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 17:39:41 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
cc:     "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        "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, 22 Jun 2018, Andy Lutomirski wrote:
> On Fri, Jun 22, 2018 at 7:28 AM Thomas Gleixner <tglx@...utronix.de> wrote:
> > > +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.

Fair enough. Though the general rule is: Fix bugs first and then do
features, unless you really need the extra step to fix it proper.

Now in that case the real question is whether this is a bug or just a
slight incorrectness which has no practical impact. If it's the latter,
then introduce the new function which does the right thing first and make
the new fs/gs base functions use it without having a blurb about preserving
bugs.

Thanks,

	tglx