SLOB: fix bogus ksize calculation

SLOB's ksize calculation was braindamaged and generally harmlessly
underreported the allocation size. But for very small buffers, it could
in fact overreport them, leading code depending on krealloc to overrun
the allocation and trample other data.

Signed-off-by: Matt Mackall <[email protected]>
Tested-by: Peter Zijlstra <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Matt Mackall

mm/slob.c

@@ -515,7 +515,7 @@ size_t ksize(const void *block)
 
 	sp = (struct slob_page *)virt_to_page(block);
 	if (slob_page(sp))
-		return ((slob_t *)block - 1)->units + SLOB_UNIT;
+		return (((slob_t *)block - 1)->units - 1) * SLOB_UNIT;
 	else
 		return sp->page.private;
 }