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

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Date: Wed, 15 May 2024 10:12:20 -0700
From: Justin Stitt <justinstitt@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Kees Cook <keescook@...omium.org>, 
	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

Hi Peter,

On Wed, May 15, 2024 at 12:36 AM 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.

FWIW,

Clang-19 allows -fwrapv and -fsanitize=signed-integer-overflow to work
together [1]

And the sanitizer was re-introduced with Commit 557f8c582a9ba8ab
("ubsan: Reintroduce signed overflow sanitizer").

>
> 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.

[1]: https://clang.llvm.org/docs/ReleaseNotes.html#sanitizers

Thanks
Justin