linux-kernel - Re: [PATCH v3 next 02/10] lib: mul_u64_u64_div_u64() Use WARN

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Message-ID: <0qs6r27q-qq9s-s676-p80q-s20po6541q16@syhkavp.arg>
Date: Sat, 14 Jun 2025 18:23:18 -0400 (EDT)
From: Nicolas Pitre <nico@...xnic.net>
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>
Subject: Re: [PATCH v3 next 02/10] lib: mul_u64_u64_div_u64() Use WARN_ONCE()
 for divide errors.

On Sat, 14 Jun 2025, David Laight wrote:

> On Sat, 14 Jun 2025 11:17:33 -0400 (EDT)
> Nicolas Pitre <nico@...xnic.net> wrote:
> 
> > On Sat, 14 Jun 2025, David Laight wrote:
> > 
> > > Do an explicit WARN_ONCE(!divisor) instead of hoping the 'undefined
> > > behaviour' the compiler generates for a compile-time 1/0 is in any
> > > way useful.
> > > 
> > > Return 0 (rather than ~(u64)0) because it is less likely to cause
> > > further serious issues.  
> > 
> > I still disagree with this patch. Whether or not what the compiler 
> > produces is useful is beside the point. What's important here is to have 
> > a coherent behavior across all division flavors and what's proposed here 
> > is not.
> > 
> > Arguably, a compile time 1/0 might not be what we want either. The 
> > compiler forces an "illegal instruction" exception when what we want is 
> > a "floating point" exception (strange to have floating point exceptions 
> > for integer divisions but that's what it is).
> > 
> > So I'd suggest the following instead:
> > 
> > ----- >8  
> > From Nicolas Pitre <npitre@...libre.com>
> > Subject: [PATCH] mul_u64_u64_div_u64(): improve division-by-zero handling
> > 
> > Forcing 1/0 at compile time makes the compiler (on x86 at least) to emit 
> > an undefined instruction to trigger the exception. But that's not what 
> > we want. Modify the code so that an actual runtime div-by-0 exception
> > is triggered to be coherent with the behavior of all the other division
> > flavors.
> > 
> > And don't use 1 for the dividend as the compiler would convert the 
> > actual division into a simple compare.
> 
> The alternative would be BUG() or BUG_ON() - but Linus really doesn't
> like those unless there is no alternative.
> 
> I'm pretty sure that both divide overflow (quotient too large) and
> divide by zero are 'Undefined behaviour' in C.
> Unless the compiler detects and does something 'strange' it becomes
> cpu architecture defined.

Exactly. Let each architecture produce what people expect of them when a 
zero divisor is encountered.

> It is actually a right PITA that many cpu trap for overflow
> and/or divide by zero (x86 traps for both, m68k traps for divide by
> zero but sets the overflow flag for overflow (with unchanged outputs),
> can't find my arm book, sparc doesn't have divide).

Some ARMs don't have divide either. But the software lib the compiler is 
relying on in those cases deals with a zero divisor already. We should 
involve the same path here.


Nicolas