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

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Date: Thu, 25 Apr 2024 00:51:22 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Kees Cook <keescook@...omium.org>
Cc: Mark Rutland <mark.rutland@....com>, Will Deacon <will@...nel.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>, Jakub Kicinski <kuba@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Arnd Bergmann <arnd@...db.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"David S. Miller" <davem@...emloft.net>,
	David Ahern <dsahern@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Uros Bizjak <ubizjak@...il.com>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
	netdev@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 1/4] locking/atomic/x86: Silence intentional wrapping
 addition

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.

Also, what drugs is involved with __builtin_add_overflow() ? Per
-fno-strict-overflow everything is 2s complement and you can just do the
unsigned add.

Over the years we've been writing code with the express knowledge that
everything wraps properly, this annotation is going to be utter pain.

As I've said before, add an explicit non-wrapping type and use that for
the cases you care about actually not wrapping.

NAK