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

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Message-ID: <f256c2d1-6f35-447d-9f21-30d0c4a2419a@t-8ch.de>
Date: Mon, 16 Jun 2025 20:58:29 +0200
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Nicolas Pitre <npitre@...libre.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	David Laight <david.laight.linux@...il.com>, 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
Subject: Re: [PATCH] mul_u64_u64_div_u64: fix the division-by-zero behavior

On 2025-06-16 14:48:44-0400, Nicolas Pitre 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.
> 
> Signed-off-by: Nicolas Pitre <npitre@...libre.com>
> 
> diff --git a/lib/math/div64.c b/lib/math/div64.c
> index 5faa29208bdb..eaa0c7e8b974 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;
> +
> +		asm ("" : "+r" (zero)); /* hide actual value from the compiler */

This is OPTIMIZER_HIDE_VAR().

> +		return ~0UL/zero;
> +	}
>  
>  	int shift = __builtin_ctzll(c);
>