linux-hardening - Re: [RFC] Mitigating unexpected arithmetic overflow

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Date: Wed, 15 May 2024 09:36:36 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Kees Cook <keescook@...omium.org>,
	Justin Stitt <justinstitt@...gle.com>,
	Mark Rutland <mark.rutland@....com>,
	linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev
Subject: Re: [RFC] Mitigating unexpected arithmetic overflow

On Wed, May 08, 2024 at 04:47:25PM -0700, Linus Torvalds wrote:
> For example, the most common case of overflow we've ever had has very
> much been array indexing. Now, sometimes that has actually been actual
> undefined behavior, because it's been overflow in signed variables,
> and those are "easy" to find in the sense that you just say "no, can't
> do that". UBSAN finds them, and that's good.

We build with -fno-strict-overflow, which implies -fwrapv, which removes
the UB from signed overflow by mandating 2s complement.

With the exception of an UBSAN bug prior to GCC-8, UBSAN will not, and
should not, warn about signed overflow when using either of these flags.