From: Philip Guenther <guenther@gmail.com>
Subject: Re: Question - %gs support on OpenBSD
To: Neth Iafin <NethIafin-obsd@proton.me>
Cc: "tech@openbsd.org" <tech@openbsd.org>
Date: Tue, 29 Sep 2026 10:09:49 -0700

On Tuesday, September 29, 2026, Neth Iafin <NethIafin-obsd@proton.me> wrote:
>
> I was wondering why OpenBSD has no ability to set %gs in userland?
>
> Is this to avoid swapgs vulnerabilities? Something else?
> I understand that supporting wine is probably below 0 on the priority list
> for OpenBSD, but I wonder about the reasoning behind %gs being locked to
> kernel.
> I assume there is no chance that setting %gs will ever come back as a
> syscall?
>

It wasn’t needed when rthreads were implemented and in the years since no
one has done the delicate surgery on the trap and syscall entry points that
would be needed to support it.

I looked at it briefly when hardware with {rd,wr}{fs,gs}base support
appeared, but the new complexity and risks that creates (speculation across
swapgs becomes a risk, etc) made it not appear to be a good trade off.
There a ground swell of WINE interest out there?


Philip Guenther