[PATCH] Move slab objects to the end of the real allocation

From: Manfred Spraul <manfred@colorfullife.com>

The real memory allocation is usually larger than the actual object size:
either due to L1 cache line padding, or due to page padding with
CONFIG_DEBUG_PAGEALLOC.  Right now objects are placed to the beginning of
the real allocation, but to trigger bugs it's better to move objects to the
end of the real allocation: that way accesses behind the end of the
allocation have a larger chance of hitting the (unmapped) next page.  The
attached patch moves the objects to align them with the end of the real
allocation.

Actually it contains 4 seperate changes:

- Do not page-pad allocations that are <= SMP_CACHE_LINE_SIZE.  This
  crashes.  Right now the limit is hardcoded to 128 bytes, but sooner or
  later an arch will appear with 256 byte cache lines.

- cleanup: redzone bytes are not accessed with inline helper functions,
  instead of magic offsets scattered throughout slab.c

- main change: move objects to the end of the allocation - trivial after
  the cleanup.

- Print old redzone value if a redzone mismatch happens: This makes it
  simpler to figure out what happened [single bit error, wrong redzone
  code, overwritten]

mm/slab.c

@@ -293,6 +293,10 @@ struct kmem_cache_s {
 	atomic_t		freehit;
 	atomic_t		freemiss;
 #endif
+#if DEBUG
+	int			dbghead;
+	int			reallen;
+#endif
 };
 
 #define CFLGS_OFF_SLAB		(0x80000000UL)
@@ -356,32 +360,68 @@ struct kmem_cache_s {
 #define POISON_AFTER	0x6b	/* for use-after-free poisoning */
 #define	POISON_END	0xa5	/* end-byte of poisoning */
 
+/* memory layout of objects:
+ * 0		: objp
+ * 0 .. cachep->dbghead - BYTES_PER_WORD - 1: padding. This ensures that
+ * 		the end of an object is aligned with the end of the real
+ * 		allocation. Catches writes behind the end of the allocation.
+ * cachep->dbghead - BYTES_PER_WORD .. cachep->dbghead - 1:
+ * 		redzone word.
+ * cachep->dbghead: The real object.
+ * cachep->objsize - 2* BYTES_PER_WORD: redzone word [BYTES_PER_WORD long]
+ * cachep->objsize - 1* BYTES_PER_WORD: last caller address [BYTES_PER_WORD long]
+ */
 static inline int obj_dbghead(kmem_cache_t *cachep)
 {
-	if (cachep->flags & SLAB_RED_ZONE)
-		return BYTES_PER_WORD;
-	return 0;
+	return cachep->dbghead;
 }
 
-static inline int obj_dbglen(kmem_cache_t *cachep)
+static inline int obj_reallen(kmem_cache_t *cachep)
 {
-	int len = 0;
+	return cachep->reallen;
+}
 
-	if (cachep->flags & SLAB_RED_ZONE) {
-		len += 2*BYTES_PER_WORD;
-	}
-	if (cachep->flags & SLAB_STORE_USER) {
-		len += BYTES_PER_WORD;
-	}
-	return len;
+static unsigned long *dbg_redzone1(kmem_cache_t *cachep, void *objp)
+{
+	BUG_ON(!(cachep->flags & SLAB_RED_ZONE));
+	return (unsigned long*) (objp+obj_dbghead(cachep)-BYTES_PER_WORD);
+}
+
+static unsigned long *dbg_redzone2(kmem_cache_t *cachep, void *objp)
+{
+	BUG_ON(!(cachep->flags & SLAB_RED_ZONE));
+	if (cachep->flags & SLAB_STORE_USER)
+		return (unsigned long*) (objp+cachep->objsize-2*BYTES_PER_WORD);
+	return (unsigned long*) (objp+cachep->objsize-BYTES_PER_WORD);
+}
+
+static void **dbg_userword(kmem_cache_t *cachep, void *objp)
+{
+	BUG_ON(!(cachep->flags & SLAB_STORE_USER));
+	return (void**)(objp+cachep->objsize-BYTES_PER_WORD);
 }
 #else
 static inline int obj_dbghead(kmem_cache_t *cachep)
 {
 	return 0;
 }
-static inline int obj_dbglen(kmem_cache_t *cachep)
+static inline int obj_reallen(kmem_cache_t *cachep)
+{
+	return cachep->objsize;
+}
+static inline unsigned long *dbg_redzone1(kmem_cache_t *cachep, void *objp)
+{
+	BUG();
+	return 0;
+}
+static inline unsigned long *dbg_redzone2(kmem_cache_t *cachep, void *objp)
+{
+	BUG();
+	return 0;
+}
+static inline void **dbg_userword(kmem_cache_t *cachep, void *objp)
 {
+	BUG();
 	return 0;
 }
 #endif
@@ -804,7 +844,7 @@ static inline void kmem_freepages (kmem_cache_t *cachep, void *addr)
 #ifdef CONFIG_DEBUG_PAGEALLOC
 static void store_stackinfo(kmem_cache_t *cachep, unsigned long *addr, unsigned long caller)
 {
-	int size = cachep->objsize-obj_dbglen(cachep);
+	int size = obj_reallen(cachep);
 
