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

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Date: Tue, 23 Jan 2024 13:54:31 -0800
From: Kees Cook <keescook@...omium.org>
To: Mark Rutland <mark.rutland@....com>
Cc: linux-hardening@...r.kernel.org, Will Deacon <will@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/82] locking/atomic/x86: Silence intentional wrapping
 addition

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@...nel.org>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Boqun Feng <boqun.feng@...il.com>
> > Cc: Mark Rutland <mark.rutland@....com>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Ingo Molnar <mingo@...hat.com>
> > Cc: Borislav Petkov <bp@...en8.de>
> > Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> > Cc: x86@...nel.org
> > Cc: "H. Peter Anvin" <hpa@...or.com>
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > ---
> >  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