linux-kernel - Re: [PATCH v4 1/7] x86/fsgsbase/64: Introduce FS/GS base helper functions

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Date:   Fri, 22 Jun 2018 07:50:34 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        Andrew Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Metzger, Markus T" <markus.t.metzger@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 1/7] x86/fsgsbase/64: Introduce FS/GS base helper functions

On Fri, Jun 22, 2018 at 7:28 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Wed, 20 Jun 2018, Chang S. Bae wrote:
> > +void write_fsbase(unsigned long fsbase)
> > +{
> > +     /* set the selector to 0 to not confuse __switch_to */
>
> 'to not confuse __switch_to' is not that helpful of a comment as it
> requires to stare into __switch_to to figure out what might get confused
> there. Please write it out why this needs to be done in technical terms.
>
> > +     loadseg(FS, 0);
> > +     wrmsrl(MSR_FS_BASE, fsbase);
> > +}
> > +
> > +void write_inactive_gsbase(unsigned long gsbase)
> > +{
> > +     /* set the selector to 0 to not confuse __switch_to */
>
> Ditto
>
> > +     loadseg(GS, 0);
> > +     wrmsrl(MSR_KERNEL_GS_BASE, gsbase);
> > +}
> > +
> > +unsigned long read_task_fsbase(struct task_struct *task)
> > +{
> > +     unsigned long fsbase;
> > +
> > +     if (task == current) {
> > +             fsbase = read_fsbase();
> > +     } else {
> > +             /*
> > +              * XXX: This will not behave as expected if called
> > +              * if fsindex != 0. This preserves an existing bug
> > +              * that will be fixed.
>
> I'm late to this party, but let me ask the obvious question:
>
>     Why is the existing bug not fixed as the first patch in the series?

IIRC that was how I did it in the old version of this code.  I think
it did it because it was less messy to fix the bug after cleaning up
the code, but I could be remembering wrong.