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Date: Thu, 9 May 2024 18:54:17 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Theodore Ts'o <tytso@mit.edu>, Kees Cook <keescook@chromium.org>,
	Justin Stitt <justinstitt@google.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Mark Rutland <mark.rutland@arm.com>,
	linux-hardening@vger.kernel.org, linux-kernel@vger.kernel.org,
	llvm@lists.linux.dev
Subject: Re: [RFC] Mitigating unexpected arithmetic overflow
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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.