[FFmpeg-devel] [PATCH] avcodec/mjpegdec: Fixes runtime error: signed integer overflow: -24543 * 2031616 cannot be represented in type 'int'
wm4
nfxjfg at googlemail.com
Fri Apr 7 13:35:37 EEST 2017
On Fri, 7 Apr 2017 12:22:07 +0200
Michael Niedermayer <michael at niedermayer.cc> wrote:
> On Fri, Apr 07, 2017 at 08:30:50AM +0200, wm4 wrote:
> > On Fri, 7 Apr 2017 02:17:37 +0200
> > Michael Niedermayer <michael at niedermayer.cc> wrote:
> >
> > > On Wed, Mar 29, 2017 at 11:47:31AM +0200, wm4 wrote:
> > > > On Mon, 27 Mar 2017 00:41:05 +0200
> > > > Michael Niedermayer <michael at niedermayer.cc> wrote:
> > > >
> > > > > On Sun, Mar 26, 2017 at 05:30:05PM -0400, Ronald S. Bultje wrote:
> > > > > > Hi,
> > > > > >
> > > > > > On Sun, Mar 26, 2017 at 3:31 PM, Michael Niedermayer <michael at niedermayer.cc
> > > > > > > wrote:
> > > > > >
> > > > > > > On Sun, Mar 26, 2017 at 07:41:58PM +0200, wm4 wrote:
> > > > > > > > On Sun, 26 Mar 2017 19:16:26 +0200
> > > > > > > > Michael Niedermayer <michael at niedermayer.cc> wrote:
> > > > > > > >
> > > > > > > > > On Sun, Mar 26, 2017 at 06:51:11PM +0200, wm4 wrote:
> > > > > > > > > > On Sun, 26 Mar 2017 18:11:01 +0200
> > > > > > > > > > Michael Niedermayer <michael at niedermayer.cc> wrote:
> > > > > > > > > >
> > > > > > > > > > > Fixes: 943/clusterfuzz-testcase-5114865297391616
> > > > > > > > > > >
> > > > > > > > > > > Found-by: continuous fuzzing process
> > > > > > > https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
> > > > > > > > > > > Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> > > > > > > > > > > ---
> > > > > > > > > > > libavcodec/mjpegdec.c | 3 ++-
> > > > > > > > > > > 1 file changed, 2 insertions(+), 1 deletion(-)
> > > > > > > > > > >
> > > > > > > > > > > diff --git a/libavcodec/mjpegdec.c b/libavcodec/mjpegdec.c
> > > > > > > > > > > index f26e8a3f9a..e08b045fe7 100644
> > > > > > > > > > > --- a/libavcodec/mjpegdec.c
> > > > > > > > > > > +++ b/libavcodec/mjpegdec.c
> > > > > > > > > > > @@ -757,7 +757,8 @@ static int decode_block_progressive(MJpegDecodeContext
> > > > > > > *s, int16_t *block,
> > > > > > > > > > > uint16_t *quant_matrix,
> > > > > > > > > > > int ss, int se, int Al, int
> > > > > > > *EOBRUN)
> > > > > > > > > > > {
> > > > > > > > > > > - int code, i, j, level, val, run;
> > > > > > > > > > > + int code, i, j, val, run;
> > > > > > > > > > > + SUINT level;
> > > > > > > > > > >
> > > > > > > > > > > if (*EOBRUN) {
> > > > > > > > > > > (*EOBRUN)--;
> > > > > > > > > >
> > > > > > > > > > Please make the type either signed or unsigned. Making it both
> > > > > > > > > > (depending on the debug level) just to make the fuzzer happy (or
> > > > > > > > > > something more complicated than that?) isn't a good idea. You
> > > > > > > probably
> > > > > > > > > > want to make it always unsigned?
> > > > > > > > >
> > > > > > > > > No, i want to make it SUINT
> > > > > > > > >
> > > > > > > > > If it is always unsigned then its not possible to detect overflows
> > > > > > > > > without explicitly checking for overflows.
> > > > > > > > > If it is SUINT then ubsan can be used to detect overflows, this is
> > > > > > > > > usefull to test files showing artifacts but no decode errors.
> > > > > > > > >
> > > > > > > >
> > > > > > > > The point of these tools (static analyzers, sanitizers, fuzzers) is to
> > > > > > > > improve the correctness of the code.
