Re: [PATCH] lib/stackdepot: use a non-instrumented version of memcmp()

[Alexander Potapenko <glider@google.com>]

On Thu, Nov 16, 2017 at 4:08 PM, Andrey Ryabinin
<aryabinin@virtuozzo.com> wrote:
>
>
> On 11/15/2017 08:34 PM, Alexander Potapenko wrote:
>> stackdepot used to call memcmp(), which compiler tools normally
>> instrument, therefore every lookup used to unnecessarily call
>> instrumented code.
>> This is somewhat ok in the case of KASAN, but under KMSAN a lot of time
>> was spent in the instrumentation.
>>
>> Signed-off-by: Alexander Potapenko <glider@google.com>
>> ---
>>  lib/stackdepot.c | 21 ++++++++++++++++++---
>>  1 file changed, 18 insertions(+), 3 deletions(-)
>>
>> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
>> index f87d138e9672..d372101e8dc2 100644
>> --- a/lib/stackdepot.c
>> +++ b/lib/stackdepot.c
>> @@ -163,6 +163,23 @@ static inline u32 hash_stack(unsigned long *entries, unsigned int size)
>>                              STACK_HASH_SEED);
>>  }
>>
>> +/* Use our own, non-instrumented version of memcmp().
>> + *
>> + * We actually don't care about the order, just the equality.
>> + */
>> +static inline
>> +int stackdepot_memcmp(const void *s1, const void *s2, unsigned int n)
>> +{
>
> Why 'void *' types? The function treats s1, s2 as array of long, also 'n' is number of longs here.
Agreed. I started with a plain memcpy, so the arg types were left over.
>> +     unsigned long *u1 = (unsigned long *)s1;
>> +     unsigned long *u2 = (unsigned long *)s2;
>> +
>> +     for ( ; n-- ; u1++, u2++) {
>> +             if (*u1 != *u2)
>> +                     return 1;
>> +     }
>> +     return 0;
>> +}
>> +
>>  /* Find a stack that is equal to the one stored in entries in the hash */
>>  static inline struct stack_record *find_stack(struct stack_record *bucket,
>>                                            unsigned long *entries, int size,
>> @@ -173,10 +190,8 @@ static inline struct stack_record *find_stack(struct stack_record *bucket,
>>       for (found = bucket; found; found = found->next) {
>>               if (found->hash == hash &&
>>                   found->size == size &&
>> -                 !memcmp(entries, found->entries,
>> -                         size * sizeof(unsigned long))) {
>> +                 !stackdepot_memcmp(entries, found->entries, size))
>>                       return found;
>> -             }
>>       }
>>       return NULL;
>>  }
>>



-- 
Alexander Potapenko
Software Engineer

Google Germany GmbH
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