SETFSGID(2) Linux Programmer's Manual SETFSGID(2)
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) and setfsgid() 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.)
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.
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 sig-
nal permission handling is slightly different. See setfsuid(2) for a
discussion of why the use of both setfsuid(2) and setfsgid() is nowa-
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 deter-
mines 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 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).
kill(2), setfsuid(2), capabilities(7), credentials(7)
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Linux 2017-09-15 SETFSGID(2)
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