an inode struct to handle them. When you regularly run
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out of inodes, you need to increase this value.
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The file inode-nr contains the first two items from
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inode-state, so we'll skip to that file...
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Inode-state contains three actual numbers and four dummies.
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The actual numbers are, in order of appearance, nr_inodes,
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nr_free_inodes and preshrink.
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Nr_inodes stands for the number of inodes the system has
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allocated, this can be slightly more than inode-max because
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Linux allocates them one pageful at a time.
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Nr_free_inodes represents the number of free inodes (?) and
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preshrink is nonzero when the nr_inodes > inode-max and the
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system needs to prune the inode list instead of allocating
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more.
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overflowgid & overflowuid
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-------------------------
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Some filesystems only support 16-bit UIDs and GIDs, although in Linux
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UIDs and GIDs are 32 bits. When one of these filesystems is mounted
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with writes enabled, any UID or GID that would exceed 65535 is translated
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to a fixed value before being written to disk.
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These sysctls allow you to change the value of the fixed UID and GID.
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The default is 65534.
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pipe-user-pages-hard
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--------------------
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Maximum total number of pages a non-privileged user may allocate for pipes.
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Once this limit is reached, no new pipes may be allocated until usage goes
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below the limit again. When set to 0, no limit is applied, which is the default
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setting.
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pipe-user-pages-soft
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--------------------
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Maximum total number of pages a non-privileged user may allocate for pipes
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before the pipe size gets limited to a single page. Once this limit is reached,
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new pipes will be limited to a single page in size for this user in order to
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limit total memory usage, and trying to increase them using fcntl() will be
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denied until usage goes below the limit again. The default value allows to
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allocate up to 1024 pipes at their default size. When set to 0, no limit is
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applied.
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protected_fifos
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---------------
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The intent of this protection is to avoid unintentional writes to
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an attacker-controlled FIFO, where a program expected to create a regular
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file.
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When set to "0", writing to FIFOs is unrestricted.
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When set to "1" don't allow O_CREAT open on FIFOs that we don't own
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in world writable sticky directories, unless they are owned by the
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owner of the directory.
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When set to "2" it also applies to group writable sticky directories.
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This protection is based on the restrictions in Openwall.
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protected_hardlinks
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--------------------
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A long-standing class of security issues is the hardlink-based
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time-of-check-time-of-use race, most commonly seen in world-writable
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directories like /tmp. The common method of exploitation of this flaw
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is to cross privilege boundaries when following a given hardlink (i.e. a
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root process follows a hardlink created by another user). Additionally,
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on systems without separated partitions, this stops unauthorized users
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from "pinning" vulnerable setuid/setgid files against being upgraded by
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the administrator, or linking to special files.
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When set to "0", hardlink creation behavior is unrestricted.
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When set to "1" hardlinks cannot be created by users if they do not
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already own the source file, or do not have read/write access to it.
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This protection is based on the restrictions in Openwall and grsecurity.
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protected_regular
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-----------------
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This protection is similar to protected_fifos, but it
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avoids writes to an attacker-controlled regular file, where a program
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expected to create one.
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When set to "0", writing to regular files is unrestricted.
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When set to "1" don't allow O_CREAT open on regular files that we
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don't own in world writable sticky directories, unless they are
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owned by the owner of the directory.
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When set to "2" it also applies to group writable sticky directories.
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protected_symlinks
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------------------
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A long-standing class of security issues is the symlink-based
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time-of-check-time-of-use race, most commonly seen in world-writable
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directories like /tmp. The common method of exploitation of this flaw
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is to cross privilege boundaries when following a given symlink (i.e. a
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root process follows a symlink belonging to another user). For a likely
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incomplete list of hundreds of examples across the years, please see:
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https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp
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When set to "0", symlink following behavior is unrestricted.
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When set to "1" symlinks are permitted to be followed only when outside
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a sticky world-writable directory, or when the uid of the symlink and
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follower match, or when the directory owner matches the symlink's owner.
