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

[Andy Lutomirski]

On Wed, Mar 21, 2018 at 3:11 PM, Bae, Chang Seok
<chang.seok.bae@xxxxxxxxx> wrote:
> On 3/20/18, 17:47, "Andy Lutomirski" <luto@xxxxxxxxxx> 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
> âmov gs (fs) ââ 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.