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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON |
SETFSGID(2) Linux Programmer's Manual SETFSGID(2)
setfsgid - set group identity used for filesystem checks
#include <sys/fsuid.h>
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 signal permission handling is slightly different. 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.
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 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)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2016-07-17 SETFSGID(2)
Pages that refer to this page: setfsuid(2), setresuid(2), syscalls(2), credentials(7), path_resolution(7), user_namespaces(7)