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fea681da | 1 | .\" Copyright (C) 1995, Thomas K. Dyas <tdyas@eden.rutgers.edu> |
d478d0de | 2 | .\" and Copyright (C) 2019, Michael Kerrisk <mtk.manpages@gmail.com> |
fea681da | 3 | .\" |
93015253 | 4 | .\" %%%LICENSE_START(VERBATIM) |
fea681da MK |
5 | .\" Permission is granted to make and distribute verbatim copies of this |
6 | .\" manual provided the copyright notice and this permission notice are | |
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9 | .\" Permission is granted to copy and distribute modified versions of this | |
10 | .\" manual under the conditions for verbatim copying, provided that the | |
11 | .\" entire resulting derived work is distributed under the terms of a | |
12 | .\" permission notice identical to this one. | |
c13182ef | 13 | .\" |
fea681da MK |
14 | .\" Since the Linux kernel and libraries are constantly changing, this |
15 | .\" manual page may be incorrect or out-of-date. The author(s) assume no | |
16 | .\" responsibility for errors or omissions, or for damages resulting from | |
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19 | .\" which is licensed free of charge, as they might when working | |
20 | .\" professionally. | |
c13182ef | 21 | .\" |
fea681da MK |
22 | .\" Formatted or processed versions of this manual, if unaccompanied by |
23 | .\" the source, must acknowledge the copyright and authors of this work. | |
4b72fb64 | 24 | .\" %%%LICENSE_END |
fea681da MK |
25 | .\" |
26 | .\" Created 1995-08-06 Thomas K. Dyas <tdyas@eden.rutgers.edu> | |
27 | .\" Modified 2000-07-01 aeb | |
28 | .\" Modified 2002-07-23 aeb | |
c11b1abf | 29 | .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com> |
fea681da MK |
30 | .\" Added notes on capability requirements |
31 | .\" | |
2e3023c4 | 32 | .TH SETFSGID 2 2019-05-09 "Linux" "Linux Programmer's Manual" |
fea681da | 33 | .SH NAME |
9ee4a2b6 | 34 | setfsgid \- set group identity used for filesystem checks |
fea681da | 35 | .SH SYNOPSIS |
0070a62d | 36 | .B #include <sys/fsuid.h> |
68e4db0a | 37 | .PP |
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38 | .BI "int setfsgid(uid_t " fsgid ); |
39 | .SH DESCRIPTION | |
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40 | On Linux, a process has both a filesystem group ID and an effective group ID. |
41 | The (Linux-specific) filesystem group ID is used | |
42 | for permissions checking when accessing filesystem objects, | |
43 | while the effective group ID is used for some other kinds | |
44 | of permissions checks (see | |
45 | .BR credentials (7)). | |
efeece04 | 46 | .PP |
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47 | Normally, the value of the process's filesystem group ID |
48 | is the same as the value of its effective group ID. | |
49 | This is so, because whenever a process's effective group ID is changed, | |
50 | the kernel also changes the filesystem group ID to be the same as | |
51 | the new value of the effective group ID. | |
52 | A process can cause the value of its filesystem group ID to diverge | |
53 | from its effective group ID by using | |
e511ffb6 | 54 | .BR setfsgid () |
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55 | to change its filesystem group ID to the value given in |
56 | .IR fsgid . | |
efeece04 | 57 | .PP |
e511ffb6 | 58 | .BR setfsgid () |
33a0ccb2 | 59 | will succeed only if the caller is the superuser or if |
fea681da | 60 | .I fsgid |
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61 | matches either the caller's real group ID, effective group ID, |
62 | saved set-group-ID, or current the filesystem user ID. | |
47297adb | 63 | .SH RETURN VALUE |
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64 | On both success and failure, |
65 | this call returns the previous filesystem group ID of the caller. | |
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66 | .SH VERSIONS |
67 | This system call is present in Linux since version 1.2. | |
68 | .\" This system call is present since Linux 1.1.44 | |
69 | .\" and in libc since libc 4.7.6. | |
47297adb | 70 | .SH CONFORMING TO |
e511ffb6 | 71 | .BR setfsgid () |
8382f16d | 72 | is Linux-specific and should not be used in programs intended |
97c1eac8 | 73 | to be portable. |
fea681da | 74 | .SH NOTES |
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75 | The filesystem group ID concept and the |
76 | .BR setfsgid () | |
77 | system call were invented for historical reasons that are | |
78 | no longer applicable on modern Linux kernels. | |
6ace3248 MK |
79 | See |
80 | .BR setfsuid (2) | |
81 | for a discussion of why the use of both | |
82 | .BR setfsuid (2) | |
83 | and | |
84 | .BR setfsgid () | |
85 | is nowadays unneeded. | |
efeece04 | 86 | .PP |
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87 | The original Linux |
88 | .BR setfsgid () | |
89 | system call supported only 16-bit group IDs. | |
c5662d5d | 90 | Subsequently, Linux 2.4 added |
0f0b04f8 MK |
91 | .BR setfsgid32 () |
92 | supporting 32-bit IDs. | |
93 | The glibc | |
94 | .BR setfsgid () | |
95 | wrapper function transparently deals with the variation across kernel versions. | |
dc439d82 | 96 | .SS C library/kernel differences |
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97 | In glibc 2.15 and earlier, |
98 | when the wrapper for this system call determines that the argument can't be | |
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99 | passed to the kernel without integer truncation (because the kernel |
100 | is old and does not support 32-bit group IDs), | |
3cc069d5 | 101 | it will return \-1 and set \fIerrno\fP to |
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102 | .B EINVAL |
103 | without attempting | |
104 | the system call. | |
a1d5f77c | 105 | .SH BUGS |
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106 | No error indications of any kind are returned to the caller, |
107 | and the fact that both successful and unsuccessful calls return | |
108 | the same value makes it impossible to directly determine | |
109 | whether the call succeeded or failed. | |
110 | Instead, the caller must resort to looking at the return value | |
111 | from a further call such as | |
112 | .IR setfsgid(\-1) | |
113 | (which will always fail), in order to determine if a preceding call to | |
114 | .BR setfsgid () | |
115 | changed the filesystem group ID. | |
a1d5f77c MK |
116 | At the very |
117 | least, | |
118 | .B EPERM | |
119 | should be returned when the call fails (because the caller lacks the | |
120 | .B CAP_SETGID | |
121 | capability). | |
47297adb | 122 | .SH SEE ALSO |
fea681da MK |
123 | .BR kill (2), |
124 | .BR setfsuid (2), | |
53a1443c MK |
125 | .BR capabilities (7), |
126 | .BR credentials (7) |