[FFmpeg-devel] [PATCH]lavf/swfdec: Reduce score when auto-detecting swf files

[Carl Eugen Hoyos]

2018-01-19 23:42 GMT+01:00 Michael Niedermayer <michael at niedermayer.cc>:
> On Fri, Jan 19, 2018 at 07:25:43PM +0100, Carl Eugen Hoyos wrote:
>> 2018-01-19 18:51 GMT+01:00 Michael Niedermayer <michael at niedermayer.cc>:
>> > On Thu, Jan 18, 2018 at 09:28:40PM +0100, Carl Eugen Hoyos wrote:
>> >> Hi!
>> >>
>> >> The probe score for swf files (with uncompressed headers) is currently
>> >> very high after testing a little more than 24bit, attached patch
>> >> reduces the score.
>> >
>> > hmm
>> > the first 24 bits are tested and all but 2 values are rejected
>> >
>> > thats 23 bits that must match
>>
>> Ok.
>>
>> > then 4 values are tested which can be from 1 to 31 bits each
>> > the tests are each either == 0 or != 0 so they are a bit weak
>> > but its at least adding 4 bits that must match and also len
>>
>> I was sure this is not equivalent to four bits on a specific
>> position (but for the patch I assumed a "best case scenario"
>> where this actually were the case).
>>
>> > itself has to be smallish so we could argue that this gets us to
>> > about 28 bits
>>
>> > and then buf[3] is checked to be < 20
>> > at this point we need about 32bits to be matching, still not huge but
>> > id think this is stronger than what file extensions prove which are
>> > MAX/2
>>
>> We return MAX/2 for many "initial 32 bits" tests and I believe
>> we made good experience (and for MAX/2 the extension is
>> ignored or do I misremember?)
>>
>
>> > The test is weakened by using 0 / not 0 / ascii as values though
>> >
>> > Have you seen an actual probe failure ?
>>
>> No, I was looking at another issue.
>
> we have many files with wrong extensions, its not uncommon

Yes, I misremembered how extensions are rated, I actually
wanted to set the return value to "AVPROBE_SCORE_EXTENSION + 1".
Would that be ok?

Carl Eugen