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

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Date: Wed, 8 May 2024 17:23:08 -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:
>
> So *that* I feel could be something where you can warn without a ton
> of compiler smarts at all. If you see an *implicit* cast to unsigned
> and then the subsequent operations wraps around, it's probably worth
> being a lot more worried about.

Side note on this part: quite often, because of C promotion rules, you
have "int" as an "intermediate" type.

IOW, while I had that example of

        int a;
        ...
        a * sizeof(xyz);

being questionably not-UB (because "int a" gets promoted to unsigned
as part of C integer promotion, and thus you really had a signed value
that was involved in unsigned wrap-around), if you have

        unsigned short a;
        ...
        a * sizeof(xyz);

then technically that 'a' is first promoted to 'int' (because all
arithmetic on types smaller than int get promoted to int), and then it
gets promoted to size_t because the multiply gets done in the bigger
type.

So in one sense that unsigned multiply may actually have involved a
cast from a signed type, but at the same time it's not at all in that
kind of "accidentally not UB" class.

I suspect most compilers would have combined the two levels of
implicit casts into just one, so at no point outside of perhaps some
very intermediate stage will it show as a signed int cast to unsigned,
but I thought I'd mention it anyway. Implicit casts get nasty not just
in assignments, but also in these kinds of situations.

I still suspect the "implicit truncating cast at assignment" is likely
a much more common case of loss of information than actual arithmetic
wrap-around, but clearly the two have commonalities.

              Linus