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

[Paul B Mahol]

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.