1 .\" Copyright (C) 1995, Thomas K. Dyas <tdyas@eden.rutgers.edu>
3 .\" %%%LICENSE_START(VERBATIM)
4 .\" Permission is granted to make and distribute verbatim copies of this
5 .\" manual provided the copyright notice and this permission notice are
6 .\" preserved on all copies.
8 .\" Permission is granted to copy and distribute modified versions of this
9 .\" manual under the conditions for verbatim copying, provided that the
10 .\" entire resulting derived work is distributed under the terms of a
11 .\" permission notice identical to this one.
13 .\" Since the Linux kernel and libraries are constantly changing, this
14 .\" manual page may be incorrect or out-of-date. The author(s) assume no
15 .\" responsibility for errors or omissions, or for damages resulting from
16 .\" the use of the information contained herein. The author(s) may not
17 .\" have taken the same level of care in the production of this manual,
18 .\" which is licensed free of charge, as they might when working
21 .\" Formatted or processed versions of this manual, if unaccompanied by
22 .\" the source, must acknowledge the copyright and authors of this work.
25 .\" Created 1995-08-06 Thomas K. Dyas <tdyas@eden.rutgers.edu>
26 .\" Modified 2000-07-01 aeb
27 .\" Modified 2002-07-23 aeb
28 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
29 .\" Added notes on capability requirements
31 .TH SETFSGID 2 2017-09-15 "Linux" "Linux Programmer's Manual"
33 setfsgid \- set group identity used for filesystem checks
35 .B #include <sys/fsuid.h>
37 .BI "int setfsgid(uid_t " fsgid );
41 changes the value of the caller's filesystem group ID\(emthe
42 group ID that the Linux kernel uses to check for all accesses
44 Normally, the value of
45 the filesystem group ID
46 will shadow the value of the effective group ID.
48 effective group ID is changed,
49 the filesystem group ID
50 will also be changed to the new value of the effective group ID.
56 are usually used only by programs such as the Linux NFS server that
57 need to change what user and group ID is used for file access without a
58 corresponding change in the real and effective user and group IDs.
59 A change in the normal user IDs for a program such as the NFS server
60 is a security hole that can expose it to unwanted signals.
64 will succeed only if the caller is the superuser or if
66 matches either the caller's real group ID, effective group ID,
67 saved set-group-ID, or current the filesystem user ID.
69 On both success and failure,
70 this call returns the previous filesystem group ID of the caller.
72 This system call is present in Linux since version 1.2.
73 .\" This system call is present since Linux 1.1.44
74 .\" and in libc since libc 4.7.6.
77 is Linux-specific and should not be used in programs intended
80 Note that at the time this system call was introduced, a process
81 could send a signal to a process with the same effective user ID.
82 Today signal permission handling is slightly different.
85 for a discussion of why the use of both
93 system call supported only 16-bit group IDs.
94 Subsequently, Linux 2.4 added
96 supporting 32-bit IDs.
99 wrapper function transparently deals with the variation across kernel versions.
100 .SS C library/kernel differences
101 In glibc 2.15 and earlier,
102 when the wrapper for this system call determines that the argument can't be
103 passed to the kernel without integer truncation (because the kernel
104 is old and does not support 32-bit group IDs),
105 it will return \-1 and set \fIerrno\fP to
110 No error indications of any kind are returned to the caller,
111 and the fact that both successful and unsuccessful calls return
112 the same value makes it impossible to directly determine
113 whether the call succeeded or failed.
114 Instead, the caller must resort to looking at the return value
115 from a further call such as
117 (which will always fail), in order to determine if a preceding call to
119 changed the filesystem group ID.
123 should be returned when the call fails (because the caller lacks the
129 .BR capabilities (7),