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Based on kernel version 3.9. Page generated on 2013-05-02 23:15 EST.

	Documentation for /proc/sys/fs/*	kernel version 2.2.10
		(c) 1998, 1999,  Rik van Riel <riel@nl.linux.org>
		(c) 2009,        Shen Feng<shen@cn.fujitsu.com>
	
	For general info and legal blurb, please look in README.
	
	==============================================================
	
	This file contains documentation for the sysctl files in
	/proc/sys/fs/ and is valid for Linux kernel version 2.2.
	
	The files in this directory can be used to tune and monitor
	miscellaneous and general things in the operation of the Linux
	kernel. Since some of the files _can_ be used to screw up your
	system, it is advisable to read both documentation and source
	before actually making adjustments.
	
	1. /proc/sys/fs
	----------------------------------------------------------
	
	Currently, these files are in /proc/sys/fs:
	- aio-max-nr
	- aio-nr
	- dentry-state
	- dquot-max
	- dquot-nr
	- file-max
	- file-nr
	- inode-max
	- inode-nr
	- inode-state
	- nr_open
	- overflowuid
	- overflowgid
	- protected_hardlinks
	- protected_symlinks
	- suid_dumpable
	- super-max
	- super-nr
	
	==============================================================
	
	aio-nr & aio-max-nr:
	
	aio-nr is the running total of the number of events specified on the
	io_setup system call for all currently active aio contexts.  If aio-nr
	reaches aio-max-nr then io_setup will fail with EAGAIN.  Note that
	raising aio-max-nr does not result in the pre-allocation or re-sizing
	of any kernel data structures.
	
	==============================================================
	
	dentry-state:
	
	From linux/fs/dentry.c:
	--------------------------------------------------------------
	struct {
	        int nr_dentry;
	        int nr_unused;
	        int age_limit;         /* age in seconds */
	        int want_pages;        /* pages requested by system */
	        int dummy[2];
	} dentry_stat = {0, 0, 45, 0,};
	-------------------------------------------------------------- 
	
	Dentries are dynamically allocated and deallocated, and
	nr_dentry seems to be 0 all the time. Hence it's safe to
	assume that only nr_unused, age_limit and want_pages are
	used. Nr_unused seems to be exactly what its name says.
	Age_limit is the age in seconds after which dcache entries
	can be reclaimed when memory is short and want_pages is
	nonzero when shrink_dcache_pages() has been called and the
	dcache isn't pruned yet.
	
	==============================================================
	
	dquot-max & dquot-nr:
	
	The file dquot-max shows the maximum number of cached disk
	quota entries.
	
	The file dquot-nr shows the number of allocated disk quota
	entries and the number of free disk quota entries.
	
	If the number of free cached disk quotas is very low and
	you have some awesome number of simultaneous system users,
	you might want to raise the limit.
	
	==============================================================
	
	file-max & file-nr:
	
	The value in file-max denotes the maximum number of file-
	handles that the Linux kernel will allocate. When you get lots
	of error messages about running out of file handles, you might
	want to increase this limit.
	
	Historically,the kernel was able to allocate file handles
	dynamically, but not to free them again. The three values in
	file-nr denote the number of allocated file handles, the number
	of allocated but unused file handles, and the maximum number of
	file handles. Linux 2.6 always reports 0 as the number of free
	file handles -- this is not an error, it just means that the
	number of allocated file handles exactly matches the number of
	used file handles.
	
	Attempts to allocate more file descriptors than file-max are
	reported with printk, look for "VFS: file-max limit <number>
	reached".
	==============================================================
	
	nr_open:
	
	This denotes the maximum number of file-handles a process can
	allocate. Default value is 1024*1024 (1048576) which should be
	enough for most machines. Actual limit depends on RLIMIT_NOFILE
	resource limit.
	
	==============================================================
	
	inode-max, inode-nr & inode-state:
	
	As with file handles, the kernel allocates the inode structures
	dynamically, but can't free them yet.
	
	The value in inode-max denotes the maximum number of inode
	handlers. This value should be 3-4 times larger than the value
	in file-max, since stdin, stdout and network sockets also
	need an inode struct to handle them. When you regularly run
	out of inodes, you need to increase this value.
	
	The file inode-nr contains the first two items from
	inode-state, so we'll skip to that file...
	
	Inode-state contains three actual numbers and four dummies.
	The actual numbers are, in order of appearance, nr_inodes,
	nr_free_inodes and preshrink.
	
	Nr_inodes stands for the number of inodes the system has
	allocated, this can be slightly more than inode-max because
	Linux allocates them one pageful at a time.
	
	Nr_free_inodes represents the number of free inodes (?) and
	preshrink is nonzero when the nr_inodes > inode-max and the
	system needs to prune the inode list instead of allocating
	more.
	
	==============================================================
	
	overflowgid & overflowuid:
	
	Some filesystems only support 16-bit UIDs and GIDs, although in Linux
	UIDs and GIDs are 32 bits. When one of these filesystems is mounted
	with writes enabled, any UID or GID that would exceed 65535 is translated
	to a fixed value before being written to disk.
	
	These sysctls allow you to change the value of the fixed UID and GID.
	The default is 65534.
	
