linux-kernel - Re: [PATCH v2] mul_u64_u64_div_u64: fix the division-by-zero behavior

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20250616222629.620830f6@pumpkin>
Date: Mon, 16 Jun 2025 22:26:29 +0100
From: David Laight <david.laight.linux@...il.com>
To: Nicolas Pitre <npitre@...libre.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Thomas Weißschuh <linux@...ssschuh.net>, Oleg Nesterov
 <oleg@...hat.com>, Uwe Kleine-König
 <u.kleine-koenig@...libre.com>, Peter Zijlstra <peterz@...radead.org>, Biju
 Das <biju.das.jz@...renesas.com>, linux-kernel@...r.kernel.org, Linus
 Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2] mul_u64_u64_div_u64: fix the division-by-zero
 behavior

On Mon, 16 Jun 2025 15:22:44 -0400 (EDT)
Nicolas Pitre <npitre@...libre.com> wrote:

> The current implementation forces a compile-time 1/0 division, which
> generates an undefined instruction (ud2 on x86) rather than a proper
> runtime division-by-zero exception.
> 
> Change to trigger an actual div-by-0 exception at runtime, consistent
> with other division operations. Use a non-1 dividend to prevent the
> compiler from optimizing the division into a comparison.

Given there is a definite 'plan' to avoid adding more BUG() to code
I'm not at all sure generating UB here is right at all.

I don't know the best solution though.
To my mind returning zero for divide by zero and ~0 for overflow is least
likely to cause real grief later on in the called code.

	David

> 
> Signed-off-by: Nicolas Pitre <npitre@...libre.com>
> ---
> 
> Change from v1 (http://lore.kernel.org/all/q2o7r916-5601-11pn-30pn-8n5ns6p079o7@onlyvoer.pbz):
> - use OPTIMIZER_HIDE_VAR() in place of the open coded incantation.
> 
> diff --git a/lib/math/div64.c b/lib/math/div64.c
> index 5faa29208bdb..bf77b9843175 100644
> --- a/lib/math/div64.c
> +++ b/lib/math/div64.c
> @@ -212,12 +212,13 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 c)
>  
>  #endif
>  
> -	/* make sure c is not zero, trigger exception otherwise */
> -#pragma GCC diagnostic push
> -#pragma GCC diagnostic ignored "-Wdiv-by-zero"
> -	if (unlikely(c == 0))
> -		return 1/0;
> -#pragma GCC diagnostic pop
> +	/* make sure c is not zero, trigger runtime exception otherwise */
> +	if (unlikely(c == 0)) {
> +		unsigned long zero = 0;
> +
> +		OPTIMIZER_HIDE_VAR(zero);
> +		return ~0UL/zero;
> +	}
>  
>  	int shift = __builtin_ctzll(c);
>