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

[Peter Zijlstra]

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.