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| .\" Created 1995-08-06 Thomas K. Dyas <tdyas@eden.rutgers.edu> |
| .\" Modified 2000-07-01 aeb |
| .\" Modified 2002-07-23 aeb |
| .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com> |
| .\" Added notes on capability requirements |
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| .TH SETFSGID 2 2021-03-22 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| setfsgid \- set group identity used for filesystem checks |
| .SH SYNOPSIS |
| .nf |
| .B #include <sys/fsuid.h> |
| .PP |
| .BI "int setfsgid(gid_t " fsgid ); |
| .fi |
| .SH DESCRIPTION |
| On Linux, a process has both a filesystem group ID and an effective group ID. |
| The (Linux-specific) filesystem group ID is used |
| for permissions checking when accessing filesystem objects, |
| while the effective group ID is used for some other kinds |
| of permissions checks (see |
| .BR credentials (7)). |
| .PP |
| Normally, the value of the process's filesystem group ID |
| is the same as the value of its effective group ID. |
| This is so, because whenever a process's effective group ID is changed, |
| the kernel also changes the filesystem group ID to be the same as |
| the new value of the effective group ID. |
| A process can cause the value of its filesystem group ID to diverge |
| from its effective group ID by using |
| .BR setfsgid () |
| to change its filesystem group ID to the value given in |
| .IR fsgid . |
| .PP |
| .BR setfsgid () |
| will succeed only if the caller is the superuser or if |
| .I fsgid |
| matches either the caller's real group ID, effective group ID, |
| saved set-group-ID, or current the filesystem user ID. |
| .SH RETURN VALUE |
| On both success and failure, |
| this call returns the previous filesystem group ID of the caller. |
| .SH VERSIONS |
| This system call is present in Linux since version 1.2. |
| .\" This system call is present since Linux 1.1.44 |
| .\" and in libc since libc 4.7.6. |
| .SH CONFORMING TO |
| .BR setfsgid () |
| is Linux-specific and should not be used in programs intended |
| to be portable. |
| .SH NOTES |
| The filesystem group ID concept and the |
| .BR setfsgid () |
| system call were invented for historical reasons that are |
| no longer applicable on modern Linux kernels. |
| See |
| .BR setfsuid (2) |
| for a discussion of why the use of both |
| .BR setfsuid (2) |
| and |
| .BR setfsgid () |
| is nowadays unneeded. |
| .PP |
| The original Linux |
| .BR setfsgid () |
| system call supported only 16-bit group IDs. |
| Subsequently, Linux 2.4 added |
| .BR setfsgid32 () |
| supporting 32-bit IDs. |
| The glibc |
| .BR setfsgid () |
| wrapper function transparently deals with the variation across kernel versions. |
| .SS C library/kernel differences |
| In glibc 2.15 and earlier, |
| when the wrapper for this system call determines that the argument can't be |
| passed to the kernel without integer truncation (because the kernel |
| is old and does not support 32-bit group IDs), |
| it will return \-1 and set \fIerrno\fP to |
| .B EINVAL |
| without attempting |
| the system call. |
| .SH BUGS |
| No error indications of any kind are returned to the caller, |
| and the fact that both successful and unsuccessful calls return |
| the same value makes it impossible to directly determine |
| whether the call succeeded or failed. |
| Instead, the caller must resort to looking at the return value |
| from a further call such as |
| .IR setfsgid(\-1) |
| (which will always fail), in order to determine if a preceding call to |
| .BR setfsgid () |
| changed the filesystem group ID. |
| At the very |
| least, |
| .B EPERM |
| should be returned when the call fails (because the caller lacks the |
| .B CAP_SETGID |
| capability). |
| .SH SEE ALSO |
| .BR kill (2), |
| .BR setfsuid (2), |
| .BR capabilities (7), |
| .BR credentials (7) |