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setfsuid.2: Rewrite for improved clarity and to hint history more explicitly 

The current text reads somewhat clumsily. Rewrite it to introduce
the eUID and fsUID in parallel, and more clearly hint at the the
historical rationale for the fsUID, which is detailed lower in
the page.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2019-03-26 05:58:54 +01:00
parent 96ed2f3ff4
commit 5d3b92e534
1 changed files with 20 additions and 14 deletions
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34
man2/setfsuid.2
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@ -1,4 +1,5 @@
.\" Copyright (C) 1995, Thomas K. Dyas <tdyas@eden.rutgers.edu>
.\" and Copyright (C) 2013, 2019, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
@ -36,29 +37,34 @@ setfsuid \- set user identity used for filesystem checks
.PP
.BI "int setfsuid(uid_t " fsuid );
.SH DESCRIPTION
The system call
On Linux, a process has both a filesystem user ID and an effective user ID.
The (Linux-specific) filesystem user ID is used
for permissions checking when accessing filesystem objects,
while the effective user ID is used for various other kinds
of permissions checks (see
.BR credentials (7)).
.PP
Normally, the value of the process's filesystem user ID
is the same as the value of its effective user ID.
This is so, because whenever a process's effective user ID is changed,
the kernel also changes the filesystem user ID to be the same as
the new value of the effective user ID.
A process can cause the value of its filesystem user ID to diverge
from its effective user ID by using
.BR setfsuid ()
changes the value of the caller's filesystem user ID\(emthe
user ID that the Linux kernel uses to check for all accesses
to the filesystem.
Normally, the value of
the filesystem user ID
will shadow the value of the effective user ID.
In fact, whenever the
effective user ID is changed,
the filesystem user ID
will also be changed to the new value of the effective user ID.
to change its filesystem user ID to the value given in
.IR fsuid .
.PP
Explicit calls to
.BR setfsuid ()
and
.BR setfsgid (2)
are usually used only by programs such as the Linux NFS server that
are (were) 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.)
is (was) a security hole that can expose it to unwanted signals.
(However, this issue is historical; see below.)
.PP
.BR setfsuid ()
will succeed only if the caller is the superuser or if