Re: [FFmpeg-devel] [RFC] Channels

[Anton Khirnov <anton@khirnov.net>]

Quoting Michael Niedermayer (2024-03-27 22:54:14)
> On Fri, Mar 22, 2024 at 11:29:31AM +0100, Anton Khirnov wrote:
> > Quoting Michael Niedermayer (2024-03-22 03:25:25)
> [...]
> > > alternative is "wont fix" for all such cases,
> > 
> > IMO it's not, in general, a bug, so EWONTFIX is the appropriate
> > response. If the user does not want us to do arbitrarily large
> > allocation, they have the appropriate OS-level mechanisms (e.g. ulimit,
> > cgroups on Linux) or av_max_alloc().
> 
> You misunderstand the issue.
> 
> the issue is coverage in the fuzzer
> 
> if your 32bit channel number is all allowed then in some decoders
> and demuxers you will in 99.9% of the cases never go beyond the
> channel processing code
> because it will timeout or hit OOM
> 
> your suggestion of ulimits, cgroups and other limits dont help
> We already have both time and space limits in the fuzzers
> 
> Below is simplifying things a bit
> 
> if 99.9% of the random 32bit channel numbers die in the channel
> processing because of the current limit. Then making the limit
> tighter will increase that percentage further.
> 
> If you want better coverage you need a channel limit that stops
> us before a resource intensive channel processing loop
> 
> you can also write down a model of this problem in a more formal way
> Ht as in time spend reading the header
> Ct time spend processing each channel after the header
> Cmax maximum number of channels that will continue execution after the header
> 
> you will see that a Cmax = 2^32 will never be able to do what s Cmax=512
> can do no matter what external limits you apply
> 
> because if you set really high external limits than 99.9% of time will be
> spend in the channel processing code because most of the time the channel
> number will be very large and nothing will stop it so little time will be
> spend for coverage afterwards
> 
> and OTOH if you set a medium outside memory/time limut then most channel
> cases will hit that limit but run the full length of the time limut
> here 99.9% of the cases will timeout and take ALOT of time leaving no
> resources for coverage after the channel code
> 
> and if you set a realls small outside memory/time limit then maybe you
> will quickly stop the channel code but now 99.999% of cases will timeout
> in the channel loop and what remains will not have enough time left to
> even execute all the code after the loop
> 
> So again if you want fuzzer coverage theres need for a channel limit of
> some sort.
> 
> The alternative is to tell everyone that we will not fix this and then
> have bad fuzzer coverage for some cases.

I understand that this is done for fuzzers, I just disagree that we
should introduce arbitrary limits to our code in order to appease them.
They should be tools for our benefit, not vice versa.

-- 
Anton Khirnov
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