Re: [FFmpeg-devel] [PATCH 00/22] Deprecate av_uninit

[Zhao Zhili <quinkblack-at-foxmail.com@ffmpeg.org>]

> On Apr 11, 2025, at 16:36, Nicolas George <george@nsup.org> wrote:
> 
> Zhao Zhili (HE12025-04-11):
>> From: Zhao Zhili <zhilizhao@tencent.com>
>> 
>> The macro is meant to suppress false uninitialized warnings. However,
>> sometimes these 'false uninitialized warnings' are really undefined
>> behavior, and leading to real issue like crash, e.g., ab792634197e.
>> 
>> For false uninitialized warnings, it can be silenced by initialization,
>> and compiler can easily optimize away unnecessary initializations.
>> 
>> av_uninit shouldn't be used in any case.
> 
> NAK, you are hiding the UBs, not fixing the bugs.

By the way, logic bug isn’t equal to UB, so I’m not hiding UB.

Who put av_uninit in the code means there is no logic bug. If there is,
the patchset fixed UB or replaced UB by deterministic logic error, which
can’t be worse.

> 
> If the author of the code put av_uninit, that means they believe the
> value will always have been initialized by the part of the code
> responsible for it. If that is not true, then it is a bug that can lead
> to an exploitable security issue or a silent data corruption.

If there is no bug for code with av_uninit, the patchset does nothing really.
If there is, the patchset fixed or makes the issue deterministic.

We don’t initialized all variables when declaration. But if there is a
sometimes-uninitialized warning, there is some reason for compiler.
Uninitialized warning isn’t the same as deprecated or unused, it should
never be ignored in my opinion.

> 
> With your changes, nothing proves that the = 0 you put there is the
> right value, the bug is still there: the code expects the value to be
> correctly set, but instead there is an arbitrary 0.
> 
> At least, with av_uninit, valgrind and fuzzing can find the bugs.
> 
> Regards,
> 
> -- 
>  Nicolas George
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