	==============================================================
	
	protected_hardlinks:
	
	A long-standing class of security issues is the hardlink-based
	time-of-check-time-of-use race, most commonly seen in world-writable
	directories like /tmp. The common method of exploitation of this flaw
	is to cross privilege boundaries when following a given hardlink (i.e. a
	root process follows a hardlink created by another user). Additionally,
	on systems without separated partitions, this stops unauthorized users
	from "pinning" vulnerable setuid/setgid files against being upgraded by
	the administrator, or linking to special files.
	
	When set to "0", hardlink creation behavior is unrestricted.
	
	When set to "1" hardlinks cannot be created by users if they do not
	already own the source file, or do not have read/write access to it.
	
	This protection is based on the restrictions in Openwall and grsecurity.
	
	==============================================================
	
	protected_symlinks:
	
	A long-standing class of security issues is the symlink-based
	time-of-check-time-of-use race, most commonly seen in world-writable
	directories like /tmp. The common method of exploitation of this flaw
	is to cross privilege boundaries when following a given symlink (i.e. a
	root process follows a symlink belonging to another user). For a likely
	incomplete list of hundreds of examples across the years, please see:
	http://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp
	
	When set to "0", symlink following behavior is unrestricted.
	
	When set to "1" symlinks are permitted to be followed only when outside
	a sticky world-writable directory, or when the uid of the symlink and
	follower match, or when the directory owner matches the symlink's owner.
	
	This protection is based on the restrictions in Openwall and grsecurity.
	
	==============================================================
	
	suid_dumpable:
	
	This value can be used to query and set the core dump mode for setuid
	or otherwise protected/tainted binaries. The modes are
	
	0 - (default) - traditional behaviour. Any process which has changed
		privilege levels or is execute only will not be dumped.
	1 - (debug) - all processes dump core when possible. The core dump is
		owned by the current user and no security is applied. This is
		intended for system debugging situations only. Ptrace is unchecked.
		This is insecure as it allows regular users to examine the memory
		contents of privileged processes.
	2 - (suidsafe) - any binary which normally would not be dumped is dumped
		anyway, but only if the "core_pattern" kernel sysctl is set to
		either a pipe handler or a fully qualified path. (For more details
		on this limitation, see CVE-2006-2451.) This mode is appropriate
		when administrators are attempting to debug problems in a normal
		environment, and either have a core dump pipe handler that knows
		to treat privileged core dumps with care, or specific directory
		defined for catching core dumps. If a core dump happens without
		a pipe handler or fully qualifid path, a message will be emitted
		to syslog warning about the lack of a correct setting.
	
	==============================================================
	
	super-max & super-nr:
	
	These numbers control the maximum number of superblocks, and
	thus the maximum number of mounted filesystems the kernel
	can have. You only need to increase super-max if you need to
	mount more filesystems than the current value in super-max
	allows you to.
	
	==============================================================
	
	aio-nr & aio-max-nr:
	
	aio-nr shows the current system-wide number of asynchronous io
	requests.  aio-max-nr allows you to change the maximum value
	aio-nr can grow to.
	
	==============================================================
	
	
	2. /proc/sys/fs/binfmt_misc
	----------------------------------------------------------
	
	Documentation for the files in /proc/sys/fs/binfmt_misc is
	in Documentation/binfmt_misc.txt.
	
	
	3. /proc/sys/fs/mqueue - POSIX message queues filesystem
	----------------------------------------------------------
	
	The "mqueue"  filesystem provides  the necessary kernel features to enable the
	creation of a  user space  library that  implements  the  POSIX message queues
	API (as noted by the  MSG tag in the  POSIX 1003.1-2001 version  of the System
	Interfaces specification.)
	
	The "mqueue" filesystem contains values for determining/setting  the amount of
	resources used by the file system.
	
	/proc/sys/fs/mqueue/queues_max is a read/write  file for  setting/getting  the
	maximum number of message queues allowed on the system.
	
	/proc/sys/fs/mqueue/msg_max  is  a  read/write file  for  setting/getting  the
	maximum number of messages in a queue value.  In fact it is the limiting value
	for another (user) limit which is set in mq_open invocation. This attribute of
	a queue must be less or equal then msg_max.
	
	/proc/sys/fs/mqueue/msgsize_max is  a read/write  file for setting/getting the
	maximum  message size value (it is every  message queue's attribute set during
	its creation).
	
	/proc/sys/fs/mqueue/msg_default is  a read/write  file for setting/getting the
	default number of messages in a queue value if attr parameter of mq_open(2) is
	NULL. If it exceed msg_max, the default value is initialized msg_max.
	
	/proc/sys/fs/mqueue/msgsize_default is a read/write file for setting/getting
	the default message size value if attr parameter of mq_open(2) is NULL. If it
	exceed msgsize_max, the default value is initialized msgsize_max.
	
	4. /proc/sys/fs/epoll - Configuration options for the epoll interface
	--------------------------------------------------------
	
	This directory contains configuration options for the epoll(7) interface.
	
	max_user_watches
	----------------
	
	Every epoll file descriptor can store a number of files to be monitored
	for event readiness. Each one of these monitored files constitutes a "watch".
	This configuration option sets the maximum number of "watches" that are
	allowed for each user.
	Each "watch" costs roughly 90 bytes on a 32bit kernel, and roughly 160 bytes
	on a 64bit one.
	The current default value for  max_user_watches  is the 1/32 of the available
	low memory, divided for the "watch" cost in bytes.

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