linux-kernel - Re: [PATCH] x86/math64: handle #DE in mul_u64_u64_div

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20250722175807.GC2845@redhat.com>
Date: Tue, 22 Jul 2025 19:58:08 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: David Laight <david.laight.linux@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"Li,Rongqing" <lirongqing@...du.com>,
	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	x86@...nel.org
Subject: Re: [PATCH] x86/math64: handle #DE in mul_u64_u64_div_u64()

On 07/22, H. Peter Anvin wrote:
>
> On July 22, 2025 3:50:35 AM PDT, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> >The generic implementation doesn't WARN... OK, I won't argue.
> >How about
> >
> >	static inline u64 mul_u64_u64_div_u64(u64 a, u64 mul, u64 div)
> >	{
> >		char ok = 0;
> >		u64 q;
> >
> >		asm ("mulq %3; 1: divq %4; movb $1,%1; 2:\n"
> >			_ASM_EXTABLE(1b, 2b)
> >			: "=a" (q), "+r" (ok)
> >			: "a" (a), "rm" (mul), "rm" (div)
> >			: "rdx");
> >
> >		if (ok)
> >			return q;
> >		BUG_ON(!div);
> >		WARN_ON_ONCE(1);
> >		return ~(u64)0;
> >	}
> >
> >?
> >
> >Oleg.
>
> Maybe the generic version *should* warn?

David is going to change the generic version to WARN.

> As far as the ok output, the Right Way™ to do it is with an asm goto instead
> of a status variable; the second best tends to be to use the flags output.

This is what I was going to do initially. But this needs
CONFIG_CC_HAS_ASM_GOTO_OUTPUT

Oleg.