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Based on kernel version 3.13. Page generated on 2014-01-20 22:04 EST.

1	Documentation 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	
5	For general info and legal blurb, please look in README.
6	
7	==============================================================
8	
9	This file contains documentation for the sysctl files in
10	/proc/sys/fs/ and is valid for Linux kernel version 2.2.
11	
12	The files in this directory can be used to tune and monitor
13	miscellaneous and general things in the operation of the Linux
14	kernel. Since some of the files _can_ be used to screw up your
15	system, it is advisable to read both documentation and source
16	before actually making adjustments.
17	
18	1. /proc/sys/fs
19	----------------------------------------------------------
20	
21	Currently, 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	
43	aio-nr & aio-max-nr:
44	
45	aio-nr is the running total of the number of events specified on the
46	io_setup system call for all currently active aio contexts.  If aio-nr
47	reaches aio-max-nr then io_setup will fail with EAGAIN.  Note that
48	raising aio-max-nr does not result in the pre-allocation or re-sizing
49	of any kernel data structures.
50	
51	==============================================================
52	
53	dentry-state:
54	
55	From linux/fs/dentry.c:
56	--------------------------------------------------------------
57	struct {
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	
66	Dentries are dynamically allocated and deallocated, and
67	nr_dentry seems to be 0 all the time. Hence it's safe to
68	assume that only nr_unused, age_limit and want_pages are
69	used. Nr_unused seems to be exactly what its name says.
70	Age_limit is the age in seconds after which dcache entries
71	can be reclaimed when memory is short and want_pages is
72	nonzero when shrink_dcache_pages() has been called and the
73	dcache isn't pruned yet.
74	
75	==============================================================
76	
77	dquot-max & dquot-nr:
78	
79	The file dquot-max shows the maximum number of cached disk
80	quota entries.
81	
82	The file dquot-nr shows the number of allocated disk quota
83	entries and the number of free disk quota entries.
84	
85	If the number of free cached disk quotas is very low and
86	you have some awesome number of simultaneous system users,
87	you might want to raise the limit.
88	
89	==============================================================
90	
91	file-max & file-nr:
92	
93	The value in file-max denotes the maximum number of file-
94	handles that the Linux kernel will allocate. When you get lots
95	of error messages about running out of file handles, you might
96	want to increase this limit.
97	
98	Historically,the kernel was able to allocate file handles
99	dynamically, but not to free them again. The three values in
100	file-nr denote the number of allocated file handles, the number
101	of allocated but unused file handles, and the maximum number of
102	file handles. Linux 2.6 always reports 0 as the number of free
103	file handles -- this is not an error, it just means that the
104	number of allocated file handles exactly matches the number of
105	used file handles.
106	
107	Attempts to allocate more file descriptors than file-max are
108	reported with printk, look for "VFS: file-max limit <number>
109	reached".
110	==============================================================
111	
112	nr_open:
113	
114	This denotes the maximum number of file-handles a process can
115	allocate. Default value is 1024*1024 (1048576) which should be
116	enough for most machines. Actual limit depends on RLIMIT_NOFILE
117	resource limit.
118	
119	==============================================================
120	
121	inode-max, inode-nr & inode-state:
122	
123	As with file handles, the kernel allocates the inode structures
124	dynamically, but can't free them yet.
125	
126	The value in inode-max denotes the maximum number of inode
127	handlers. This value should be 3-4 times larger than the value
128	in file-max, since stdin, stdout and network sockets also
129	need an inode struct to handle them. When you regularly run
130	out of inodes, you need to increase this value.
131	
132	The file inode-nr contains the first two items from
133	inode-state, so we'll skip to that file...
134	
135	Inode-state contains three actual numbers and four dummies.
136	The actual numbers are, in order of appearance, nr_inodes,
137	nr_free_inodes and preshrink.
138	
139	Nr_inodes stands for the number of inodes the system has
140	allocated, this can be slightly more than inode-max because
141	Linux allocates them one pageful at a time.
142	
143	Nr_free_inodes represents the number of free inodes (?) and
144	preshrink is nonzero when the nr_inodes > inode-max and the
145	system needs to prune the inode list instead of allocating
146	more.
147	
148	==============================================================
149	
150	overflowgid & overflowuid:
151	
152	Some filesystems only support 16-bit UIDs and GIDs, although in Linux
153	UIDs and GIDs are 32 bits. When one of these filesystems is mounted
154	with writes enabled, any UID or GID that would exceed 65535 is translated
155	to a fixed value before being written to disk.
