cpuset: zero malloc - fix for old cpusets

The cpuset code to present a list of tasks using a cpuset to user space could
write to an array that it had kmalloc'd, after a kmalloc request of zero size.

The problem was that the code didn't check for writes past the allocated end
of the array until -after- the first write.

This is a race condition that is likely rare -- it would only show up if a
cpuset went from being empty to having a task in it, during the brief time
between the allocation and the first write.

Prior to roughly 2.6.22 kernels, this was also a benign problem, because a
zero kmalloc returned a few usable bytes anyway, and no harm was done with the
bogus write.

With the 2.6.22 kernel changes to make issue a warning if code tries to write
to the location returned from a zero size allocation, this problem is no
longer benign.  This cpuset code would occassionally trigger that warning.

The fix is trivial -- check before storing into the array, not after, whether
the array is big enough to hold the store.

Cc: "Eric W. Biederman" <[email protected]>
Cc: "Serge E. Hallyn" <[email protected]>
Cc: Balbir Singh <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Herbert Poetzl <[email protected]>
Cc: Kirill Korotaev <[email protected]>
Cc: Paul Menage <[email protected]>
Cc: Srivatsa Vaddagiri <[email protected]>
Cc: Christoph Lameter <[email protected]>
Signed-off-by: Paul Jackson <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

kernel/cpuset.c

@@ -1682,9 +1682,9 @@ static int pid_array_load(pid_t *pidarray, int npids, struct cpuset *cs)
 
 	do_each_thread(g, p) {
 		if (p->cpuset == cs) {
-			pidarray[n++] = p->pid;
 			if (unlikely(n == npids))
 				goto array_full;
+			pidarray[n++] = p->pid;
 		}
 	} while_each_thread(g, p);