 	addr = (unsigned long *)&((char*)addr)[obj_dbghead(cachep)];
 
@@ -836,7 +876,7 @@ static void store_stackinfo(kmem_cache_t *cachep, unsigned long *addr, unsigned
 
 static void poison_obj(kmem_cache_t *cachep, void *addr, unsigned char val)
 {
-	int size = cachep->objsize-obj_dbglen(cachep);
+	int size = obj_reallen(cachep);
 	addr = &((char*)addr)[obj_dbghead(cachep)];
 
 	memset(addr, val, size);
@@ -858,47 +898,42 @@ static void *scan_poisoned_obj(unsigned char* addr, unsigned int size)
 	return NULL;
 }
 
-static void check_poison_obj(kmem_cache_t *cachep, void *addr)
+static void check_poison_obj(kmem_cache_t *cachep, void *objp)
 {
 	void *end;
-	int size = cachep->objsize-obj_dbglen(cachep);
+	void *realobj;
+	int size = obj_reallen(cachep);
 
-	addr = &((char*)addr)[obj_dbghead(cachep)];
+	realobj = objp+obj_dbghead(cachep);
 
-	end = scan_poisoned_obj(addr, size);
+	end = scan_poisoned_obj(realobj, size);
 	if (end) {
 		int s;
 		printk(KERN_ERR "Slab corruption: start=%p, expend=%p, "
-				"problemat=%p\n", addr, addr+size-1, end);
+				"problemat=%p\n", realobj, realobj+size-1, end);
 		if (cachep->flags & SLAB_STORE_USER) {
-			void *pc;
-
-			if (cachep->flags & SLAB_RED_ZONE)
-				pc = *(void**)(addr+size+BYTES_PER_WORD);
-			else
-				pc = *(void**)(addr+size);
-			printk(KERN_ERR "Last user: [<%p>]", pc);
-			print_symbol("(%s)", (unsigned long)pc);
+			printk(KERN_ERR "Last user: [<%p>]", *dbg_userword(cachep, objp));
+			print_symbol("(%s)", (unsigned long)*dbg_userword(cachep, objp));
 			printk("\n");
 		}
 		printk(KERN_ERR "Data: ");
 		for (s = 0; s < size; s++) {
-			if (((char*)addr)[s] == POISON_BEFORE)
+			if (((char*)realobj)[s] == POISON_BEFORE)
 				printk(".");
-			else if (((char*)addr)[s] == POISON_AFTER)
+			else if (((char*)realobj)[s] == POISON_AFTER)
 				printk("*");
 			else
-				printk("%02X ", ((unsigned char*)addr)[s]);
+				printk("%02X ", ((unsigned char*)realobj)[s]);
 		}
 		printk("\n");
 		printk(KERN_ERR "Next: ");
 		for (; s < size + 32; s++) {
-			if (((char*)addr)[s] == POISON_BEFORE)
+			if (((char*)realobj)[s] == POISON_BEFORE)
 				printk(".");
-			else if (((char*)addr)[s] == POISON_AFTER)
+			else if (((char*)realobj)[s] == POISON_AFTER)
 				printk("*");
 			else
-				printk("%02X ", ((unsigned char*)addr)[s]);
+				printk("%02X ", ((unsigned char*)realobj)[s]);
 		}
 		printk("\n");
 		slab_error(cachep, "object was modified after freeing");
@@ -916,7 +951,6 @@ static void slab_destroy (kmem_cache_t *cachep, struct slab *slabp)
 	int i;
 	for (i = 0; i < cachep->num; i++) {
 		void *objp = slabp->s_mem + cachep->objsize * i;
-		int objlen = cachep->objsize;
 
