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

[Bae, Chang Seok]

On 3/21/18, 18:41, "Andy Lutomirski" <l...@kernel.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).