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From: Justin Stitt <justinstitt@google.com>
Date: Wed, 15 May 2024 10:12:20 -0700
Message-ID: <CAFhGd8qKObvqbLJWvkG+Wfuy0gpWiTsSmFrBO-NzC-akB6bhzA@mail.gmail.com>
Subject: Re: [RFC] Mitigating unexpected arithmetic overflow
To: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, Kees Cook <keescook@chromium.org>, 
	Mark Rutland <mark.rutland@arm.com>, linux-hardening@vger.kernel.org, 
	linux-kernel@vger.kernel.org, llvm@lists.linux.dev
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Hi Peter,

On Wed, May 15, 2024 at 12:36=E2=80=AFAM Peter Zijlstra <peterz@infradead.o=
rg> 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=3Dsigned-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