 		if (cachep->flags & SLAB_POISON) {
 #ifdef CONFIG_DEBUG_PAGEALLOC
@@ -928,21 +962,16 @@ static void slab_destroy (kmem_cache_t *cachep, struct slab *slabp)
 			check_poison_obj(cachep, objp);
 #endif
 		}
-		if (cachep->flags & SLAB_STORE_USER)
-			objlen -= BYTES_PER_WORD;
-
 		if (cachep->flags & SLAB_RED_ZONE) {
-			if (*((unsigned long*)(objp)) != RED_INACTIVE)
+			if (*dbg_redzone1(cachep, objp) != RED_INACTIVE)
 				slab_error(cachep, "start of a freed object "
 							"was overwritten");
-			if (*((unsigned long*)(objp + objlen - BYTES_PER_WORD))
-				       	!= RED_INACTIVE)
+			if (*dbg_redzone2(cachep, objp) != RED_INACTIVE)
 				slab_error(cachep, "end of a freed object "
 							"was overwritten");
-			objp += BYTES_PER_WORD;
 		}
 		if (cachep->dtor && !(cachep->flags & SLAB_POISON))
-			(cachep->dtor)(objp, cachep, 0);
+			(cachep->dtor)(objp+obj_dbghead(cachep), cachep, 0);
 	}
 #else
 	if (cachep->dtor) {
@@ -1020,10 +1049,6 @@ kmem_cache_create (const char *name, size_t size, size_t offset,
 	}
 
 #if FORCED_DEBUG
-#ifdef CONFIG_DEBUG_PAGEALLOC
-	if (size < PAGE_SIZE-3*BYTES_PER_WORD && size > 128)
-		size = PAGE_SIZE-3*BYTES_PER_WORD;
-#endif
 	/*
 	 * Enable redzoning and last user accounting, except
 	 * - for caches with forced alignment: redzoning would violate the
@@ -1054,6 +1079,9 @@ kmem_cache_create (const char *name, size_t size, size_t offset,
 		goto opps;
 	memset(cachep, 0, sizeof(kmem_cache_t));
 
+#if DEBUG
+	cachep->reallen = size;
+#endif
 	/* Check that size is in terms of words.  This is needed to avoid
 	 * unaligned accesses for some archs when redzoning is used, and makes
 	 * sure any on-slab bufctl's are also correctly aligned.
@@ -1071,12 +1099,20 @@ kmem_cache_create (const char *name, size_t size, size_t offset,
 		 * when redzoning.
 		 */
 		flags &= ~SLAB_HWCACHE_ALIGN;
-		size += 2*BYTES_PER_WORD;	/* words for redzone */
+		/* add space for red zone words */
+		cachep->dbghead += BYTES_PER_WORD;
+		size += 2*BYTES_PER_WORD;
 	}
 	if (flags & SLAB_STORE_USER) {
 		flags &= ~SLAB_HWCACHE_ALIGN;
-		size += BYTES_PER_WORD;		/* word for kfree caller address */
+		size += BYTES_PER_WORD; /* add space */
 	}
+#if FORCED_DEBUG && defined(CONFIG_DEBUG_PAGEALLOC)
+	if (size > 128 && cachep->reallen > L1_CACHE_BYTES && size < PAGE_SIZE) {
+		cachep->dbghead += PAGE_SIZE - size;
+		size = PAGE_SIZE;
+	}
+#endif
 #endif
 	align = BYTES_PER_WORD;
 	if (flags & SLAB_HWCACHE_ALIGN)
@@ -1444,20 +1480,15 @@ static void cache_init_objs (kmem_cache_t * cachep,
 	for (i = 0; i < cachep->num; i++) {
 		void* objp = slabp->s_mem+cachep->objsize*i;
 #if DEBUG
-		int objlen = cachep->objsize;
 		/* need to poison the objs? */
 		if (cachep->flags & SLAB_POISON)
 			poison_obj(cachep, objp, POISON_BEFORE);
-		if (cachep->flags & SLAB_STORE_USER) {
-			objlen -= BYTES_PER_WORD;
-			((unsigned long*)(objp+objlen))[0] = 0;
-		}
+		if (cachep->flags & SLAB_STORE_USER)
+			*dbg_userword(cachep, objp) = NULL;
 
