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

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Date: Wed, 8 May 2024 17:06:07 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Kees Cook <keescook@...omium.org>
Cc: Justin Stitt <justinstitt@...gle.com>, Peter Zijlstra <peterz@...radead.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

On Wed, 8 May 2024 at 16:47, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> But again, maybe it would be worth looking into, at least for some
> limited cases. To go back to your particular example, it might be
> worth trying to limit it for unusual type sizes like implicit casts to
> 'u16' or bitfields: not because those type sizes are magical, but
> because it might be a way to limit the pain.

I think it would be interesting in general to have some kind of
warning for "implicit cast drops bits".

I fear that we'd have an enormous about of them, and maybe they'd be
unsolvable without making the code *much* uglier (and sometimes the
fix might be to add an explicit cast to document intentionally dropped
bits, but explicit casts have their own issues).

So that's why I feel like it might be interesting to see if limiting
such checks to only "unusual" types might give us a manageable list of
worrisome places.

I think it *would* have flagged the perf u16 overflow case. Again: I
don't think that was actually ever overflow or wrap-around in the
arithmetic sense, and it was purely a "assignment to smaller field
truncated bits" issue.

Which again is most definitely not undefined behavior, but I would not
be at all surprised if it was a much more common issue than actual
arithmetic overflow is.

               Linus