SETFSGID
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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NAME
setfsgid - set group identity used for filesystem checks
SYNOPSIS
#include <sys/fsuid.h>
int setfsgid(uid_t fsgid);
DESCRIPTION
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.
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
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.
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
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).
SEE ALSO
kill(2),
setfsuid(2),
capabilities(7),
credentials(7)
COLOPHON
This page is part of release 4.13 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- VERSIONS
-
- CONFORMING TO
-
- NOTES
-
- C library/kernel differences
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
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