Re: [FFmpeg-devel] [PATCH 1/5] lavu/common.h: Fix UB in av_clipl_int32_c()

From: Michael Niedermayer <michael@niedermayer.cc>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH 1/5] lavu/common.h: Fix UB in av_clipl_int32_c()
Date: Thu, 30 May 2024 21:07:13 +0200
Message-ID: <20240530190713.GB2821752@pb2> (raw)
In-Reply-To: <3421890.a5N6fE6ukZ@basile.remlab.net>

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On Thu, May 30, 2024 at 08:49:12PM +0300, Rémi Denis-Courmont wrote:
> Le torstaina 30. toukokuuta 2024, 19.48.13 EEST Tomas Härdin a écrit :
> > > > Are you saying that UB is acceptable? You know the compiler is free
> > > > to
> > > > assume signed arithmetic doesn't overflow, right? If so then what
> > > > other
> > > > UB might we accept?
> > > 
> > > He did not say that... He said we should switch to a better
> > > implementation rather than trying to fix the existing potentially
> > > buggy one.
> > 
> > I have a fix for demonstrable UB and Rémi is problematizing it.
> 
> Andreas made cosmetic arguments against this patch before I had even seen the 
> patch, forget comment on it.
> 

> > It is not a "theoretical" UB - that's not how UB works.
> 
> It is a *theoretical* UB if you can not prove that it leads to misbehaviour in 

If the function doesnt get called with values triggering UB then its not UB.

If the function gets called with values triggering the signed overflow then its UB
And its a bug unless the applications intended behavior is undefined.

also i would not bet on that the function produces the correct output for
input values that trigger UB on every platform

The case where this really could be a problem is if its used with compile
time constants that would trigger the overflow because in these cases
the optimizer can assume the whole codepath leading to it can be
removed.

IMHO we should simply fix UB instead of arguing over how bad it could be
or when.

thx

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

Those who would give up essential Liberty, to purchase a little
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