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

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Date:   Wed, 21 Mar 2018 15:11:35 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     Andy Lutomirski <luto@...nel.org>
CC:     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 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. If ptracer changes GS index between PTRACE_GETREGS 
and PTRACE_SETREG, the tracee does not have GS base from GDT/LDT 
when it resumes, with the patch [1]. Task’s GS base is preserved at schedule-in, 
when FSGSBASE enabled.

Code-wise, no clean implementation was found from the existing putreg(). 
putreg() goes in an independently isolated way, while the backward 
compatibility needs something to put one from the other value. 
I would like to know a better way without introducing putregs() (and 
reversely setting them).

[1] https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/fsgsbase&id=cfaf9911b88930ca6e4d0173fe8a58d2ea4ee6fb