Re: [PATCH v2 02/29] powerpc: Remove PT_NOTE workaround

Michael Ellerman
2019-10-11T06:07:04Z

Kees Cook <keescook@chromium.org> writes:
> In preparation for moving NOTES into RO_DATA, remove the PT_NOTE
> workaround since the kernel requires at least gcc 4.6 now.
>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> arch/powerpc/kernel/vmlinux.lds.S | 24 ++----------------------
> 1 file changed, 2 insertions(+), 22 deletions(-)

Acked-by: Michael Ellerman <mpe@ellerman.id.au>

For the archives, Joel tried a similar patch a while back which caused
some problems, see:

https://lore.kernel.org/linuxppc-dev/20190321003253.22100-1-joel@jms.id.au/

and a v2:

https://lore.kernel.org/linuxppc-dev/20190329064453.12761-1-joel@jms.id.au/

This is similar to his v2. The only outstanding comment on his v2 was
from Segher:
(And I do not know if there are any tools that expect the notes in a phdr,
or even specifically the second phdr).

But this patch solves that by not changing the note.

cheers

> diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
> index 81e672654789..a3c8492b2b19 100644
> --- a/arch/powerpc/kernel/vmlinux.lds.S
> +++ b/arch/powerpc/kernel/vmlinux.lds.S
> @@ -20,20 +20,6 @@ ENTRY(_stext)
> PHDRS {
> kernel PT_LOAD FLAGS(7); /* RWX */
> note PT_NOTE FLAGS(0);
> - dummy PT_NOTE FLAGS(0);
> -
> - /* binutils < 2.18 has a bug that makes it misbehave when taking an
> - ELF file with all segments at load address 0 as input. This
> - happens when running "strip" on vmlinux, because of the AT() magic
> - in this linker script. People using GCC >= 4.2 won't run into
> - this problem, because the "build-id" support will put some data
> - into the "notes" segment (at a non-zero load address).
> -
> - To work around this, we force some data into both the "dummy"
> - segment and the kernel segment, so the dummy segment will get a
> - non-zero load address. It's not enough to always create the
> - "notes" segment, since if nothing gets assigned to it, its load
> - address will be zero. */
> }
>
> #ifdef CONFIG_PPC64
> @@ -178,14 +164,8 @@ SECTIONS
> EXCEPTION_TABLE(0)
>
> NOTES :kernel :note
> -
> - /* The dummy segment contents for the bug workaround mentioned above
> - near PHDRS. */
> - .dummy : AT(ADDR(.dummy) - LOAD_OFFSET) {
> - LONG(0)
> - LONG(0)
> - LONG(0)
> - } :kernel :dummy
> + /* Restore program header away from PT_NOTE. */
> + .dummy : { *(.dummy) } :kernel
>
> /*
> * Init sections discarded at runtime
> --
> 2.17.1