Re: [PATCH v4 1/4] x86/signal/64: Add a comment about sigcontext->fs and gs

[Andy Lutomirski]

On Feb 16, 2016 12:42 AM, "Ingo Molnar" <mingo@xxxxxxxxxx> wrote:
>
>
> * Andy Lutomirski <luto@xxxxxxxxxx> 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 <luto@xxxxxxxxxx>
> > ---
> >  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