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

[Carl Eugen Hoyos]

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.

Carl Eugen