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

[David Laight]

On Mon, 16 Jun 2025 15:22:44 -0400 (EDT)
Nicolas Pitre <npitre@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> ---
> 
> Change from v1 (http://lore.kernel.org/all/q2o7r916-5601-11pn-30pn-8n5ns6p079o7@xxxxxxxxxxxx):
> - 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);
>