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Re: [PATCH] x86/atomic: Fix UB on various atomic helpers
- To: "Jan Beulich" <jbeulich@xxxxxxxx>
- From: "Alejandro Vallejo" <alejandro.garciavallejo@xxxxxxx>
- Date: Wed, 14 Oct 2026 17:38:33 +0200
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- Cc: "Andrew Cooper" <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger@xxxxxxxxxxxxxx>, "Teddy Astie" <teddy.astie@xxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Wed, 14 Oct 2026 15:38:49 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Wed Oct 14, 2026 at 4:33 PM CEST, Jan Beulich wrote:
> On 14.10.2026 15:35, Alejandro Vallejo wrote:
> > --- a/xen/arch/x86/include/asm/atomic.h
> > +++ b/xen/arch/x86/include/asm/atomic.h
> > @@ -122,7 +122,7 @@ static inline void atomic_add(int i, atomic_t *v)
> >
> > static inline int atomic_add_return(int i, atomic_t *v)
> > {
> > - return i + arch_fetch_and_add(&v->counter, i);
> > + return (unsigned int)i + arch_fetch_and_add(&v->counter, i);
> > }
>
> I'm concerned seeing this kind of change: It's merely papering over an
> issue in the caller. There's not going to be UB if the caller passes
> suitable values. And the atomic types hardly are meant to be used in
> contexts where overflow could occur.
>
> Furthermore different parts of the C spec say different things here [1],
> [2]. Hence whether overflow in signed calculations is undefined or
> implementation-defined doesn't become entirely clear, according to my
> reading.
I won't dwell on interpreting the standard, because I only care how
GCC and Clang developers interpreted it; and that's a hard trap on
signed overflow with sanitisers enabled.
I can live without this patch (it's just paranoia), but thinking about
it some more, perhaps we ought to pass -fno-strict-overflow, like Linux
does, to the compiler. This brings forward post-C23 overflow semantics
and removes this whole class of UB wholesale. Here and elsewhere.
Thoughts?
Cheers,
Alejandro
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