Re: [PATCH 2/2] x86: Discard .note.gnu.property sections in vmlinux

[Kees Cook <keescook@chromium.org>]

On Thu, Jan 30, 2020 at 09:51:38AM -0800, H.J. Lu wrote:
> On Mon, Jan 27, 2020 at 3:34 PM Kees Cook <keescook@chromium.org> wrote:
> >
> > On Fri, Jan 24, 2020 at 10:18:19AM -0800, H.J. Lu wrote:
> > > With the command-line option, -mx86-used-note=yes, the x86 assembler
> > > in binutils 2.32 and above generates a program property note in a note
> > > section, .note.gnu.property, to encode used x86 ISAs and features.
> > > But x86 kernel linker script only contains a signle NOTE segment:
> > >
> > > PHDRS {
> > >  text PT_LOAD FLAGS(5);
> > >  data PT_LOAD FLAGS(6);
> > >  percpu PT_LOAD FLAGS(6);
> > >  init PT_LOAD FLAGS(7);
> > >  note PT_NOTE FLAGS(0);
> > > }
> > > SECTIONS
> > > {
> > > ...
> > >  .notes : AT(ADDR(.notes) - 0xffffffff80000000) { __start_notes = .; KEEP(*(.not
> > > e.*)) __stop_notes = .; } :text :note
> > > ...
> > > }
> > >
> > > which may not be incompatible with note.gnu.property sections.  Since

I don't understand this. "may not be incompatible"? Is there an error
generated? If so, what does it look like?

> > > note.gnu.property section in kernel image is unused, this patch discards
> > > .note.gnu.property sections in kernel linker script by adding
> > >
> > >  /DISCARD/ : {
> > >   *(.note.gnu.property)
> > >  }
> >
> > I think this is happening in the wrong place? Shouldn't this be in the
> > DISCARDS macro in include/asm-generic/vmlinux.lds.h instead?
> 
> Please read my commit message closely.   We can't discard .note.gnu.property
> sections by adding .note.gnu.property to default discarded sections
> since default
> discarded sections are placed AFTER .notes sections in x86 kernel
> linker scripts.

I see what you mean now, /DISCARD/ happens after the NOTES macro (now in
the RO_DATA macro). To this end, I think this should be in
include/asm-generic/vmlinux.lds.h in the NOTES macro? It's x86-specific
right now, but why not make this future-proof?

I'd like to avoid as much arch-specific linker stuff as we can. I spent
a lot of time trying to clean up NOTES specifically. :)

> +	/* .note.gnu.property sections should be discarded */

This comment should say _why_ -- the script already shows _what_ is
happening...

> +	/DISCARD/ : {
> +		*(.note.gnu.property)
> +	}

-Kees

-- 
Kees Cook