 		if (cachep->flags & SLAB_RED_ZONE) {
-			*((unsigned long*)(objp)) = RED_INACTIVE;
-			objp += BYTES_PER_WORD;
-			objlen -= 2* BYTES_PER_WORD;
-			*((unsigned long*)(objp + objlen)) = RED_INACTIVE;
+			*dbg_redzone1(cachep, objp) = RED_INACTIVE;
+			*dbg_redzone2(cachep, objp) = RED_INACTIVE;
 		}
 		/*
 		 * Constructors are not allowed to allocate memory from
@@ -1465,14 +1496,13 @@ static void cache_init_objs (kmem_cache_t * cachep,
 		 * Otherwise, deadlock. They must also be threaded.
 		 */
 		if (cachep->ctor && !(cachep->flags & SLAB_POISON))
-			cachep->ctor(objp, cachep, ctor_flags);
+			cachep->ctor(objp+obj_dbghead(cachep), cachep, ctor_flags);
 
 		if (cachep->flags & SLAB_RED_ZONE) {
-			if (*((unsigned long*)(objp + objlen)) != RED_INACTIVE)
+			if (*dbg_redzone2(cachep, objp) != RED_INACTIVE)
 				slab_error(cachep, "constructor overwrote the"
 							" end of an object");
-			objp -= BYTES_PER_WORD;
-			if (*((unsigned long*)(objp)) != RED_INACTIVE)
+			if (*dbg_redzone1(cachep, objp) != RED_INACTIVE)
 				slab_error(cachep, "constructor overwrote the"
 							" start of an object");
 		}
@@ -1623,9 +1653,9 @@ static inline void *cache_free_debugcheck (kmem_cache_t * cachep, void * objp, v
 #if DEBUG
 	struct page *page;
 	unsigned int objnr;
-	int objlen = cachep->objsize;
 	struct slab *slabp;
 
+	objp -= obj_dbghead(cachep);
 	kfree_debugcheck(objp);
 	page = virt_to_page(objp);
 
@@ -1638,21 +1668,18 @@ static inline void *cache_free_debugcheck (kmem_cache_t * cachep, void * objp, v
 	}
 	slabp = GET_PAGE_SLAB(page);
 
-	if (cachep->flags & SLAB_STORE_USER) {
-		objlen -= BYTES_PER_WORD;
-	}
 	if (cachep->flags & SLAB_RED_ZONE) {
-		objp -= BYTES_PER_WORD;
-		if (xchg((unsigned long *)objp, RED_INACTIVE) != RED_ACTIVE)
-			slab_error(cachep, "double free, or memory before"
-						" object was overwritten");
-		if (xchg((unsigned long *)(objp+objlen-BYTES_PER_WORD), RED_INACTIVE) != RED_ACTIVE)
-			slab_error(cachep, "double free, or memory after "
+		if (*dbg_redzone1(cachep, objp) != RED_ACTIVE || *dbg_redzone2(cachep, objp) != RED_ACTIVE) {
+			slab_error(cachep, "double free, or memory outside"
 						" object was overwritten");
+			printk(KERN_ERR "%p: redzone 1: 0x%lx, redzone 2: 0x%lx.\n",
+					objp, *dbg_redzone1(cachep, objp), *dbg_redzone2(cachep, objp));
+		}
+		*dbg_redzone1(cachep, objp) = RED_INACTIVE;
+		*dbg_redzone2(cachep, objp) = RED_INACTIVE;
 	}
-	if (cachep->flags & SLAB_STORE_USER) {
-		*((void**)(objp+objlen)) = caller;
-	}
+	if (cachep->flags & SLAB_STORE_USER)
+		*dbg_userword(cachep, objp) = caller;
 
 	objnr = (objp-slabp->s_mem)/cachep->objsize;
 
