mm, slub, kasan: enable user tracking by default with KASAN=y

It's recommended to have slub's user tracking enabled with CONFIG_KASAN,
because:

a) User tracking disables slab merging which improves
    detecting out-of-bounds accesses.
b) User tracking metadata acts as redzone which also improves
    detecting out-of-bounds accesses.
c) User tracking provides additional information about object.
    This information helps to understand bugs.

Currently it is not enabled by default.  Besides recompiling the kernel
with KASAN and reinstalling it, user also have to change the boot cmdline,
which is not very handy.

Enable slub user tracking by default with KASAN=y, since there is no good
reason to not do this.

[akpm@linux-foundation.org: little fixes, per David]
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 5 insertions, 5 deletions

diff --git a/Documentation/kasan.txt b/Documentation/kasan.txt
index 94c881579374..aa1e0c91e368 100644
--- a/Documentation/kasan.txt
+++ b/Documentation/kasan.txt
@@ -28,8 +28,7 @@ the latter is 1.1 - 2 times faster. Inline instrumentation requires a GCC
 version 5.0 or later.
 
 Currently KASAN works only with the SLUB memory allocator.
-For better bug detection and nicer report, enable CONFIG_STACKTRACE and put
-at least 'slub_debug=U' in the boot cmdline.
+For better bug detection and nicer reporting, enable CONFIG_STACKTRACE.
 
 To disable instrumentation for specific files or directories, add a line
 similar to the following to the respective kernel Makefile:
diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
index 39f24d6721e5..0fee5acd5aa0 100644
--- a/lib/Kconfig.kasan
+++ b/lib/Kconfig.kasan
@@ -15,8 +15,7 @@ config KASAN
           global variables requires gcc 5.0 or later.
           This feature consumes about 1/8 of available memory and brings about
           ~x3 performance slowdown.
-          For better error detection enable CONFIG_STACKTRACE,
-          and add slub_debug=U to boot cmdline.
+          For better error detection enable CONFIG_STACKTRACE.
 
 choice
         prompt "Instrumentation type"
diff --git a/mm/slub.c b/mm/slub.c
index 423dbe77d145..75a5fa92ac2a 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -459,8 +459,10 @@ static void get_map(struct kmem_cache *s, struct page *page, unsigned long *map)
 /*
  * Debug settings:
  */
-#ifdef CONFIG_SLUB_DEBUG_ON
+#if defined(CONFIG_SLUB_DEBUG_ON)
 static int slub_debug = DEBUG_DEFAULT_FLAGS;
+#elif defined(CONFIG_KASAN)
+static int slub_debug = SLAB_STORE_USER;
 #else
 static int slub_debug;
 #endif