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

[Andy Lutomirski]

On Thu, Mar 22, 2018 at 3:45 PM, Bae, Chang Seok
<chang.seok.bae@xxxxxxxxx> wrote:
> On 3/21/18, 18:41, "Andy Lutomirski" <luto@xxxxxxxxxx> 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?