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setfsgid.2: Rewrite for improved clarity and defer to setfsuid() for details 

Rewrite for improved clarity and defer to setfsuid(2) for the
rationale of the fsGID rather than repeating the same details
in this page.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2019-03-26 06:11:58 +01:00
parent 5d3b92e534
commit d478d0defe
1 changed files with 20 additions and 24 deletions
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44
man2/setfsgid.2
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@ -1,4 +1,5 @@
.\" Copyright (C) 1995, Thomas K. Dyas <tdyas@eden.rutgers.edu>
.\" and Copyright (C) 2019, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
@ -36,29 +37,23 @@ setfsgid \- set group identity used for filesystem checks
.PP
.BI "int setfsgid(uid_t " fsgid );
.SH DESCRIPTION
The system call
.BR setfsgid ()
changes the value of the caller's filesystem group ID\(emthe
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.
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 accessing filesystem objects,
while the effective group ID is used for some other kinds
of permissions checks (see
.BR credentials (7)).
.PP
Explicit calls to
.BR setfsuid (2)
and
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
.BR 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.)
to change its filesystem group ID to the value given in
.IR fsgid .
.PP
.BR setfsgid ()
will succeed only if the caller is the superuser or if
@ -77,9 +72,10 @@ 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.
.SH 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.
The filesystem group ID concept and the
.BR setfsgid ()
system call were invented for historical reasons that are
no longer applicable on modern Linux kernels.
See
.BR setfsuid (2)
for a discussion of why the use of both