Re: [PATCH v8 2/3] mm, kasan: align free_meta_offset on sizeof(void*)

[Alexander Potapenko]

On Mon, Aug 1, 2016 at 4:55 PM, Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote:
>
>
> On 07/28/2016 06:31 PM, Alexander Potapenko wrote:
>> When free_meta_offset is not zero, it is usually aligned on 4 bytes,
>> because the size of preceding kasan_alloc_meta is aligned on 4 bytes.
>> As a result, accesses to kasan_free_meta fields may be misaligned.
>>
>> Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
>> ---
>>  mm/kasan/kasan.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
>> index 6845f92..0379551 100644
>> --- a/mm/kasan/kasan.c
>> +++ b/mm/kasan/kasan.c
>> @@ -390,7 +390,8 @@ void kasan_cache_create(struct kmem_cache *cache, size_t *size,
>>       /* Add free meta. */
>>       if (cache->flags & SLAB_DESTROY_BY_RCU || cache->ctor ||
>>           cache->object_size < sizeof(struct kasan_free_meta)) {
>> -             cache->kasan_info.free_meta_offset = *size;
>> +             cache->kasan_info.free_meta_offset =
>> +                     ALIGN(*size, sizeof(void *));
>
> This cannot work.
Well, it does, at least on my tests.
> I slightly changed metadata layout in http://lkml.kernel.org/g/<1470062715-14077-5-git-send-email-aryabinin@xxxxxxxxxxxxx>
> which should also fix UBSAN's complain.
Thanks, will take a look.
>>               *size += sizeof(struct kasan_free_meta);
>>       }
>>       redzone_adjust = optimal_redzone(cache->object_size) -
>>



-- 
Alexander Potapenko
Software Engineer

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