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Date: Thu, 16 May 2024 06:30:32 -0700
From: Kees Cook <kees@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>,
 Linus Torvalds <torvalds@linux-foundation.org>
CC: Kees Cook <keescook@chromium.org>, Justin Stitt <justinstitt@google.com>,
 Mark Rutland <mark.rutland@arm.com>, linux-hardening@vger.kernel.org,
 linux-kernel@vger.kernel.org, llvm@lists.linux.dev
Subject: Re: [RFC] Mitigating unexpected arithmetic overflow
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On May 15, 2024 12:36:36 AM PDT, Peter Zijlstra <peterz@infradead=2Eorg> w=
rote:
>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=2E Now, sometimes that has actually been actua=
l
>> 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"=2E UBSAN finds them, and that's good=2E
>
>We build with -fno-strict-overflow, which implies -fwrapv, which removes
>the UB from signed overflow by mandating 2s complement=2E

I am a broken record=2E :) This is _not_ about undefined behavior=2E

This is about finding a way to make the intent of C authors unambiguous=2E=
 That overflow wraps is well defined=2E It is not always _desired_=2E C has=
 no way to distinguish between the two cases=2E

-Kees

--=20
Kees Cook