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

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Date:   Thu, 16 Nov 2017 18:08:52 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Alexander Potapenko <glider@...gle.com>, dvyukov@...gle.com,
        akpm@...ux-foundation.org
Cc:     kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] lib/stackdepot: use a non-instrumented version of
 memcmp()



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@...gle.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.

> +	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;
>  }
>