linux-kernel - Re: [PATCH v5 06/39] x86/fpu: Add helper for modifying xstate

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Message-ID: <Y9qs7iMmk3MYmfhL@zn.tnic>
Date:   Wed, 1 Feb 2023 19:18:22 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH v5 06/39] x86/fpu: Add helper for modifying xstate

On Wed, Feb 01, 2023 at 05:31:50PM +0000, Edgecombe, Rick P wrote:
> On Wed, 2023-02-01 at 12:01 +0100, Borislav Petkov wrote:
> > On Thu, Jan 19, 2023 at 01:22:44PM -0800, Rick Edgecombe wrote:
> > > +void fpregs_lock_and_load(void)
> > > +{
> > > +     /*
> > > +      * fpregs_lock() only disables preemption (mostly). So
> > > modifying state
> > > +      * in an interrupt could screw up some in progress fpregs
> > > operation,
> > > +      * but appear to work. Warn about it.
> > 
> > I don't like comments where it sounds like we don't know what we're
> > doing. "Appear to work"?
> 
> I can change it. This patch started with the observation that modifying
> xstate from the kernel had been gotten wrong a couple times in the
> past, so that is what this is referencing. Since then, the fancy
> automatic solution got boiled down to this helper and a couple
> warnings.

Yeah, but that comment right now reads like: modifying in interrupt
context can corrupt fpregs and you should not do it but it kinda works,
by chance. Thus encouraging people to keep doing that.

I guess "but appear to work" can go and then it is fine.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette