kasan: improve slab object description

Changes slab object description from:

  Object at ffff880068388540, in cache kmalloc-128 size: 128

to:

  The buggy address belongs to the object at ffff880068388540
   which belongs to the cache kmalloc-128 of size 128
  The buggy address is located 123 bytes inside of
   128-byte region [ffff880068388540, ffff8800683885c0)

Makes it more explanatory and adds information about relative offset of
the accessed address to the start of the object.

Link: http://lkml.kernel.org/r/20170302134851.101218-7-andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Acked-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Alexander Potapenko <glider@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/kasan/report.c

@@ -198,18 +198,49 @@ static struct page *addr_to_page(const void *addr)
 	return NULL;
 }
 
-static void describe_object(struct kmem_cache *cache, void *object)
+static void describe_object_addr(struct kmem_cache *cache, void *object,
+				const void *addr)
+{
+	unsigned long access_addr = (unsigned long)addr;
+	unsigned long object_addr = (unsigned long)object;
+	const char *rel_type;
+	int rel_bytes;
+
+	pr_err("The buggy address belongs to the object at %p\n"
+	       " which belongs to the cache %s of size %d\n",
+		object, cache->name, cache->object_size);
+
+	if (!addr)
+		return;
+
+	if (access_addr < object_addr) {
+		rel_type = "to the left";
+		rel_bytes = object_addr - access_addr;
+	} else if (access_addr >= object_addr + cache->object_size) {
+		rel_type = "to the right";
+		rel_bytes = access_addr - (object_addr + cache->object_size);
+	} else {
+		rel_type = "inside";
+		rel_bytes = access_addr - object_addr;
+	}
+
+	pr_err("The buggy address is located %d bytes %s of\n"
+	       " %d-byte region [%p, %p)\n",
+		rel_bytes, rel_type, cache->object_size, (void *)object_addr,
+		(void *)(object_addr + cache->object_size));
+}
+
+static void describe_object(struct kmem_cache *cache, void *object,
+				const void *addr)
 {
 	struct kasan_alloc_meta *alloc_info = get_alloc_info(cache, object);
 
-	pr_err("Object at %p, in cache %s size: %d\n", object, cache->name,
-		cache->object_size);
+	if (cache->flags & SLAB_KASAN) {
+		print_track(&alloc_info->alloc_track, "Allocated");
+		print_track(&alloc_info->free_track, "Freed");
+	}
 
-	if (!(cache->flags & SLAB_KASAN))
-		return;
-
-	print_track(&alloc_info->alloc_track, "Allocated");
-	print_track(&alloc_info->free_track, "Freed");
+	describe_object_addr(cache, object, addr);
 }
 
 void kasan_report_double_free(struct kmem_cache *cache, void *object,
@@ -221,13 +252,13 @@ void kasan_report_double_free(struct kmem_cache *cache, void *object,
 	pr_err("BUG: Double free or freeing an invalid pointer\n");
 	pr_err("Unexpected shadow byte: 0x%hhX\n", shadow);
 	dump_stack();
-	describe_object(cache, object);
+	describe_object(cache, object, NULL);
 	kasan_end_report(&flags);
 }
 
 static void print_address_description(struct kasan_access_info *info)
 {
-	const void *addr = info->access_addr;
+	void *addr = (void *)info->access_addr;
 	struct page *page = addr_to_page(addr);
 
 	if (page)
@@ -237,9 +268,9 @@ static void print_address_description(struct kasan_access_info *info)
 
 	if (page && PageSlab(page)) {
 		struct kmem_cache *cache = page->slab_cache;
-		void *object = nearest_obj(cache, page,	(void *)addr);
+		void *object = nearest_obj(cache, page,	addr);
 
-		describe_object(cache, object);
+		describe_object(cache, object, addr);
 	}
 
 	if (kernel_or_module_addr(addr)) {