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

Kees Cook
2020-01-27T23:34:43Z

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
> 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?

> before .notes sections. Since .exit.text and .exit.data sections are
> discarded at runtime, it undefines EXIT_TEXT and EXIT_DATA to exclude
> .exit.text and .exit.data sections from default discarded sections.

This looks like a separate issue (though maybe related to DISCARDS)?

-Kees

>
> Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> ---
> arch/x86/kernel/vmlinux.lds.S | 11 +++++++++++
> 1 file changed, 11 insertions(+)
>
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index 3a1a819da137..6c6cc26b0177 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -150,6 +150,11 @@ SECTIONS
> _etext = .;
> . = ALIGN(PAGE_SIZE);
>
> + /* .note.gnu.property sections should be discarded */
> + /DISCARD/ : {
> + *(.note.gnu.property)
> + }
> +
> X86_ALIGN_RODATA_BEGIN
> RO_DATA(PAGE_SIZE)
> X86_ALIGN_RODATA_END
> @@ -413,6 +418,12 @@ 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)
> --
> 2.24.1
>

--
Kees Cook