mtab: fix test for a link to a location in /proc

/etc/mtab can be a link to a file in /proc. If it is, we should not
attempt to update /etc/mtab.

The original test used "! -w" as part of the test.  This does not
work since everything is writeable by root.

Thanks to Robin Johnson for the suggestion of using readlink -f and the
regular expression.

Reported-By: junkmailnotread@yahoo.com
X-Gentoo-Bug: 370037
X-Gentoo-Bug-URL: http://bugs.gentoo.org/show_bug.cgi?id=370037

init.d/mtab.in

@@ -12,9 +12,10 @@ depend()
 
 start()
 {
-	# /etc/mtab could be a symlink to /proc/mounts
+	# /etc/mtab could be a symlink to a location in /proc
-	if [ ! -w /etc/mtab -a -L /etc/mtab ]; then
+	if readlink -f /etc/mtab | grep -sq '^/proc/\(self\|[0-9]\+\)/mounts$'
-		einfo "Skipping mtab update (non writeable symlink)"
+	then
+		einfo "Skipping mtab update (link points to location in /proc)"
 		return 0
 	fi