Re: [PATCH v5 23/36] arm/build: Explicitly keep .ARM.attributes sections

From: Nick Desaulniers <ndesaulniers@google.com>
To: Kees Cook <keescook@chromium.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Will Deacon <will@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	Peter Collingbourne <pcc@google.com>,
	James Morse <james.morse@arm.com>, Borislav Petkov <bp@suse.de>,
	Ingo Molnar <mingo@redhat.com>,
	Russell King <linux@armlinux.org.uk>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Arvind Sankar <nivedita@alum.mit.edu>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Arnd Bergmann <arnd@arndb.de>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	linux-arch <linux-arch@vger.kernel.org>,
	linux-efi <linux-efi@vger.kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v5 23/36] arm/build: Explicitly keep .ARM.attributes sections
Date: Mon, 3 Aug 2020 12:02:07 -0700	[thread overview]
Message-ID: <CAKwvOdn11z+iFQZC54JvQHC=NFX1FsoRMw2a-2P=5sQ6FKwbnw@mail.gmail.com> (raw)
In-Reply-To: <20200731230820.1742553-24-keescook@chromium.org>

On Fri, Jul 31, 2020 at 4:18 PM Kees Cook <keescook@chromium.org> wrote:
>
> In preparation for adding --orphan-handling=warn, explicitly keep the
> .ARM.attributes section by expanding the existing ELF_DETAILS macro into
> ARM_DETAILS.
>
> Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
> Link: https://lore.kernel.org/lkml/CAKwvOdk-racgq5pxsoGS6Vtifbtrk5fmkmnoLxrQMaOvV0nPWw@mail.gmail.com/
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  arch/arm/include/asm/vmlinux.lds.h | 4 ++++
>  arch/arm/kernel/vmlinux-xip.lds.S  | 2 +-
>  arch/arm/kernel/vmlinux.lds.S      | 2 +-
>  3 files changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm/include/asm/vmlinux.lds.h b/arch/arm/include/asm/vmlinux.lds.h
> index a08f4301b718..c4af5182ab48 100644
> --- a/arch/arm/include/asm/vmlinux.lds.h
> +++ b/arch/arm/include/asm/vmlinux.lds.h
> @@ -52,6 +52,10 @@
>                 ARM_MMU_DISCARD(*(__ex_table))                          \
>                 COMMON_DISCARDS
>
> +#define ARM_DETAILS                                                    \
> +               ELF_DETAILS                                             \
> +               .ARM.attributes 0 : { *(.ARM.attributes) }

I had to look up what the `0` meant:
https://sourceware.org/binutils/docs/ld/Output-Section-Attributes.html#Output-Section-Attributes
mentions it's an "address" and
https://ftp.gnu.org/old-gnu/Manuals/ld-2.9.1/html_chapter/ld_3.html#SEC21
mentions it as "start" (an address).
Unless we need those, can we drop them? (Sorry for the resulting churn
that would cause).  I think the NO_LOAD stuff makes more sense, but
I'm curious if the kernel checks for that.

> +
>  #define ARM_STUBS_TEXT                                                 \
>                 *(.gnu.warning)                                         \
>                 *(.glue_7)                                              \
> diff --git a/arch/arm/kernel/vmlinux-xip.lds.S b/arch/arm/kernel/vmlinux-xip.lds.S
> index 904c31fa20ed..57fcbf55f913 100644
> --- a/arch/arm/kernel/vmlinux-xip.lds.S
> +++ b/arch/arm/kernel/vmlinux-xip.lds.S
> @@ -150,7 +150,7 @@ SECTIONS
>         _end = .;
>
>         STABS_DEBUG
> -       ELF_DETAILS
> +       ARM_DETAILS
>  }
>
>  /*
> diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
> index bb950c896a67..1d3d3b599635 100644
> --- a/arch/arm/kernel/vmlinux.lds.S
> +++ b/arch/arm/kernel/vmlinux.lds.S
> @@ -149,7 +149,7 @@ SECTIONS
>         _end = .;
>
>         STABS_DEBUG
> -       ELF_DETAILS
> +       ARM_DETAILS
>  }
>
>  #ifdef CONFIG_STRICT_KERNEL_RWX
> --
> 2.25.1
>

-- 
Thanks,
~Nick Desaulniers