On Feb 16, 2016 12:42 AM, "Ingo Molnar" <mi...@kernel.org> wrote:
>
>
> * Andy Lutomirski <l...@kernel.org> wrote:
>
> > These fields have a strange history. This tries to document it.
> >
> > This borrows from 9a036b93a344 ("x86/signal/64: Remove 'fs' and 'gs'
> > from sigcontext"), which was reverted by ed596cde9425 ("Revert x86
> > sigcontext cleanups").
> >
> > Signed-off-by: Andy Lutomirski <l...@kernel.org>
> > ---
> > arch/x86/include/uapi/asm/sigcontext.h | 19 +++++++++++++++++++
> > 1 file changed, 19 insertions(+)
> >
> > diff --git a/arch/x86/include/uapi/asm/sigcontext.h
> > b/arch/x86/include/uapi/asm/sigcontext.h
> > index d485232f1e9f..47dae8150520 100644
> > --- a/arch/x86/include/uapi/asm/sigcontext.h
> > +++ b/arch/x86/include/uapi/asm/sigcontext.h
> > @@ -341,6 +341,25 @@ struct sigcontext {
> > __u64 rip;
> > __u64 eflags; /* RFLAGS */
> > __u16 cs;
> > +
> > + /*
> > + * Prior to 2.5.64 ("[PATCH] x86-64 updates for 2.5.64-bk3"),
> > + * Linux saved and restored fs and gs in these slots. This
> > + * was counterproductive, as fsbase and gsbase were never
> > + * saved, so arch_prctl was presumably unreliable.
> > + *
> > + * If these slots are ever needed for any other purpose, there
> > + * is some risk that very old 64-bit binaries could get
> > + * confused. I doubt that many such binaries still work,
> > + * though, since the same patch in 2.5.64 also removed the
> > + * 64-bit set_thread_area syscall, so it appears that there is
> > + * no TLS API beyond modify_ldt that works in both pre- and
> > + * post-2.5.64 kernels.
> > + *
> > + * There is at least one additional concern if these slots are
> > + * recycled for another purpose: some DOSEMU versions stash fs
> > + * and gs in these slots manually.
> > + */
> > __u16 gs;
> > __u16 fs;
>
> So I think this comment should be a lot more assertive: it should state that
> due
> to these old legacies that user-space learned to rely on the kernel must not
> touch
> these fields. I.e. it is an ABI - no ifs and whens.
We could still touch them to a limited extent. For example, we could
save FS and GS there (but we probably can't restore them).
I'll improve the comment.
>
> We should also rename them to __dosemu_gs_reserved/__dosemu_fs_reserved or so.
I suspect that DOSEMU won't build if we do that. In any event, I
think it should be a separate patch so that it can be trivially
reverted if needed.
--Andy
