linux-kernel - Re: [PATCH v4 next 2/9] lib: mul_u64_u64_div_u64() Combine overflow and divide by zero checks

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Message-ID: <p8126roo-43s3-8266-5n3s-42969sso38s8@onlyvoer.pbz>
Date: Wed, 29 Oct 2025 14:02:10 -0400 (EDT)
From: Nicolas Pitre <npitre@...libre.com>
To: David Laight <david.laight.linux@...il.com>
cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, 
    u.kleine-koenig@...libre.com, Oleg Nesterov <oleg@...hat.com>, 
    Peter Zijlstra <peterz@...radead.org>, 
    Biju Das <biju.das.jz@...renesas.com>, Borislav Petkov <bp@...en8.de>, 
    Dave Hansen <dave.hansen@...ux.intel.com>, 
    "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>, 
    Thomas Gleixner <tglx@...utronix.de>, Li RongQing <lirongqing@...du.com>, 
    Yu Kuai <yukuai3@...wei.com>, Khazhismel Kumykov <khazhy@...omium.org>, 
    Jens Axboe <axboe@...nel.dk>, x86@...nel.org
Subject: Re: [PATCH v4 next 2/9] lib: mul_u64_u64_div_u64() Combine overflow
 and divide by zero checks

On Wed, 29 Oct 2025, David Laight wrote:

> Since the overflow check always triggers when the divisor is zero
> move the check for divide by zero inside the overflow check.
> This means there is only one test in the normal path.
> 
> Signed-off-by: David Laight <david.laight.linux@...il.com>

Reviewed-by: Nicolas Pitre <npitre@...libre.com>

> ---
> 
> V3 contained a different patch 2 that did different chenges to the
> error paths.
> 
>  lib/math/div64.c | 19 +++++++++----------
>  1 file changed, 9 insertions(+), 10 deletions(-)
> 
> diff --git a/lib/math/div64.c b/lib/math/div64.c
> index 0ebff850fd4d..1092f41e878e 100644
> --- a/lib/math/div64.c
> +++ b/lib/math/div64.c
> @@ -212,12 +212,16 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
>  
>  #endif
>  
> -	/* make sure d is not zero, trigger runtime exception otherwise */
> -	if (unlikely(d == 0)) {
> -		unsigned long zero = 0;
> +	if (unlikely(n_hi >= d)) {
> +		/* trigger runtime exception if divisor is zero */
> +		if (d == 0) {
> +			unsigned long zero = 0;
>  
> -		OPTIMIZER_HIDE_VAR(zero);
> -		return ~0UL/zero;
> +			OPTIMIZER_HIDE_VAR(zero);
> +			return ~0UL/zero;
> +		}
> +		/* overflow: result is unrepresentable in a u64 */
> +		return ~0ULL;
>  	}
>  
>  	int shift = __builtin_ctzll(d);
> @@ -234,11 +238,6 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
>  		 */
>  	}
>  
> -	if (n_hi >= d) {
> -		/* overflow: result is unrepresentable in a u64 */
> -		return -1;
> -	}
> -
>  	/* Do the full 128 by 64 bits division */
>  
>  	shift = __builtin_clzll(d);
> -- 
> 2.39.5
> 
>