[FFmpeg-devel] [PATCH] avformat/movenc: suppress -Wstrict-overflow warnings

[Ganesh Ajjanagadde]

On Sun, Sep 27, 2015 at 9:18 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> On Sun, Sep 27, 2015 at 01:23:03PM -0400, Ganesh Ajjanagadde wrote:
>> On Sun, Sep 27, 2015 at 12:58 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
>> > On Sat, Sep 26, 2015 at 10:55:26PM -0400, Ganesh Ajjanagadde wrote:
>> >> On Sat, Sep 26, 2015 at 10:32 PM, Ronald S. Bultje <rsbultje at gmail.com> wrote:
>> >> > Hi,
>> >> >
>> >> > On Sat, Sep 26, 2015 at 7:19 PM, Ganesh Ajjanagadde <gajjanag at mit.edu>
>> >> > wrote:
>> >> >
>> >> >> On Sat, Sep 26, 2015 at 7:11 PM, Michael Niedermayer <michaelni at gmx.at>
>> >> >> wrote:
>> >> >> > On Fri, Sep 18, 2015 at 05:15:50PM -0400, Ganesh Ajjanagadde wrote:
>> >> >> >> This patch results in identical behavior of movenc, and suppresses
>> >> >> -Wstrict-overflow
>> >> >> >> warnings observed in GCC 5.2.
>> >> >> >> I have manually checked that all usages are safe, and overflow
>> >> >> possibility does
>> >> >> >> not exist with this expression rewrite.
>> >> >> >>
>> >> >> >> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
>> >> >> >> ---
>> >> >> >>  libavformat/movenc.c | 2 +-
>> >> >> >>  1 file changed, 1 insertion(+), 1 deletion(-)
>> >> >> >>
>> >> >> >> diff --git a/libavformat/movenc.c b/libavformat/movenc.c
>> >> >> >> index af03d1e..6e4a1a6 100644
>> >> >> >> --- a/libavformat/movenc.c
>> >> >> >> +++ b/libavformat/movenc.c
>> >> >> >> @@ -854,7 +854,7 @@ static int get_cluster_duration(MOVTrack *track,
>> >> >> int cluster_idx)
>> >> >> >>  {
>> >> >> >>      int64_t next_dts;
>> >> >> >>
>> >> >> >> -    if (cluster_idx >= track->entry)
>> >> >> >> +    if (cluster_idx - track->entry >= 0)
>> >> >> >
>> >> >> > i do not understand what this fixes or why
>> >> >> > also plese quote the actual warnings which are fixed in the commit
>> >> >> > message
>> >> >>
>> >> >> I have posted v2 with a more detailed commit message. It should be
>> >> >> self explanatory.
>> >> >
>> >> >
>> >> > Even with the new message, it's still not clear to me what's being fixed.
>> >> > What does the warning check for? What is the problem in the initial
>> >> > expression?
>> >>
>> >> Compilers make transformations on the statements in order to possibly
>> >> get better performance when compiled with optimizations. However, some
>> >> of these optimizations require assumptions in the code. In particular,
>> >> the compiler is internally rewriting cluster_idx >= track->entry to
>> >> cluster_idx - track->entry >= 0 internally for some reason (I am not
>> >> an asm/instruction set guy, so I can't comment why it likes this).
>> >> However, such a transformation is NOT always safe as integer
>> >> arithmetic can overflow (try e.g extreme values close to INT_MIN,
>> >> INT_MAX). The warning is spit out since the compiler can't be sure
>> >> that this is safe, but it still wants to do it (I suspect only the
>> >> -O3/-O2 level that try this, can check if you want).
>> >
>> > iam not sure i understand correctly but
>> > if the compiler changes the code and then warns that what it just
>> > did might be unsafe then the compiler is broken
>>
>> https://stackoverflow.com/questions/12984861/dont-understand-assuming-signed-overflow-warning
>> - gives a detailed explanation.
>>
>> Some more info: this is triggered only when -finline-functions is
>> enabled (done by default on -O3, not enabled by default on -O2).
>> -finline-functions tries to inline stuff even when "inline" keyword is
>> absent (like in this case).
>> As for the warning, http://linux.die.net/man/1/gcc - search for
>> -Wstrict-overflow. It is enabled due to -Wall, and as the man page
>> suggests, it depends on optimization level as we can see in this
>> example.
>> I do consider the compiler broken in this case, but then again
>> compilers are broken in so many different ways it is not even funny:
>> see e.g -Warray-bounds, can't use the ISO C correct { 0 } initializer
>> for compound data types, etc.
>>
>> If you don't like this, we should add a -Wnostrict-overflow either to
>> configure, or a local enable/disable via pragmas/macros. I don't like
>> either of these as compared to this simple workaround:
>> 1. -Wnostrict-overflow: FFmpeg with the amount of integer arithmetic
>> being done should benefit from this warning in general, so disabling
>> it globally may be bad.
>
> how many actual bugs has Wstrict-overflow found ?

No idea; maybe a good place to check is the Google fuzzing effort
where many bugs were fixed.

>
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