1SETFSGID(2) Linux Programmer's Manual SETFSGID(2)
2
6 setfsgid - set group identity used for filesystem checks
7
9 #include <sys/fsuid.h>
10 int setfsgid(uid_t fsgid);
12
14 On Linux, a process has both a filesystem group ID and an effective
15 group ID. The (Linux-specific) filesystem group ID is used for permis‐
16 sions checking when accessing filesystem objects, while the effective
17 group ID is used for some other kinds of permissions checks (see cre‐
18 dentials(7)).
19 Normally, the value of the process's filesystem group ID is the same as
21 the value of its effective group ID. This is so, because whenever a
22 process's effective group ID is changed, the kernel also changes the
23 filesystem group ID to be the same as the new value of the effective
24 group ID. A process can cause the value of its filesystem group ID to
25 diverge from its effective group ID by using setfsgid() to change its
26 filesystem group ID to the value given in fsgid.
27 setfsgid() will succeed only if the caller is the superuser or if fsgid
29 matches either the caller's real group ID, effective group ID, saved
30 set-group-ID, or current the filesystem user ID.
31
33 On both success and failure, this call returns the previous filesystem
34 group ID of the caller.
35
37 This system call is present in Linux since version 1.2.
38
40 setfsgid() is Linux-specific and should not be used in programs
41 intended to be portable.
42
44 The filesystem group ID concept and the setfsgid() system call were
45 invented for historical reasons that are no longer applicable on modern
46 Linux kernels. See setfsuid(2) for a discussion of why the use of both
47 setfsuid(2) and setfsgid() is nowadays unneeded.
48 The original Linux setfsgid() system call supported only 16-bit group
50 IDs. Subsequently, Linux 2.4 added setfsgid32() supporting 32-bit IDs.
51 The glibc setfsgid() wrapper function transparently deals with the
52 variation across kernel versions.
53 C library/kernel differences
55 In glibc 2.15 and earlier, when the wrapper for this system call deter‐
56 mines that the argument can't be passed to the kernel without integer
57 truncation (because the kernel is old and does not support 32-bit group
58 IDs), it will return -1 and set errno to EINVAL without attempting the
59 system call.
60
62 No error indications of any kind are returned to the caller, and the
63 fact that both successful and unsuccessful calls return the same value
64 makes it impossible to directly determine whether the call succeeded or
65 failed. Instead, the caller must resort to looking at the return value
66 from a further call such as setfsgid(-1) (which will always fail), in
67 order to determine if a preceding call to setfsgid() changed the
68 filesystem group ID. At the very least, EPERM should be returned when
69 the call fails (because the caller lacks the CAP_SETGID capability).
70
72 kill(2), setfsuid(2), capabilities(7), credentials(7)
73
75 This page is part of release 5.04 of the Linux man-pages project. A
76 description of the project, information about reporting bugs, and the
77 latest version of this page, can be found at
78 https://www.kernel.org/doc/man-pages/.
79Linux 2019-05-09 SETFSGID(2)