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

[Ganesh Ajjanagadde]

On Sun, Sep 27, 2015 at 9:39 PM, Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
> 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.

See e.g your commit: 09ef98f1ae3c8a4e08b66f41c3bd97dd7b07405f -
Wstrict-overflow is indeed useful.
I am thus convinced that we should retain it.
Given the fact that local suppression is not worth it for just 2
instances and also that the patch does not reduce readability in any
way, I think this patch and the one for xface are ok.

>
>>
>> [...]
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