Re: [FFmpeg-devel] [PATCH] avformat/vpk: fix divide by zero

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Kacper Michajlow:
> On Fri, 9 Aug 2024 at 22:51, Michael Niedermayer <michael@niedermayer.cc> wrote:
>>
>> On Wed, Aug 07, 2024 at 03:42:46PM +0200, Kacper Michajłow wrote:
>>> Can happen after calling avformat_find_stream_info() when the codec
>>> fails to open, but return value is 0 and subsequent uses of this context
>>> have zero value in channel number.
>>>
>>> Found by OSS-Fuzz.
>>>
>>> Signed-off-by: Kacper Michajłow <kasper93@gmail.com>
>>> ---
>>>  libavformat/vpk.c | 2 ++
>>>  1 file changed, 2 insertions(+)
>>>
>>> diff --git a/libavformat/vpk.c b/libavformat/vpk.c
>>> index 001ad33555..aa98ef2dd4 100644
>>> --- a/libavformat/vpk.c
>>> +++ b/libavformat/vpk.c
>>> @@ -86,6 +86,8 @@ static int vpk_read_packet(AVFormatContext *s, AVPacket *pkt)
>>>
>>>      vpk->current_block++;
>>>      if (vpk->current_block == vpk->block_count) {
>>> +        if (par->ch_layout.nb_channels <= 0)
>>> +            return AVERROR_INVALIDDATA;
>>>          unsigned size = vpk->last_block_size / par->ch_layout.nb_channels;
>>>          unsigned skip = (par->block_align - vpk->last_block_size) / par->ch_layout.nb_channels;
>>>          uint64_t pos = avio_tell(s->pb);
>>
>> iam not sure if a parser or other should replace a valid set of
>> parameters by an invalid
>> (this patch implies that such a action occured)
>>
>> can you explain more detailedly by what and why channels is set to 0 ?
>>
> 
> You are right, it might be better to improve this to not override the
> params. Let me explain what happens, I didn't read through the whole
> avformat_find_stream_info() to know what would be the best approach
> yet. I will try to look at it, but if you have immediate ideas, that
> would be nice.
> 
> 1. avformat_open_input() sets nb_channels to 108
> 
> 2. Just after that we call avformat_find_stream_info(avfc, NULL); this
> returns 0 (success), but as a result it overrides params already
> present in the context.
> log for reference, during the find stream info call
> [ffmpeg/demuxer] vpk: Before avformat_find_stream_info() pos:
> 538976288 bytes read:21 seeks:1 nb_streams:1
> [ffmpeg/demuxer] vpk: Failed to open codec in avformat_find_stream_info
> [lavf] mp_seek(0x512000018090, 0, size)
> [lavf] 0=mp_read(0x512000018090, 0x7fe4c7ce8800, 50000000), pos:
> 538976288, eof:1
> [lavf] 0=mp_read(0x512000018090, 0x52d00000a400, 32768), pos: 538976288, eof:1
> [ffmpeg/audio] adpcm_psx: Decoder requires channel layout to be set
> [ffmpeg/demuxer] vpk: Failed to open codec in avformat_find_stream_info
> [lavf] mp_seek(0x512000018090, 0, size)
> [lavf] mp_seek(0x512000018090, 0, size)
> [lavf] mp_seek(0x512000018090, 0, size)
> [ffmpeg/demuxer] vpk: stream 0: start_time: NOPTS duration: 0.069852
> [ffmpeg/demuxer] vpk: format: start_time: NOPTS duration: 0.069852
> (estimate from stream) bitrate=2 kb/s
> [ffmpeg/demuxer] vpk: Could not find codec parameters for stream 0
> (Audio: adpcm_psx, 538976288 Hz, 0 channels): unspecified sample
> format
> [ffmpeg/demuxer] Consider increasing the value for the
> 'analyzeduration' (0) and 'probesize' (5000000) options
> [ffmpeg/demuxer] vpk: After avformat_find_stream_info() pos: 538976288
> bytes read:21 seeks:1 frames:0
> 
> 3. the nb_channels value is cleared in avformat_find_stream_info() ->
> avcodec_parameters_from_context() -> codec_parameters_reset() and
> remains 0.

This seems like the error: Why is AVCodecParameters being set from an
AVCodecContext if the codec could not be successfully opened?

> 
> 4. as we can see there were errors, but it still returns success, so we proceed.
> 
> 5. on the next av_seek_frame() which goes to vpk_read_packet() it will
> fail because nb_channels is 0 at this point.
> 
> Sorry for only a high level overview, but at this moment, I'm not sure
> how exactly it is supposed to work. I thought it might be intended to
> override headers parameters later if we know better later on, that's
> why my initial patch only tackled this case.
> 

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