Re: [FFmpeg-devel] [PATCH 2/3] avformat/evcdec: Check that enough data has been read

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

James Almer:
> On 7/6/2023 6:08 PM, Andreas Rheinhardt wrote:
>> Fixes potential use of uninitialized values
>> in evc_read_nal_unit_length().
>>
>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
>> ---
>>   libavformat/evcdec.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/libavformat/evcdec.c b/libavformat/evcdec.c
>> index 9886542311..0f464930f7 100644
>> --- a/libavformat/evcdec.c
>> +++ b/libavformat/evcdec.c
>> @@ -162,6 +162,8 @@ static int evc_read_packet(AVFormatContext *s,
>> AVPacket *pkt)
>>           ret = avio_read(s->pb, buf, EVC_NALU_LENGTH_PREFIX_SIZE);
>>           if (ret < 0)
>>               return ret;
>> +        if (ret != EVC_NALU_LENGTH_PREFIX_SIZE)
>> +            return AVERROR_INVALIDDATA;
> 
> There's a ffio_ensure_seekback() for EVC_NALU_LENGTH_PREFIX_SIZE bytes
> immediately before the avio_read() call. Shouldn't that be enough to
> guarantee that much can be read?
> 

ffio_ensure_seekback() ensures that the read buffer is big enough so
that reading EVC_NALU_LENGTH_PREFIX_SIZE bytes does not lead to a reset
of the buffer; it does not imply that the buffer already contains
EVC_NALU_LENGTH_PREFIX_SIZE bytes. In fact, there is not a single
codepath in ffio_ensure_seekback() that actually reads further input.
(If EVC_NALU_LENGTH_PREFIX_SIZE bytes are not available in the buffer,
then the buf_size <= s->buffer_size codepath will likely be taken in the
non-seekable case (in the seekable case, ffio_ensure_seekback() does
even less, namely nothing).)

> Also, you can just pass ret to evc_read_nal_unit_length() below instead
> of adding this check here. It will return an error if it's <
> EVC_NALU_LENGTH_PREFIX_SIZE.
> 

It will actually return 0 which the caller will transform into an error.
I do not want to rely on this behaviour.
(Why did you add two inline functions of the same name in different evc
headers?)

>>             nalu_size = evc_read_nal_unit_length(buf,
>> EVC_NALU_LENGTH_PREFIX_SIZE);
>>           if (!nalu_size || nalu_size > INT_MAX)

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