> > > > > > >
> > > > > > > > SUINT is still defined to "int" if
> > > > > > > > CHECKED is not defined
> > > > > > >
> > > > > > > no
> > > > > > >
> > > > > > > internal.h:
> > > > > > > #ifdef CHECKED
> > > > > > > #define SUINT int
> > > > > > > #define SUINT32 int32_t
> > > > > > > #else
> > > > > > > #define SUINT unsigned
> > > > > > > #define SUINT32 uint32_t
> > > > > > > #endif
> > > > > > >
> > > > > > > I belive the rest of your mail assumes this condition is backward to
> > > > > > > how it is
> > > > > > >
> > > > > > > SUINT is not there to make any tools happy its there to allow finding
> > > > > > > overflows in debug more while having valid c code in normal builds
> > > > > >
> > > > > >
> > > > >
> > > > > > Why don't we want to detect overflows in debug mode?
> > > > >
> > > > > like wm4 you read "#if A" as "#if not A", all your mail and questions
> > > > > seem based on reading the condition for SUINT flipped around
> > > > >
> > > > > in DEBUG mode CHECKED is enabled, SUINT is int and overflows are
> > > > > undefined behaviour which can be detected easily with ubsan.
> > > > >
> > > > > This allows us to debug samples producing artifacts but no decode
> > > > > errors due to overflows.
> > > > >
> > > > >
> > > > > in normal mode CHECKED is disabled, SUINT is unsigned and overflows
> > > > > are defined behavior. There must be no undefined behavior in releases
> > > > >
> > > > > maybe you belive everyone is using debug mode and the fuzzers run in
> > > > > debug mode. Maybe this is why everyone belives the condition is
> > > > > backward
> > > >
> > > > How would we know this? Maybe I've been assuming too much in order to
> > > > try to make sense out of it.
> > > >
> > > > > I might be wrong but unless you manually pass -DDEBUG you dont use
> > > > > debug mode, adding -DDEBUG is how our debug mode fate client tests that
> > > > > mode
> > > >
> > >
> > > > But why do you want to detect overflows in debug mode, if in release
> > >
> > > To debug.
> > > Like i want compiler warnings to "debug"
> > > or coverity warnings to find and correct bugs. == "debug"
> > >
> > > If a undamaged file triggers an overflow thats very likely a bug and
> > > a easy to fix one if one knows about the overflow.
> > > If one doesnt know of that overflow it can be very hard to find
> >
>
> > Then it's not a bug by definition, and you can ignore it.
>
> overflows have been bugs in the past
> warnings have pointed to bugs in the past
> ubsan has pointed to bugs in the past
What I'm saying is:
Either an overflow is a bug. Then you should not apply hacks like
making all types unsigned just to hide these bugs not show up on
sanitizers or static analyzers. And you shouldn't add another hack just
to make them show up again in debug mode. You could argue that this is
some kind of "hardening" technique (with the second hack still allowing
debugging it), but how about let's not? Just fix overflows when you
find them.
Or the overflow is not a bug. Then the type should always be unsigned,
because overflows would be relatively meaningless. Why make the source
code harder to follow and add noise in debug mode just because of a
vague idea that unknown overflows could occur and which could be bugs?
If overflows are a problem, make the type always signed, and always fix
overflows as you encounter them.
As it looks like, SUINT only hides bugs, and serves the questionable
idea that it's better to hide bugs for the sake of silencing undefined
behavior.
>
> The example is about a overflow bug causing wrong decoder output
> ubsan would find
> that is in the most generic sense, the example would fit any such
> bug and such bugs were found that way in the past.
>
> if you say these are not bugs by definition even though its
> quite obvious they are bugs then whatever you use as definition must
> be flawed. Or you misunderstood what i wrote.
>
> Again the key points are
> * ubsan does and did help finding bugs which otherwise are hard to find
> * using unsigned prevents ubsan from working
> * adding checks and warnings would slow the code down and become messy
> * using signed without checks is undefined behavior which is not ok for
> use of the code as its not valid C
> * using SUINT avoids the undefined behavior, avoids peppering the code
> with checks and warnings and still allows us to use ubsan to detect
> overflows when we need to
>
> Id like to apply the patch unless you or someone else objects.
>
> thanks
>
> [...]
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