setfsgid - set group identity used for filesystem checks
int setfsgid(uid_t fsgid);
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 credentials(7)).
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 setfsgid() to change its filesystem
group ID to the value given in fsgid.
setfsgid() will succeed only if the caller is the superuser
or if fsgid matches either the caller's real group ID, effective
group ID, saved set-group-ID, or current the filesystem user ID.
On both success and failure, this call returns the previous filesystem group ID
of the caller.
This system call is present in Linux since version 1.2.
setfsgid() is Linux-specific and should not be used in programs intended
to be portable.
The filesystem group ID concept and the setfsgid() system call were
invented for historical reasons that are no longer applicable on modern Linux
kernels. See setfsuid(2) for a discussion of why the use of both
setfsuid(2) and setfsgid() is nowadays unneeded.
The original Linux setfsgid() system call supported only
16-bit group IDs. Subsequently, Linux 2.4 added setfsgid32()
supporting 32-bit IDs. The glibc setfsgid() wrapper function
transparently deals with the variation across kernel versions.
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 errno to EINVAL without attempting the system call.
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 setfsgid(-1) (which will always fail), in order to
determine if a preceding call to setfsgid() changed the filesystem
group ID. At the very least, EPERM should be returned when the call
fails (because the caller lacks the CAP_SETGID capability).
This page is part of release 5.01 of the Linux man-pages project. A
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version of this page, can be found at https://www.kernel.org/doc/man-pages/.