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>

diff --git a/man2/setfsgid.2 b/man2/setfsgid.2
index ad81c92f0ef72eb07285f793be8e7a79e24298f8..3397fa006ea6392a07b599239ea5aa0747f20898 100644 (file)
--- a/man2/setfsgid.2
+++ b/man2/setfsgid.2
@@ -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