linux-kernel - Re: [PATCH v4 2/7] x86/fsgsbase/64: Make ptrace read FS/GS base accurately

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Date:   Fri, 22 Jun 2018 16:37:14 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Chang S. Bae" <chang.seok.bae@...el.com>
cc:     Andy 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>,
        Markus T Metzger <markus.t.metzger@...el.com>,
        Ravi Shankar <ravi.v.shankar@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 2/7] x86/fsgsbase/64: Make ptrace read FS/GS base
 accurately

On Wed, 20 Jun 2018, Chang S. Bae wrote:

> From: Andy Lutomirski <luto@...nel.org>
> 
> ptrace can read FS/GS base using the register access API
> (PTRACE_PEEKUSER, etc) or PTRACE_ARCH_PRCTL.  Make both of these
> mechanisms return the actual FS/GS base.

That's the 'BUG' fix, right? Ok, not something which needs to be urgently
backported it seems. It's surely a correctness issue, which improved
debugability, but surely not something crucial.

So the right ordering of the patches is:

   1) Introduce task_seg_base() as a standalong patch w/o using it.

   2) Add the new fsgs helpers and use task_seg_base() right away instead
      of adding this misleading 'preserve bug' comment just to remove it
      one patch later.

   .....

Thanks,

	tglx