linux-kernel - Re: [PATCH 8/9] x86/fpu: correctly check for kthreads

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Message-ID: <20190814133910.GC51963@lakrids.cambridge.arm.com>
Date:   Wed, 14 Aug 2019 14:39:10 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, ak@...ux.intel.com,
        akpm@...ux-foundation.org, bp@...e.de, catalin.marinas@....com,
        davem@...emloft.net, hch@....de, kan.liang@...el.com,
        mingo@...nel.org, peterz@...radead.org, riel@...riel.com,
        will@...nel.org
Subject: Re: [PATCH 8/9] x86/fpu: correctly check for kthreads

On Wed, Aug 14, 2019 at 03:07:08PM +0200, Sebastian Andrzej Siewior wrote:
> On 2019-08-14 11:41:30 [+0100], Mark Rutland wrote:
> > Per commit:
> > 
> >   0cecca9d03c964ab ("x86/fpu: Eager switch PKRU state")
> > 
> > ... switch_fpu_state() is trying to distinguish user threads from
> > kthreads, such that kthreads consistently use init_pkru_value. It does
> > do by looking at current->mm.
> > 
> > In general, a non-NULL current->mm doesn't imply that current is a
> > kthread, as kthreads can install an mm via use_mm(), and so it's
> > preferable to use is_kthread() to determine whether a thread is a
> > kthread.
> 
> I think this was missed in commit.
> 	8d3289f2fa1e0 ("x86/fpu: Don't use current->mm to check for a kthread")

Yup, though if it's a bug it's been a bug since commit:

  0cecca9d03c964ab ("x86/fpu: Eager switch PKRU state")

... which I guess the fixes tag would have to mention.

> 
> A kthread with use_mm() would load here non-existing FPU state.
> 
> Acked-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

Given the above, should I add the fixes tag (for 0cecca9d03c964ab)?

Thanks,
Mark.