linux-kernel - Re: [PATCH v3 7/7] x86/ldt,ptrace: provide regset access to the LDT

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Message-ID: <CALCETrUdNZ9xUBi7116uU5dvufH9TcBTzcq5MvLoG7M7rcqLOg@mail.gmail.com>
Date:   Fri, 22 Jun 2018 08:30:43 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     Andrew Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <h.peter.anvin@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        "Metzger, Markus T" <markus.t.metzger@...el.com>
Subject: Re: [PATCH v3 7/7] x86/ldt,ptrace: provide regset access to the LDT

On Fri, Jun 22, 2018 at 8:06 AM <hpa@...or.com> wrote:
>
> On June 22, 2018 7:49:13 AM PDT, Andy Lutomirski <luto@...nel.org> wrote:
> >On Thu, Jun 21, 2018 at 2:18 PM H. Peter Anvin, Intel
> ><h.peter.anvin@...el.com> wrote:
> >>
> >> From: "H. Peter Anvin" <hpa@...ux.intel.com>
> >>
> >> Provide ptrace/regset access to the LDT, if one exists.  This
> >> interface provides both read and write access. The write code is
> >> unified with modify_ldt(); the read code doesn't have enough
> >> similarity so it has been kept made separate.
> >
> >For this and for the GDT, you've chosen to use struct user_desc as
> >your format instead of using a native hardware descriptor format.  Any
> >particular reason why?  If nothing else, it will bloat core files a
> >bit more than needed.
>
> I did because REGSET_TLS was implemented that way, and that is simply a subset of the GDT (which made the same code trivially applicable to both.) modify_ldt() does it *both* ways for extra fun (one for reading, and one for writing.)
>
> ->active is defined as "beyond this point the regset contains only the default value", which seemed appropriate in this case.

I saw that definition too.  But I'm still very unclear as to what, if
anything, the code actually does :)

--Andy