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

[Michael Niedermayer]

On Sat, Jan 20, 2018 at 02:36:08AM +0100, Carl Eugen Hoyos wrote:
> 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?

yes

thx

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Democracy is the form of government in which you can choose your dictator
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