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

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Date: Thu, 9 May 2024 18:54:17 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Theodore Ts'o <tytso@....edu>, Kees Cook <keescook@...omium.org>,
	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 Thu, May 09, 2024 at 08:38:28AM -0700, Linus Torvalds wrote:

> Going the other way is similar:
> 
>         all_bits = low_bits + ((u64) high_bits << 16) << 16);
> 
> and again, the compiler will recognize this idiom and do the right
> thing (and if 'all_bits' is only 32-bit, the compiler will optimize
> the high bit noise away).

Umm...  That would make sense if it was
	all_bits = low_bits + ((T) high_bits << 16) << 16);
with possibly 32bit T.  But the way you wrote that (with u64) it's
pointless - u64 _can_ be shifted by 32 just fine.