linux-kernel - Re: [PATCH 14/15] x86/fsgsbase/64: Support legacy behavior when FS/GS updated by ptracer

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Date:   Thu, 22 Mar 2018 16:07:53 +0000
From:   Andy Lutomirski <luto@...nel.org>
To:     "Bae, Chang Seok" <chang.seok.bae@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Metzger, Markus T" <markus.t.metzger@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 14/15] x86/fsgsbase/64: Support legacy behavior when FS/GS
 updated by ptracer

On Thu, Mar 22, 2018 at 3:45 PM, Bae, Chang Seok
<chang.seok.bae@...el.com> wrote:
> On 3/21/18, 18:41, "Andy Lutomirski" <luto@...nel.org> wrote:
>> mov to gs changes GSBASE even if GS was unchanged.
> In GDB, ptrace (syscall) doesn't happen when FS/GS unchanged as
> its (context) cache seems to be first checked. This does not allow to
> preserve GSBASE as you know.
>
>> But it's not clear to me that you've identified any case where
>> emulating this behavior is useful.
> One argument I heard is (if debugging a legacy application) user
> might want to (indirectly) access LDT during inferior call and this
> mov to fs/gs has been useful (maybe needed).
>
>

But your patch doesn't actually do this, since gdb will just do
SETREGS anyway, right?