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

[Michael Niedermayer]

On Tue, Feb 03, 2015 at 09:44:13PM +0100, Reimar Döffinger wrote:
> 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

applied


> 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...

i would suggest:

@@ -57,7 +57,7 @@ typedef struct {

 static inline int64_t bs_get_v(const uint8_t **bs)
 {
-    int64_t v = 0;
+    uint64_t v = 0;
     int br = 0;
     int c;


thanks

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Let us carefully observe those good qualities wherein our enemies excel us
and endeavor to excel them, by avoiding what is faulty, and imitating what
is excellent in them. -- Plutarch
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