Re: [PATCH] Discard .note.gnu.property sections in generic NOTES

[Kees Cook <keescook@chromium.org>]

On Thu, Jan 30, 2020 at 02:43:37PM -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
> kernel linker script only contains a single 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
> ...
> }
> 
> The NOTE segment generated by kernel linker script is aligned to 4 bytes.
> But .note.gnu.property section must be aligned to 8 bytes on x86-64 and
> we get
> 
> [hjl@gnu-skx-1 linux]$ readelf -n vmlinux
> 
> Displaying notes found in: .notes
>   Owner                Data size Description
>   Xen                  0x00000006 Unknown note type: (0x00000006)
>    description data: 6c 69 6e 75 78 00
>   Xen                  0x00000004 Unknown note type: (0x00000007)
>    description data: 32 2e 36 00
>   xen-3.0              0x00000005 Unknown note type: (0x006e6558)
>    description data: 08 00 00 00 03
> readelf: Warning: note with invalid namesz and/or descsz found at offset 0x50
> readelf: Warning:  type: 0xffffffff, namesize: 0x006e6558, descsize:
> 0x80000000, alignment: 8
> [hjl@gnu-skx-1 linux]$
> 
> Since note.gnu.property section in kernel image is never used, this patch
> discards .note.gnu.property sections in kernel linker script by adding
> 
> /DISCARD/ : {
>   *(.note.gnu.property)
> }
> 
> before kernel NOTE segment in generic NOTES.
> 
> Signed-off-by: H.J. Lu <hjl.tools@gmail.com>

Thanks!

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
>  include/asm-generic/vmlinux.lds.h | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 6b943fb8c5fd..6659a7c07c84 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -818,7 +818,14 @@
>  #define TRACEDATA
>  #endif
>  
> +/*
> + * Discard .note.gnu.property sections which are unused and have
> + * different alignment requirement from kernel note sections.
> + */
>  #define NOTES								\
> +	/DISCARD/ : {							\
> +		*(.note.gnu.property)					\
> +	}								\
>  	.notes : AT(ADDR(.notes) - LOAD_OFFSET) {			\
>  		__start_notes = .;					\
>  		KEEP(*(.note.*))					\
> -- 
> 2.24.1
> 

-- 
Kees Cook