[FFmpeg-devel] [PATCH] avcodec/get_bits: consider bit_size of 0 an error

[Paul B Mahol]

On 10/28/13, Ronald S. Bultje <rsbultje at gmail.com> wrote:
> Hi,
>
> On Mon, Oct 28, 2013 at 2:17 PM, Paul B Mahol <onemda at gmail.com> wrote:
>
>> On 10/28/13, Ronald S. Bultje <rsbultje at gmail.com> wrote:
>> > Hi,
>> >
>> > On Mon, Oct 28, 2013 at 1:35 PM, Paul B Mahol <onemda at gmail.com> wrote:
>> >
>> >> On 10/28/13, Reimar Doeffinger <Reimar.Doeffinger at gmx.de> wrote:
>> >> > On Mon, Oct 28, 2013 at 05:04:38PM +0000, Paul B Mahol wrote:
>> >> >> On 10/28/13, Reimar Doeffinger <Reimar.Doeffinger at gmx.de> wrote:
>> >> >> > On Mon, Oct 28, 2013 at 04:52:27PM +0000, Paul B Mahol wrote:
>> >> >> >> Signed-off-by: Paul B Mahol <onemda at gmail.com>
>> >> >> >
>> >> >> > Could you add the reason for this change to the commit message?
>> >> >> > In theory being able to use 0 might work as an optimization in
>> >> >> > some cases, so I don't think supporting it is as nonsense as it
>> >> >> > might seem.
>> >> >>
>> >> >> What kind of optimization?
>> >> >
>> >> > Purely theoretical: You have a value A, depending on that following
>> >> > value B is either not encoded, encoded with 2, 5 or 8 bits.
>> >> > If get_bits supports 0 bits then you can just do something like
>> >> > get_bits(... (int){0, 2, 5, 8}[A]);
>> >> > if 0 is not allowed, you have to explicitly skip the get_bits for
>> >> > A == 0.
>> >> >
>> >> >> If return value is never checked and bit_size is negative number
>> >> >> next
>> >> >> get_bits will happily crash.
>> >> >
>> >> > But negative number is already checked, your patch is about checking
>> >> > 0?!
>> >>
>> >> Yes, because here is what happens for 0 case:
>> >>
>> >> buffer is not set to NULL (for negative case this causes crash)
>> >>
>> >> get_bits may return 1 even if bit count is 0, and it will happily
>> >> return it forever (until  whatever buffer points to is changed to 0)
>> >>
>> >> so something like
>> >>
>> >> while(get_bits1(gb)) {
>> >> }
>> >>
>> >> loops forever.
>> >>
>> >
>> > That is a bug in the caller.
>>
>> Are you saying I should use get_bits_left ?
>
>
> In this particular case, no; If you buffer is padded with
> FF_INPUT_BUFFER_PADDING_SIZE zero bytes, at EOF, get_bits should always
> return 0.

For sample from bug #3089 it does not return always 0, as bit size is set to 0,
thus checked bit reader never reach padded bytes.

>
> More generally, yes, because I don't see why we wouldn't support inverted
> VLC schemes (while(!get_bits(gb)) { .. }), and to support that, you need
> some explicit exit clause (think e.g. golomb codes).
>
> Ronald
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