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

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Date: Thu, 9 May 2024 21:06:54 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Linus Torvalds' <torvalds@...ux-foundation.org>, Theodore Ts'o
	<tytso@....edu>
CC: 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-hardening@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"llvm@...ts.linux.dev" <llvm@...ts.linux.dev>
Subject: RE: [RFC] Mitigating unexpected arithmetic overflow

...
> 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).

On a 32bit system the compiler might not do the expected thing.

I had terrible trouble with the 32bit div_u64_u32() code I was
playing with getting the compiler to do 'something sensible' for
that.
I just couldn't get it to stop generating two 64bit values
(in two register pairs) and then oring them together.
I didn't try using a union - that might work.
On x64 the asm "A" (edx:eax) and "a" and "d" constraints will DTRT
but force the values into edx:eax which is ok if you are doing divides.

	David

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