linux-kernel - Re: [PATCH v3 2/6] x86/uaccess: Avoid barrier_nospec() in 64-bit __get

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Message-ID: <CAHk-=wiL0TepguMNaR65ZdkkiBEoi4hTE7PwG3bBO1c5SOXmWw@mail.gmail.com>
Date: Fri, 22 Nov 2024 11:13:05 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: David Laight <David.Laight@...lab.com>, "x86@...nel.org" <x86@...nel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Borislav Petkov <bp@...en8.de>, Peter Zijlstra <peterz@...radead.org>, 
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, Waiman Long <longman@...hat.com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Ingo Molnar <mingo@...hat.com>, 
	Michael Ellerman <mpe@...erman.id.au>, 
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>, Andrew Cooper <andrew.cooper3@...rix.com>, 
	Mark Rutland <mark.rutland@....com>, "Kirill A . Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH v3 2/6] x86/uaccess: Avoid barrier_nospec() in 64-bit __get_user()

On Thu, 21 Nov 2024 at 19:57, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> Anyway, that was a long way of saying: I really think we should just
> special-case the (few) important cases that get reported. Because any
> *big* improvements will come not from just inlining.

Looking around at the futex code some more, I note:

 - the cmpxchg case and futex ops use an explicit barrier too, which is bad

 - we'd actually be better off inlining not just the user access, but
the whole futex_get_value_locked(), because then the compiler will be
able to do CSE on the user address masking, and only do it once
(several places do multiple different futex_get_value_locked() calls).

iow, I think the fix for the futex case ends up being a patch
something like the attached.

[ Annoyingly, we need "can_do_masked_user_access()" even on x86,
because the 32-bit case doesn't do the address masking trick ]

I've only compiled it so far, about to actually boot into it. Pray for me,

               Linus

View attachment "patch.diff" of type "text/x-patch" (4488 bytes)