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

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Message-ID: <25882715-FE44-44C0-BB9B-57F2E7D1F0F9@kernel.org>
Date: Thu, 16 May 2024 06:30:32 -0700
From: Kees Cook <kees@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>,
 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 May 15, 2024 12:36:36 AM PDT, Peter Zijlstra <peterz@...radead.org> wrote:
>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.

I am a broken record. :) This is _not_ about undefined behavior.

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

-Kees

-- 
Kees Cook