Re: [PATCH 1/4] locking/atomic/x86: Silence intentional wrapping addition

[Peter Zijlstra]

On Thu, Apr 25, 2024 at 12:54:36AM +0200, Peter Zijlstra wrote:
> On Wed, Apr 24, 2024 at 03:45:07PM -0700, Kees Cook wrote:
> > On Thu, Apr 25, 2024 at 12:41:41AM +0200, Peter Zijlstra wrote:
> > > On Wed, Apr 24, 2024 at 12:17:34PM -0700, Kees Cook wrote:
> > > 
> > > > @@ -82,7 +83,7 @@ static __always_inline bool arch_atomic_add_negative(int i, atomic_t *v)
> > > >  
> > > >  static __always_inline int arch_atomic_add_return(int i, atomic_t *v)
> > > >  {
> > > > -	return i + xadd(&v->counter, i);
> > > > +	return wrapping_add(int, i, xadd(&v->counter, i));
> > > >  }
> > > >  #define arch_atomic_add_return arch_atomic_add_return
> > > 
> > > this is going to get old *real* quick :-/
> > > 
> > > This must be the ugliest possible way to annotate all this, and then
> > > litter the kernel with all this... urgh.
> > 
> > I'm expecting to have explicit wrapping type annotations soon[1], but for
> > the atomics, it's kind of a wash on how intrusive the annotations get. I
> > had originally wanted to mark the function (as I did in other cases)
> > rather than using the helper, but Mark preferred it this way. I'm happy
> > to do whatever! :)
> > 
> > -Kees
> > 
> > [1] https://github.com/llvm/llvm-project/pull/86618
> 
> This is arse-about-face. Signed stuff wraps per -fno-strict-overflow.
> We've been writing code for years under that assumption.
> 
> You want to mark the non-wrapping case.

That is, anything that actively warns about signed overflow when build
with -fno-strict-overflow is a bug. If you want this warning you have to
explicitly mark things.

Signed overflow is not UB, is not a bug.

Now, it might be unexpected in some places, but fundamentally we run on
2s complement and expect 2s complement. If you want more, mark it so.