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 01:40:38 +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 Wed, Mar 21, 2018 at 3:11 PM, Bae, Chang Seok
<chang.seok.bae@...el.com> wrote:
> On 3/20/18, 17:47, "Andy Lutomirski" <luto@...nel.org> wrote:
>>    If I've understood all your emails right, when you looked at existing
>>    ptrace users, you found that all of them that write to gs and/or
>>    gs_base do it as part of a putregs call that writes them at the same
>>    time.  If so, then your patch does exactly the same thing that my old
>>    patches did, but your patch is much more complicated.  So why did you
>>    add all that complexity?
>
> What is tried to be provided is backward compatibility by emulating
> “mov gs (fs) …” when index is only changed and preserve a (given) base value
> in other cases.

mov to gs changes GSBASE even if GS was unchanged.

But it's not clear to me that you've identified any case where
emulating this behavior is useful.