setfsgid - set group identity used for filesystem checks
int setfsgid(uid_t fsgid);
The system call setfsgid
() changes the value of the caller's filesystem
group ID—the group ID that the Linux kernel uses to check for all
accesses to the filesystem. Normally, the value of the filesystem group ID
will shadow the value of the effective group ID. In fact, whenever the
effective group ID is changed, the filesystem group ID will also be changed to
the new value of the effective group ID.
Explicit calls to setfsuid(2)
() are usually used only
by programs such as the Linux NFS server that need to change what user and
group ID is used for file access without a corresponding change in the real
and effective user and group IDs. A change in the normal user IDs for a
program such as the NFS server is a security hole that can expose it to
unwanted signals. (But see below.)
() will succeed only if the caller is the superuser or if
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.
() is Linux-specific and should not be used in programs intended
to be portable.
Note that at the time this system call was introduced, a process could send a
signal to a process with the same effective user ID. Today signal permission
handling is slightly different. See setfsuid(2)
for a discussion of why
the use of both setfsuid(2)
() 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.
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), they will return -1
and set errno
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
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