Re: [PATCH 10/82] locking/atomic/x86: Silence intentional wrapping addition

[Kees Cook]

On Tue, Jan 23, 2024 at 09:27:26AM +0000, Mark Rutland wrote:
> On Mon, Jan 22, 2024 at 04:26:45PM -0800, Kees Cook wrote:
> > Annotate atomic_add_return() to avoid signed overflow instrumentation.
> > It is expected to wrap around.
> > 
> > Cc: Will Deacon <will@xxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> > Cc: Mark Rutland <mark.rutland@xxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxxxx>
> > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Cc: x86@xxxxxxxxxx
> > Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > ---
> >  arch/x86/include/asm/atomic.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/include/asm/atomic.h b/arch/x86/include/asm/atomic.h
> > index 55a55ec04350..4120cdd87da8 100644
> > --- a/arch/x86/include/asm/atomic.h
> > +++ b/arch/x86/include/asm/atomic.h
> > @@ -80,7 +80,7 @@ static __always_inline bool arch_atomic_add_negative(int i, atomic_t *v)
> >  }
> >  #define arch_atomic_add_negative arch_atomic_add_negative
> >  
> > -static __always_inline int arch_atomic_add_return(int i, atomic_t *v)
> > +static __always_inline __signed_wrap int arch_atomic_add_return(int i, atomic_t *v)
> >  {
> >  	return i + xadd(&v->counter, i);
> >  }
> 
> I think that here (and in the arm64 patch) it'd be better to use add_wrap() on
> the specific statement, i.e. have:
> 
> static __always_inline int arch_atomic_add_return(int i, atomic_t *v)
> {
> 	return add_wrap(i, xadd(&v->counter, i));
> }
> 
> ... since otherwise the annotation could applly to the '+' or something else
> (e.g. if the 'xadd() part is a special macro), and the annotation might
> unexpectedly hide things if we add other statements here in future.

Okay, sure, I can do that. I may have some header inclusion problems,
but I'll give it a shot.

-- 
Kees Cook