lkml.org 
[lkml]   [2016]   [Oct]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH] lib: bump stackdepot capacity from 16MB to 128MB
Date
KASAN uses stackdepot to memorize stacks for all kmalloc/kfree calls.
Current stackdepot capacity is 16MB (1024 top level entries x
4 pages on second level). Size of each stack is (num_frames + 3) *
sizeof(long). Which gives us ~84K stacks. This capacity was chosen
empirically and it is enough to run kernel normally. However,
when lots of configs are enabled and a fuzzer tries to maximize
code coverage, it easily hits the limit within tens of minutes.
I've tested for long a time with number of top level entries bumped 4x
(4096). And I think I've seen overflow only once. But I don't have
all configs enabled and code coverage has not reached maximum yet.
So bump it 8x to 8192. Since we have two-level table, memory cost
of this is very moderate -- currently the top-level table is 8KB,
with this patch it is 64KB, which is negligible under KASAN.

Here is some approx math.
128MB allows us to memorize ~670K stacks (assuming stack is ~200b).
I've grepped kernel for kmalloc|kfree|kmem_cache_alloc|kmem_cache_free|
kzalloc|kstrdup|kstrndup|kmemdup and it gives ~60K matches.
Most of alloc/free call sites are reachable with only one stack.
But some utility functions can have large fanout. Assuming average
fanout is 5x, total number of alloc/free stacks is ~300K.

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Cc: kasan-dev@googlegroups.com
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: sploving1@gmail.com
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
---
lib/stackdepot.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index 60f77f1..4d830e2 100644
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -50,7 +50,7 @@
STACK_ALLOC_ALIGN)
#define STACK_ALLOC_INDEX_BITS (DEPOT_STACK_BITS - \
STACK_ALLOC_NULL_PROTECTION_BITS - STACK_ALLOC_OFFSET_BITS)
-#define STACK_ALLOC_SLABS_CAP 1024
+#define STACK_ALLOC_SLABS_CAP 8192
#define STACK_ALLOC_MAX_SLABS \
(((1LL << (STACK_ALLOC_INDEX_BITS)) < STACK_ALLOC_SLABS_CAP) ? \
(1LL << (STACK_ALLOC_INDEX_BITS)) : STACK_ALLOC_SLABS_CAP)
--
2.8.0.rc3.226.g39d4020
\
 
 \ /
  Last update: 2016-10-14 17:22    [W:0.029 / U:0.084 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site