Re: [PATCH][next] ALSA: vmaster: Use flex_array_size() helper in memcpy()

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

On 7/30/20 03:41, Takashi Iwai wrote:
> On Thu, 30 Jul 2020 00:18:29 +0200,
> Gustavo A. R. Silva wrote:
>>
>> Make use of the flex_array_size() helper to calculate the size of a
>> flexible array member within an enclosing structure.
>>
>> This helper offers defense-in-depth against potential integer overflows
>> and makes it explicitly clear that we are dealing with a flexible array
>> member.
>>
>> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
>> ---
>>  sound/core/vmaster.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/sound/core/vmaster.c b/sound/core/vmaster.c
>> index ab36f9898711..21ce4082cb5f 100644
>> --- a/sound/core/vmaster.c
>> +++ b/sound/core/vmaster.c
>> @@ -262,7 +262,8 @@ int _snd_ctl_add_follower(struct snd_kcontrol *master,
>>  		return -ENOMEM;
>>  	srec->kctl = follower;
>>  	srec->follower = *follower;
>> -	memcpy(srec->follower.vd, follower->vd, follower->count * sizeof(*follower->vd));
>> +	memcpy(srec->follower.vd, follower->vd, flex_array_size(srec, follower.vd,
>> +								srec->follower.count));
> 
> Changing from follower->count to srec->follower.count isn't obvious,
> so it should have been mentioned in the log; other than that, the code
> change looks good.
> 

Yeah; you're right. I'll include that in the changelog text.

> But since the API isn't in Linus tree yet, I'll postpone the merge
> until the API reaches to upstream.  Maybe I can merge this during RC1
> merge window, as those are trivial.
> 

The API is already in mainline, see:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/overflow.h#n320

> 
> BTW, looking at those patterns, I wonder whether it'd make sense to
> make the whole memset() call as a macro like
> 
> 	safe_copy_array(srec->follower.vd, follower->vd, follower->count);
> 
> ?
> 

Yep; I was actually thinking about that, yesterday. I might implement it
for the next development cycle. :)

Thanks for the feedback.
--
Gustavo