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

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Message-ID: <8s7595q6-208q-r756-qsrs-4ononrp3n7o0@onlyvoer.pbz>
Date: Mon, 16 Jun 2025 15:02:47 -0400 (EDT)
From: Nicolas Pitre <npitre@...libre.com>
To: Thomas Weißschuh <linux@...ssschuh.net>
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 Mon, 16 Jun 2025, Thomas Weißschuh wrote:

> 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().

Excellent! I thought I'd seen that somewhere but couldn't remember it.


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