[PATCH][2.4] Slab after-before poisoning backport

[Faik Uygur]

This is a trivial patch that backports Andrew Morton's slab poisoning
modification in 2.5. Patch makes it easier to understand if the BUG() 
at slab.c caused by a freed or an uninitialized slab object usage.

--- linux-2.4.21-pre5/mm/slab.c.orig	Tue Mar  4 22:47:21 2003
+++ linux-2.4.21-pre5/mm/slab.c	Tue Mar  4 22:45:51 2003
@@ -297,8 +297,9 @@
 #define	RED_MAGIC2	0x170FC2A5UL	/* when obj is inactive */
 
 /* ...and for poisoning */
-#define	POISON_BYTE	0x5a		/* byte value for poisoning */
-#define	POISON_END	0xa5		/* end-byte of poisoning */
+#define	POISON_BEFORE	0x5a	/* for use-uninitialised poisoning */
+#define	POISON_AFTER	0x6b	/* for use-after-free poisoning */
+#define	POISON_END	0xa5	/* end-byte of poisoning */
 
 #endif
 
@@ -522,14 +523,15 @@
 }
 
 #if DEBUG
-static inline void kmem_poison_obj (kmem_cache_t *cachep, void *addr)
+static inline void kmem_poison_obj (kmem_cache_t *cachep, void *addr, 
+				    unsigned char val)
 {
 	int size = cachep->objsize;
 	if (cachep->flags & SLAB_RED_ZONE) {
 		addr += BYTES_PER_WORD;
 		size -= 2*BYTES_PER_WORD;
 	}
-	memset(addr, POISON_BYTE, size);
+	memset(addr, val, size);
 	*(unsigned char *)(addr+size-1) = POISON_END;
 }
 
@@ -1083,7 +1085,7 @@
 			objp -= BYTES_PER_WORD;
 		if (cachep->flags & SLAB_POISON)
 			/* need to poison the objs */
-			kmem_poison_obj(cachep, objp);
+			kmem_poison_obj(cachep, objp, POISON_BEFORE);
 		if (cachep->flags & SLAB_RED_ZONE) {
 			if (*((unsigned long*)(objp)) != RED_MAGIC1)
 				BUG();
@@ -1443,7 +1445,7 @@
 			BUG();
 	}
 	if (cachep->flags & SLAB_POISON)
-		kmem_poison_obj(cachep, objp);
+		kmem_poison_obj(cachep, objp, POISON_AFTER);
 	if (kmem_extra_free_checks(cachep, slabp, objp))
 		return;
 #endif