linux-kernel - Re: [PATCH v2 02/29] powerpc: Remove PT

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Message-ID: <878sprx1br.fsf@mpe.ellerman.id.au>
Date:   Fri, 11 Oct 2019 17:07:04 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Kees Cook <keescook@...omium.org>, Borislav Petkov <bp@...en8.de>
Cc:     Kees Cook <keescook@...omium.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Will Deacon <will@...nel.org>, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, linux-alpha@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-c6x-dev@...ux-c6x.org,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Michal Simek <monstr@...str.eu>, linux-parisc@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, Joel Stanley <joel@....id.au>,
        Segher Boessenkool <segher@...nel.crashing.org>
Subject: Re: [PATCH v2 02/29] powerpc: Remove PT_NOTE workaround

Kees Cook <keescook@...omium.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@...omium.org>
> ---
>  arch/powerpc/kernel/vmlinux.lds.S | 24 ++----------------------
>  1 file changed, 2 insertions(+), 22 deletions(-)

Acked-by: Michael Ellerman <mpe@...erman.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