[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 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 ?

>
> Ronald
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