Re: [PATCH] x86: Don't discard .exit.text and .exit.data at link-time

[Kees Cook <keescook@chromium.org>]

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
+#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).

-Kees

> > > --- a/arch/x86/kernel/vmlinux.lds.S
> > > +++ b/arch/x86/kernel/vmlinux.lds.S
> > > @@ -416,6 +416,12 @@ SECTIONS
> > >       STABS_DEBUG
> > >       DWARF_DEBUG
> > >
> > > +     /* Sections to be discarded.  EXIT_TEXT and EXIT_DATA discard at runtime.
> > > +      * not link time.  */
> > > +#undef EXIT_TEXT
> > > +#define EXIT_TEXT
> > > +#undef EXIT_DATA
> > > +#define EXIT_DATA
> > >       DISCARDS
> > >       /DISCARD/ : {
> > >               *(.eh_frame)

-- 
Kees Cook