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

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Message-ID: <20240424230500.GG12673@noisy.programming.kicks-ass.net>
Date: Thu, 25 Apr 2024 01:05:00 +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 12:54:36AM +0200, Peter Zijlstra wrote:
> 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.

That is, anything that actively warns about signed overflow when build
with -fno-strict-overflow is a bug. If you want this warning you have to
explicitly mark things.

Signed overflow is not UB, is not a bug.

Now, it might be unexpected in some places, but fundamentally we run on
2s complement and expect 2s complement. If you want more, mark it so.