156	
157	These sysctls allow you to change the value of the fixed UID and GID.
158	The default is 65534.
159	
160	==============================================================
161	
162	protected_hardlinks:
163	
164	A long-standing class of security issues is the hardlink-based
165	time-of-check-time-of-use race, most commonly seen in world-writable
166	directories like /tmp. The common method of exploitation of this flaw
167	is to cross privilege boundaries when following a given hardlink (i.e. a
168	root process follows a hardlink created by another user). Additionally,
169	on systems without separated partitions, this stops unauthorized users
170	from "pinning" vulnerable setuid/setgid files against being upgraded by
171	the administrator, or linking to special files.
172	
173	When set to "0", hardlink creation behavior is unrestricted.
174	
175	When set to "1" hardlinks cannot be created by users if they do not
176	already own the source file, or do not have read/write access to it.
177	
178	This protection is based on the restrictions in Openwall and grsecurity.
179	
180	==============================================================
181	
182	protected_symlinks:
183	
184	A long-standing class of security issues is the symlink-based
185	time-of-check-time-of-use race, most commonly seen in world-writable
186	directories like /tmp. The common method of exploitation of this flaw
187	is to cross privilege boundaries when following a given symlink (i.e. a
188	root process follows a symlink belonging to another user). For a likely
189	incomplete list of hundreds of examples across the years, please see:
190	http://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp
191	
192	When set to "0", symlink following behavior is unrestricted.
193	
194	When set to "1" symlinks are permitted to be followed only when outside
195	a sticky world-writable directory, or when the uid of the symlink and
196	follower match, or when the directory owner matches the symlink's owner.
197	
198	This protection is based on the restrictions in Openwall and grsecurity.
199	
200	==============================================================
201	
202	suid_dumpable:
203	
204	This value can be used to query and set the core dump mode for setuid
205	or otherwise protected/tainted binaries. The modes are
206	
207	0 - (default) - traditional behaviour. Any process which has changed
208		privilege levels or is execute only will not be dumped.
209	1 - (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.
214	2 - (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	
227	super-max & super-nr:
228	
229	These numbers control the maximum number of superblocks, and
230	thus the maximum number of mounted filesystems the kernel
231	can have. You only need to increase super-max if you need to
232	mount more filesystems than the current value in super-max
233	allows you to.
234	
235	==============================================================
236	
237	aio-nr & aio-max-nr:
238	
239	aio-nr shows the current system-wide number of asynchronous io
240	requests.  aio-max-nr allows you to change the maximum value
241	aio-nr can grow to.
242	
243	==============================================================
244	
245	
246	2. /proc/sys/fs/binfmt_misc
247	----------------------------------------------------------
248	
249	Documentation for the files in /proc/sys/fs/binfmt_misc is
250	in Documentation/binfmt_misc.txt.
251	
252	
253	3. /proc/sys/fs/mqueue - POSIX message queues filesystem
254	----------------------------------------------------------
255	
256	The "mqueue"  filesystem provides  the necessary kernel features to enable the
257	creation of a  user space  library that  implements  the  POSIX message queues
258	API (as noted by the  MSG tag in the  POSIX 1003.1-2001 version  of the System
259	Interfaces specification.)
260	
261	The "mqueue" filesystem contains values for determining/setting  the amount of
262	resources used by the file system.
263	
264	/proc/sys/fs/mqueue/queues_max is a read/write  file for  setting/getting  the
265	maximum 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
268	maximum number of messages in a queue value.  In fact it is the limiting value
269	for another (user) limit which is set in mq_open invocation. This attribute of
270	a 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
273	maximum  message size value (it is every  message queue's attribute set during
274	its creation).
275	
276	/proc/sys/fs/mqueue/msg_default is  a read/write  file for setting/getting the
277	default number of messages in a queue value if attr parameter of mq_open(2) is
278	NULL. 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
281	the default message size value if attr parameter of mq_open(2) is NULL. If it
282	exceed msgsize_max, the default value is initialized msgsize_max.
283	
284	4. /proc/sys/fs/epoll - Configuration options for the epoll interface
285	--------------------------------------------------------
286	
287	This directory contains configuration options for the epoll(7) interface.
288	
289	max_user_watches
290	----------------
291	
292	Every epoll file descriptor can store a number of files to be monitored
293	for event readiness. Each one of these monitored files constitutes a "watch".
294	This configuration option sets the maximum number of "watches" that are
295	allowed for each user.
296	Each "watch" costs roughly 90 bytes on a 32bit kernel, and roughly 160 bytes
297	on a 64bit one.
298	The current default value for  max_user_watches  is the 1/32 of the available
299	low memory, divided for the "watch" cost in bytes.
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