64-bit TEB on WoW64

[Andrew Wesie]

Ah, thanks. It didn't occur to me that set_thread_area and modify_ldt
would have different behavior w.r.t. allowing code segments.

I'll play around with it and see what happens.


On Sun, May 19, 2019 at 5:42 PM Zebediah Figura <z.figura12 at gmail.com> wrote:
>
> On 5/19/19 4:58 PM, Andrew Wesie wrote:
> > On Sun, May 19, 2019 at 4:07 PM Ken Thomases <ken at codeweavers.com> wrote:
> >>
> >> Perhaps 4: use a different code segment so that the %cs segment selector doesn't look like a WoW64 environment.  Kind of specific to this particular app's behavior, so not great, but maybe passable.
> >>
> >
> > This possibility was considered but I didn't think it was possible to
> > do this on Linux any more without patching the kernel. Notably from
> > the man page for set_thread_area: "Since Linux 3.19, set_thread_area()
> > cannot be used to write non-present segments, 16-bit segments, or code
> > segments, although clearing a segment is still acceptable." The
> > associated source code:
> > https://elixir.bootlin.com/linux/v3.19/source/arch/x86/kernel/tls.c#L63.
> >
> > Please let me know if my understanding is incorrect.
> >
> >
>
> Presumably we could use the LDT [and modify_ldt(2)] instead, as we
> already do for NE segmentation?
>
>