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Date: Thu, 25 Apr 2024 11:19:15 +0100
From: Mark Rutland <mark.rutland@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Kees Cook <keescook@chromium.org>, Will Deacon <will@kernel.org>,
	Boqun Feng <boqun.feng@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
	"H. Peter Anvin" <hpa@zytor.com>, Jakub Kicinski <kuba@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	"David S. Miller" <davem@davemloft.net>,
	David Ahern <dsahern@kernel.org>,
	Eric Dumazet <edumazet@google.com>, Paolo Abeni <pabeni@redhat.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Uros Bizjak <ubizjak@gmail.com>, linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org,
	netdev@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: Re: [PATCH 1/4] locking/atomic/x86: Silence intentional wrapping
 addition
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On Thu, Apr 25, 2024 at 11:28:12AM +0200, Peter Zijlstra wrote:
> On Wed, Apr 24, 2024 at 04:30:50PM -0700, Kees Cook wrote:
> 
> > > 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.
> > 
> > This is confusing UB with "overflow detection". We're doing the latter.
> 
> Well, all of this is confusing to me because it is not presented
> coherently.
> 
> The traditional 'must not let signed overflow' is because of the UB
> nonsense, which we fixed.
> 
> > > 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.
> > 
> > Regular C never provided us with enough choice in types to be able to
> > select the overflow resolution strategy. :( So we're stuck mixing
> > expectations into our types.
> 
> Traditionally C has explicit wrapping for unsigned and UB on signed. We
> fixed the UB, so now expect wrapping for everything.
> 
> You want to add overflow, so you should make that a special and preserve
> semantics for existing code.
> 
> Also I would very strongly suggest you add an overflow qualifier to the
> type system and please provide sane means of qualifier manipulation --
> stripping qualifiers is painful :/

I agree that having an overflow/nooverflow qualifier that's separate from
signed/unsigned would make more sense than inferring that from signed vs
unsigned.

Mark.