[FFmpeg-devel] [PATCH 1/2] avformat/mpc8: fix broken pointer math

[Reimar Döffinger]

On Tue, Feb 03, 2015 at 07:04:11PM +0100, wm4 wrote:
> This could overflow and crash at least on 32 bit systems.
> ---
>  libavformat/mpc8.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/libavformat/mpc8.c b/libavformat/mpc8.c
> index a15dc25..d6ca338 100644
> --- a/libavformat/mpc8.c
> +++ b/libavformat/mpc8.c
> @@ -91,7 +91,7 @@ static int mpc8_probe(AVProbeData *p)
>          size = bs_get_v(&bs);
>          if (size < 2)
>              return 0;
> -        if (bs + size - 2 >= bs_end)
> +        if (size >= bs_end - bs + 2)
>              return AVPROBE_SCORE_EXTENSION - 1; // seems to be valid MPC but no header yet

Seems ok to me, but for consistency/ease of checking
I'd suggest changing while (bs < bs_end + 3) to this style
as well.
However there is one more issue:
bs_get_v can overflow.
And as the compiler can assume that signed overflow
will not happen, that case is not certain to be
caught by the size < 2 check, and thus these cases
might escape all checks.
Stupid question: Why do we support size values of whole
64 bits?
Nobody has invented any storage media where one could store that much.
Changing the limit in bs_get_v from 10 to e.g. 6 or 7 should
avoid the issue...