s is 8MB; on 64k page boxes, this
is 128MB.

The problem is that the new binary layout (selected in
arch_pick_mmap_layout) places the mapping segment 128MB or the stack
rlimit away from the top of the process memory, whichever is larger.  If
you chose an rlimit of less than 128MB (most defaults are in the 8Mb
range) then you can end up having your entire stack randomized away.

The fix is to make randomize_stack_top() only steal at most 8MB, which this
patch does.  However, I have to point out that even with this, your stack
rlimit might not be exactly what you get if it's > 128MB, because you're
still losing the random offset of up to 8MB.

The true fix should be to leave an explicit gap for the randomization plus
a buffer when determining mmap_base, but that would involve fixing all the
architectures.

Cc: Arjan van de Ven <arjan@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
	##