rework test for mounted /proc

The previous test assumed that we could always rely on the minor fault
counter to change between reads of /proc/self/stat, but we found that
this is not the case.

The new test compares two reads of /proc/self/environ for which we have
set the same environment variable to two different values.
If the comparison shows the two reads have the same contents, we know
that /proc is not working.

I would like to thank Robin Johnson and Mike Frysinger for their input
for this patch.

X-Gentoo-Bug: 348416
X-Gentoo-Bug-URL: http://bugs.gentoo.org/show_bug.cgi?id=348416

sh/init.sh.Linux.in

@@ -65,27 +65,21 @@ mount_svcdir()
 # By default VServer already has /proc mounted, but OpenVZ does not!
 # However, some of our users have an old proc image in /proc
 # NFC how they managed that, but the end result means we have to test if
-# /proc actually works or not. We to this by comparing two reads of
-# /proc/self/stat. They will not match, because at least the minor fault count
-# field (field 10) should have changed.
-#
-# We can use any file here that fills the following requirements:
-# - changes between sequential reads
-# - is world-readable (not blocked in hardened kernel)
-# - Is only a single line (ergo entire check is doable with no forks)
+# /proc actually works or not. We do this by comparing two reads of
+# /proc/self/environ for which we have set the variable VAR to two
+# different values. If the comparison comes back equal, we know that
+# /proc is not working.
 mountproc=true
-f=/proc/self/stat
+f=/proc/self/environ
 if [ -e $f ]; then
-	exec 9<$f ; read a <&9 ; exec 9<&-
-	exec 9<$f ; read b <&9 ; exec 9<&-
-	if [ "$a" = "$b" ]; then
+	if [ "$(VAR=a cat $f)" = "$(VAR=b cat $f)" ]; then
 		eerror "You have cruft in /proc that should be deleted"
 	else
 		einfo "/proc is already mounted, skipping"
 		mountproc=false
 	fi
 fi
-unset a b f
+unset f
 
 if $mountproc; then
 	procfs="proc"