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This protection is based on the restrictions in Openwall and grsecurity.
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suid_dumpable:
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--------------
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This value can be used to query and set the core dump mode for setuid
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or otherwise protected/tainted binaries. The modes are
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= ========== ===============================================================
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0 (default) traditional behaviour. Any process which has changed
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privilege levels or is execute only will not be dumped.
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1 (debug) all processes dump core when possible. The core dump is
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owned by the current user and no security is applied. This is
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intended for system debugging situations only.
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Ptrace is unchecked.
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This is insecure as it allows regular users to examine the
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memory contents of privileged processes.
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2 (suidsafe) any binary which normally would not be dumped is dumped
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anyway, but only if the "core_pattern" kernel sysctl is set to
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either a pipe handler or a fully qualified path. (For more
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details on this limitation, see CVE-2006-2451.) This mode is
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appropriate when administrators are attempting to debug
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problems in a normal environment, and either have a core dump
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pipe handler that knows to treat privileged core dumps with
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care, or specific directory defined for catching core dumps.
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If a core dump happens without a pipe handler or fully
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qualified path, a message will be emitted to syslog warning
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about the lack of a correct setting.
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= ========== ===============================================================
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super-max & super-nr
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--------------------
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These numbers control the maximum number of superblocks, and
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thus the maximum number of mounted filesystems the kernel
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can have. You only need to increase super-max if you need to
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mount more filesystems than the current value in super-max
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allows you to.
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aio-nr & aio-max-nr
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-------------------
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aio-nr shows the current system-wide number of asynchronous io
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requests. aio-max-nr allows you to change the maximum value
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aio-nr can grow to.
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mount-max
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---------
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This denotes the maximum number of mounts that may exist
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in a mount namespace.
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2. /proc/sys/fs/binfmt_misc
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===========================
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Documentation for the files in /proc/sys/fs/binfmt_misc is
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in Documentation/admin-guide/binfmt-misc.rst.
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3. /proc/sys/fs/mqueue - POSIX message queues filesystem
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========================================================
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The "mqueue" filesystem provides the necessary kernel features to enable the
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creation of a user space library that implements the POSIX message queues
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API (as noted by the MSG tag in the POSIX 1003.1-2001 version of the System
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Interfaces specification.)
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The "mqueue" filesystem contains values for determining/setting the amount of
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resources used by the file system.
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/proc/sys/fs/mqueue/queues_max is a read/write file for setting/getting the
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maximum number of message queues allowed on the system.
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/proc/sys/fs/mqueue/msg_max is a read/write file for setting/getting the
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maximum number of messages in a queue value. In fact it is the limiting value
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for another (user) limit which is set in mq_open invocation. This attribute of
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a queue must be less or equal then msg_max.
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/proc/sys/fs/mqueue/msgsize_max is a read/write file for setting/getting the
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maximum message size value (it is every message queue's attribute set during
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its creation).
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/proc/sys/fs/mqueue/msg_default is a read/write file for setting/getting the
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default number of messages in a queue value if attr parameter of mq_open(2) is
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NULL. If it exceed msg_max, the default value is initialized msg_max.
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/proc/sys/fs/mqueue/msgsize_default is a read/write file for setting/getting
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the default message size value if attr parameter of mq_open(2) is NULL. If it
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exceed msgsize_max, the default value is initialized msgsize_max.
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4. /proc/sys/fs/epoll - Configuration options for the epoll interface
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=====================================================================
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This directory contains configuration options for the epoll(7) interface.
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max_user_watches
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----------------
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Every epoll file descriptor can store a number of files to be monitored
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for event readiness. Each one of these monitored files constitutes a "watch".
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This configuration option sets the maximum number of "watches" that are
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allowed for each user.
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Each "watch" costs roughly 90 bytes on a 32bit kernel, and roughly 160 bytes
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on a 64bit one.
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The current default value for max_user_watches is the 1/32 of the available
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low memory, divided for the "watch" cost in bytes.
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