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

H.J. Lu
2020-01-30T20:06:45Z

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
> +#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.

Thanks.

--
H.J.