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SETFSGID(2) Linux Programmer's Manual SETFSGID(2)
NAME
setfsgid - set group identity used for filesystem checks
SYNOPSIS
#include <sys/fsuid.h>
int setfsgid(gid_t fsgid);
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 ac‐
cessing 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.
RETURN VALUE
On both success and failure, this call returns the previous filesystem group ID of the caller.
VERSIONS
This system call is present in Linux since version 1.2.
CONFORMING TO
setfsgid() is Linux-specific and should not be used in programs intended to be portable.
NOTES
The filesystem group ID concept and the setfsgid() system call were invented for historical reasons that are no longer applicable on modern Linux kernels.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 setfs‐
gid() wrapper function transparently deals with the variation across kernel versions.
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 errno to EINVAL without attempting the system call.
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 setfs‐
gid(-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).
SEE ALSO
kill(2), setfsuid(2), capabilities(7), credentials(7)
Linux 2021-03-22 SETFSGID(2)