[FFmpeg-devel] [PATCH 05/11] avformat/nutenc: Don't pass NULL to memcmp

[Andreas Rheinhardt]

Carl Eugen Hoyos:
> Am Fr., 20. Sept. 2019 um 00:30 Uhr schrieb Andreas Rheinhardt
> <andreas.rheinhardt at gmail.com>:
>>
>> Fixes lots of FATE tests, e.g. lavf-nut, as well as the nut part of
>> ticket #7980.
>>
>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
>> ---
>> This patch is made to match the previous behaviour; whether the previous
>> behaviour is correct at all if the header length is zero is unknown to
>> me.
>>
>>  libavformat/nutenc.c | 5 +++--
>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/libavformat/nutenc.c b/libavformat/nutenc.c
>> index e9a3bb49db..dc714eb809 100644
>> --- a/libavformat/nutenc.c
>> +++ b/libavformat/nutenc.c
>> @@ -791,8 +791,9 @@ static int get_needed_flags(NUTContext *nut, StreamContext *nus, FrameCode *fc,
>>          flags |= FLAG_CHECKSUM;
>>      if (pkt->size < nut->header_len[fc->header_idx] ||
>>          (pkt->size > 4096 && fc->header_idx)        ||
>> -        memcmp(pkt->data, nut->header[fc->header_idx],
>> -               nut->header_len[fc->header_idx]))
>> +        (nut->header_len[fc->header_idx] > 0 &&
>> +         memcmp(pkt->data, nut->header[fc->header_idx],
>> +                nut->header_len[fc->header_idx])))
>>          flags |= FLAG_HEADER_IDX;
> 
> Is this different from the patch I sent?
> (Why does it look different)
> https://patchwork.ffmpeg.org/patch/13782/
> 
> Carl Eugen

Yes.
a) Your patch checks whether nut->header[fc->header_idx] is NULL.
b) Michael's proposal is to check for fc->header_idx instead. This
would work, because it is the index of the only used pointer in
nut->header that is NULL, and this proposal has the advantage of one
dereferencing less than a).
c) My* patch checks whether nut->header_len[fc->header_idx] is zero.
Given that nut->header_len[fc->header_idx] is already used in the very
first check, my solution should not entail a dereference at all.

Furthermore, b) is very specific to the nutenc situation here and a)
might hide a situation where nut->header_len[fc->header_idx] is > 0,
but nut->header[fc->header_idx] == NULL, which should of course not
happen. And it singles out one of the two pointer arguments.

- Andreas

*: It is also what Reimar proposed (I was completely unaware of the
earlier patch when I submitted mine).