linux-kernel - Re: [PATCH 1/2] compiler.h: Introduce ptr

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] [day] [month] [year] [list]

Message-ID: <68dc00b3-1ca1-42bc-8f1e-78ace10e4d64@rowland.harvard.edu>
Date: Wed, 2 Oct 2024 21:50:27 -0400
From: 'Alan Stern' <stern@...land.harvard.edu>
To: David Laight <David.Laight@...lab.com>
Cc: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Will Deacon <will@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Boqun Feng <boqun.feng@...il.com>, John Stultz <jstultz@...gle.com>,
	Neeraj Upadhyay <Neeraj.Upadhyay@....com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Josh Triplett <josh@...htriplett.org>,
	Uladzislau Rezki <urezki@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>, Ingo Molnar <mingo@...hat.com>,
	Waiman Long <longman@...hat.com>,
	Mark Rutland <mark.rutland@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vlastimil Babka <vbabka@...e.cz>,
	"maged.michael@...il.com" <maged.michael@...il.com>,
	Mateusz Guzik <mjguzik@...il.com>, Gary Guo <gary@...yguo.net>,
	"rcu@...r.kernel.org" <rcu@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"lkmm@...ts.linux.dev" <lkmm@...ts.linux.dev>
Subject: Re: [PATCH 1/2] compiler.h: Introduce ptr_eq() to preserve address
 dependency

On Wed, Oct 02, 2024 at 03:24:45PM +0000, David Laight wrote:
> I think I know what you are trying to do, and you just fail.
> Whether something can work is another matter, but that code
> can't ever work.
> 
> Inside if (a == b) the compiler will always use the same register
> for references to a and b - because it knows they have the same value.

According to the other people in this discussion who have actually tried 
using this code, it _does_ work (at least some of the time).

However, I'm not one of those people and so I leave it up to them to 
decide how to respond to this critique.

Alan

> Possibly something like:
> 	c = b;
> 	OPTIMISER_HIDE_VAR(c);
> 	if (a == c) {
> 		*b
> will ensure that there isn't a speculative load from *a.
> You'll get at least one register-register move - but they are safe.
> Otherwise you'll need to put the condition inside an asm block.
> 
> 	David