bug#13125: Fix permissions bugs with setgid directories etc.

[Paul Eggert]

Tags: patch security

Emacs sometimes mishandles the permissions of files: even if
backup-by-copying-when-mismatch is set, Emacs sometimes replaces a
rewritten file with a file that has the wrong user or group.

Here's some background.

In several places Emacs assumes that on 4.2BSD hosts, a newly created
file is given a group ID equal to its parent directory, and that on
non-4.2BSD hosts the new files are given Emacs's group ID.  Although
this was true long ago, it hasn't been true for many years.  Most
commonly, the old 4.2BSD behavior is now selected by the setgid bit on
directories.  But on some hosts, the behavior is selected as a mount
flag, or (as in 4.2BSD) it's a property of the operating system.  On
network file systems the behavior is sometimes selected by the file
server, sometimes by the client.

To add to the mess, on FreeBSD systems, the setuid bit of directories
can control whether there's a similar inheritance of file ownership.
Luckily this problem is a bit simpler, in that it's not a property
of the OS or a mount flag, as far as I know.

I'm attaching a patch, which changes file-attributes so that it now
outputs a placeholder value instead of the old 9th attribute member,
since the value is rarely needed and almost nobody seems to be using
it or caring that it's wrong.  Instead, the patch moves this
functionality to file-ownership-preserved-p via a new argument GROUP.
The patch also adds new functions group-gid and group-real-gid for use
with the backup-file heuristic.

This patch is relative to trunk bzr 111160.

setgiddir.txt
Description: Text document