On Thu, Jan 30, 2020 at 12:06 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Thu, Jan 30, 2020 at 11:58 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > On Thu, Jan 30, 2020 at 11:45:15AM -0800, H.J. Lu wrote:
> > > On Thu, Jan 30, 2020 at 11:40 AM Kees Cook <keescook@chromium.org> wrote:
> > > >
> > > > On Thu, Jan 30, 2020 at 10:00:48AM -0800, H.J. Lu wrote:
> > > > > Since .exit.text and .exit.data sections are discarded at runtime, we
> > > > > should undefine EXIT_TEXT and EXIT_DATA to exclude .exit.text and
> > > > > .exit.data sections from default discarded sections.
> > > >
> > > > This is just a correctness fix, yes? The EXIT_TEXT and EXIT_DATA were
> > > > already included before the /DISCARD/ section here, so there's no
> > > > behavioral change with this patch, correct?
> > >
> > > That is correct.  I was confused by EXIT_TEXT and EXIT_DATA in generic
> > > DISCARDS.   My patch just makes it more explicit.
> >
> > Okay, so to that end and because this isn't arch-specific, I'd like to
> > see this be a behavioral flag, and then the generic DISCARDS macro can
> > be adjusted. This lets all architectures implement this without having
> > to scatter undef/define lines in each arch.
> >
> > Something like this:
> >
> > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> > index e00f41aa8ec4..f242d3b4814d 100644
> > --- a/include/asm-generic/vmlinux.lds.h
> > +++ b/include/asm-generic/vmlinux.lds.h
> > @@ -894,11 +894,17 @@
> >   * section definitions so that such archs put those in earlier section
> >   * definitions.
> >   */
> > -#define DISCARDS                                                       \
> > -       /DISCARD/ : {                                                   \
> > +#ifdef RUNTIME_DISCARD_EXIT
> > +#define EXIT_DISCARDS
> > +#else
> > +#define EXIT_DISCARDS                                                  \
> >         EXIT_TEXT                                                       \
> >         EXIT_DATA                                                       \
> > -       EXIT_CALL                                                       \
> > +       EXIT_CALL
> > +#endif

We should keep EXIT_CALL in DISCARDS.  Only .exit.text and .exit.data
sections are discarded at runtime.

> > +#define DISCARDS                                                       \
> > +       /DISCARD/ : {                                                   \
> > +       EXIT_DISCARDS                                                   \
> >         *(.discard)                                                     \
> >         *(.discard.*)                                                   \
> >         *(.modinfo)                                                     \
> >
> > Then x86 and all other architectures that do this can just use
> > #define RUNTIME_DISCARD_EXIT
> > at the top (like EMITS_PT_NOTE, etc).
> >
>
> It should work.

Here is the patch to add RUNTIME_DISCARD_EXIT to generic DISCARDS.

-- 
H.J.