Re: [PATCH v2 2/2] arm/boot: Warn on orphan section placement

From: Nick Desaulniers <ndesaulniers@google.com>
To: Kees Cook <keescook@chromium.org>
Cc: Russell King <linux@armlinux.org.uk>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Will Deacon <will@kernel.org>, Ard Biesheuvel <ardb@kernel.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 2/2] arm/boot: Warn on orphan section placement
Date: Tue, 23 Jun 2020 17:08:30 -0700	[thread overview]
Message-ID: <CAKwvOdnvgHS4bUbf58m9dpZp7=0dYLVQ=EW-LDY=u5a5_AfO9A@mail.gmail.com> (raw)
In-Reply-To: <20200622204915.2987555-3-keescook@chromium.org>

On Mon, Jun 22, 2020 at 1:49 PM Kees Cook <keescook@chromium.org> wrote:
>
> We don't want to depend on the linker's orphan section placement
> heuristics as these can vary between linkers, and may change between
> versions. All sections need to be explicitly named in the linker
> script.
>
> Use common macros for debug sections, discards, and text stubs. Add
> discards for unwanted .note, and .rel sections. Finally, enable orphan
> section warning.
>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  arch/arm/boot/compressed/Makefile      |  2 ++
>  arch/arm/boot/compressed/vmlinux.lds.S | 17 +++++++----------
>  2 files changed, 9 insertions(+), 10 deletions(-)
>
> diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
> index 00602a6fba04..b8a97d81662d 100644
> --- a/arch/arm/boot/compressed/Makefile
> +++ b/arch/arm/boot/compressed/Makefile
> @@ -128,6 +128,8 @@ endif
>  LDFLAGS_vmlinux += --no-undefined
>  # Delete all temporary local symbols
>  LDFLAGS_vmlinux += -X
> +# Report orphan sections
> +LDFLAGS_vmlinux += --orphan-handling=warn
>  # Next argument is a linker script
>  LDFLAGS_vmlinux += -T
>
> diff --git a/arch/arm/boot/compressed/vmlinux.lds.S b/arch/arm/boot/compressed/vmlinux.lds.S
> index 09ac33f52814..c2a8509f876f 100644
> --- a/arch/arm/boot/compressed/vmlinux.lds.S
> +++ b/arch/arm/boot/compressed/vmlinux.lds.S
> @@ -2,6 +2,7 @@
>  /*
>   *  Copyright (C) 2000 Russell King
>   */
> +#include <asm/vmlinux.lds.h>
>
>  #ifdef CONFIG_CPU_ENDIAN_BE8
>  #define ZIMAGE_MAGIC(x) ( (((x) >> 24) & 0x000000ff) | \
> @@ -17,8 +18,11 @@ ENTRY(_start)
>  SECTIONS
>  {
>    /DISCARD/ : {
> +    ARM_COMMON_DISCARD
>      *(.ARM.exidx*)
>      *(.ARM.extab*)
> +    *(.note.*)
> +    *(.rel.*)

.rel.* is the only case I'm curious about.  Why do we want it in
vmlinux, but not the compressed image?  Should `.rel.*` just be part
of ARM_COMMON_DISCARD from the previous patch?

>      /*
>       * Discard any r/w data - this produces a link error if we have any,
>       * which is required for PIC decompression.  Local data generates
> @@ -36,9 +40,7 @@ SECTIONS
>      *(.start)
>      *(.text)
>      *(.text.*)
> -    *(.gnu.warning)
> -    *(.glue_7t)
> -    *(.glue_7)
> +    ARM_STUBS_TEXT
>    }
>    .table : ALIGN(4) {
>      _table_start = .;
> @@ -128,12 +130,7 @@ SECTIONS
>    PROVIDE(__pecoff_data_size = ALIGN(512) - ADDR(.data));
>    PROVIDE(__pecoff_end = ALIGN(512));
>
> -  .stab 0              : { *(.stab) }
> -  .stabstr 0           : { *(.stabstr) }
> -  .stab.excl 0         : { *(.stab.excl) }
> -  .stab.exclstr 0      : { *(.stab.exclstr) }
> -  .stab.index 0                : { *(.stab.index) }
> -  .stab.indexstr 0     : { *(.stab.indexstr) }
> -  .comment 0           : { *(.comment) }
> +  STABS_DEBUG
> +  DWARF_DEBUG
>  }
>  ASSERT(_edata_real == _edata, "error: zImage file size is incorrect");
> --
> 2.25.1
>

-- 
Thanks,
~Nick Desaulniers