@@ -1825,8 +1852,6 @@ cache_alloc_debugcheck_after(kmem_cache_t *cachep,
 			unsigned long flags, void *objp, void *caller)
 {
 #if DEBUG
-	int objlen = cachep->objsize;
-
 	if (!objp)	
 		return objp;
  	if (cachep->flags & SLAB_POISON) {
@@ -1840,24 +1865,20 @@ cache_alloc_debugcheck_after(kmem_cache_t *cachep,
 #endif
 		poison_obj(cachep, objp, POISON_BEFORE);
 	}
-	if (cachep->flags & SLAB_STORE_USER) {
-		objlen -= BYTES_PER_WORD;
-		*((void **)(objp+objlen)) = caller;
-	}
+	if (cachep->flags & SLAB_STORE_USER)
+		*dbg_userword(cachep, objp) = caller;
 
 	if (cachep->flags & SLAB_RED_ZONE) {
-		/* Set alloc red-zone, and check old one. */
-		if (xchg((unsigned long *)objp, RED_ACTIVE) != RED_INACTIVE) {
-			slab_error(cachep, "memory before object was "
-						"overwritten");
-		}
-		if (xchg((unsigned long *)(objp+objlen - BYTES_PER_WORD),
-				       	RED_ACTIVE) != RED_INACTIVE) {
-			slab_error(cachep, "memory after object was "
-						"overwritten");
+		if (*dbg_redzone1(cachep, objp) != RED_INACTIVE || *dbg_redzone2(cachep, objp) != RED_INACTIVE) {
+			slab_error(cachep, "double free, or memory outside"
+						" object was overwritten");
+			printk(KERN_ERR "%p: redzone 1: 0x%lx, redzone 2: 0x%lx.\n",
+					objp, *dbg_redzone1(cachep, objp), *dbg_redzone2(cachep, objp));
 		}
-		objp += BYTES_PER_WORD;
+		*dbg_redzone1(cachep, objp) = RED_ACTIVE;
+		*dbg_redzone2(cachep, objp) = RED_ACTIVE;
 	}
+	objp += obj_dbghead(cachep);
 	if (cachep->ctor && cachep->flags & SLAB_POISON) {
 		unsigned long	ctor_flags = SLAB_CTOR_CONSTRUCTOR;
 
@@ -2175,7 +2196,7 @@ free_percpu(const void *objp)
 
 unsigned int kmem_cache_size(kmem_cache_t *cachep)
 {
-	return cachep->objsize-obj_dbglen(cachep);
+	return obj_reallen(cachep);
 }
 
 kmem_cache_t * kmem_find_general_cachep (size_t size, int gfpflags)
@@ -2763,12 +2784,17 @@ void ptrinfo(unsigned long addr)
 		if (objnr >= c->num) {
 			printk("Bad obj number.\n");
 		} else {
-			kernel_map_pages(virt_to_page(objp), c->objsize/PAGE_SIZE, 1);
+			kernel_map_pages(virt_to_page(objp),
+					c->objsize/PAGE_SIZE, 1);
+
+			if (c->flags & SLAB_RED_ZONE)
+				printk("redzone: 0x%lx/0x%lx.\n",
+					*dbg_redzone1(c, objp),
+					*dbg_redzone2(c, objp));
 
-			printk("redzone: %lxh/%lxh/%lxh.\n",
-				((unsigned long*)objp)[0],
-				((unsigned long*)(objp+c->objsize))[-2],
-				((unsigned long*)(objp+c->objsize))[-1]);
+			if (c->flags & SLAB_STORE_USER)
+				printk("Last user: %p.\n",
+					*dbg_userword(c, objp));
 		}
 		spin_unlock_irqrestore(&c->spinlock, flags);