linux-kernel - Re: [PATCH] arch: x86: power: cpu: init %gs before __restore_processor

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Message-ID: <20200918222531.GD7443@duo.ucw.cz>
Date:   Sat, 19 Sep 2020 00:25:31 +0200
From:   Pavel Machek <pavel@...x.de>
To:     Roman Kiryanov <rkir@...gle.com>
Cc:     Borislav Petkov <bp@...en8.de>, rjw@...ysocki.net,
        Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
        x86@...nel.org, linux-pm@...r.kernel.org,
        Greg KH <gregkh@...uxfoundation.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Alistair Delva <adelva@...gle.com>,
        Haitao Shan <hshan@...gle.com>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arch: x86: power: cpu: init %gs before
 __restore_processor_state (clang)

On Tue 2020-09-15 11:36:13, Roman Kiryanov wrote:
> On Tue, Sep 15, 2020 at 11:27 AM Borislav Petkov <bp@...en8.de> wrote:
> > > I believe the kernel makes a questionable assumption on how clang
> > > uses registers (gs will not be used if stack protection is disabled).
> > > Both kernel and clang behaves unfortunate here.
> >
> > If the kernel is at fault here and this same thing happens with GCC,
> > sure, but this is a clang-specific fix.
> 
> This is fair. Unfortunately I am not an x86 asm expert. I expect the proper
> fix should land into arch/x86/kernel/acpi/wakeup_64.S to init %gs
> (maybe some more registers) before "jmp restore_processor_state".

That would certainly be nicer / more acceptable solution than patch
being proposed here.

Code was written with assumption compiler random C code would not use
%gs. If that's no longer true, fixing it in wakeup_64.S _with comments
explaining what goes on_ might be solution.

Best regards,
									Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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