linux-kernel - Re: [PATCH v2 next 2/4] lib: mul_u64_u64_div_u64() Use BUG

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Message-ID: <4qprn4oq-qo64-r3n9-9rs7-345o552qo3s5@onlyvoer.pbz>
Date: Tue, 20 May 2025 18:28:08 -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>
Subject: Re: [PATCH v2 next 2/4] lib: mul_u64_u64_div_u64() Use BUG_ON() for
 divide by zero

On Tue, 20 May 2025, David Laight wrote:

> On Mon, 19 May 2025 22:21:15 -0400 (EDT)
> Nicolas Pitre <npitre@...libre.com> wrote:
> 
> > On Sun, 18 May 2025, David Laight wrote:
> > 
> > > Do an explicit BUG_ON(!divisor) instead of hoping the 'undefined
> > > behaviour' the compiler generated for a compile-time 1/0 is in any
> > > way useful.
> > > 
> > > It may be better to define the function to return ~(u64)0 for
> > > divide by zero.
> > > 
> > > Signed-off-by: David Laight <david.laight.linux@...il.com>
> > > ---
> > > 
> > > A new change for v2 of the patchset.
> > > Whereas gcc inserts (IIRC) 'ud2' clang is likely to let the code
> > > continue and generate 'random' results for any 'undefined bahaviour'.  
> > 
> > clang does exactly the same as gcc.
> 
> Did you see the recent 'rant' from Linus about the way clang handles UB.
> I'm pretty sure 'divide by zero' is UB, valid options include.
> - Jump to random location in the current function (what Linus was ranting).
> - Jump to any random location with any register values.
> - Enable user access to all kernel memory.
> - Permanently damage your cpu.
> - Make your computer catch fire.
> - Send an ICBM to some unspecified location.

LOL

> If you want a 'divide by zero' error you better generate one. eg:
> 	int n = 0;
> 	OPTIMSER_HIDE_VAR(n);
> 	i = 1 / n;

As you say then. As long as the resulting behavior is coherent for all 
division flavors on a given architecture.


Nicolas