Re: [PATCH v2 2/2] arm64/build: Warn on orphan section placement

From: Will Deacon <will@kernel.org>
To: Kees Cook <keescook@chromium.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Ard Biesheuvel <ardb@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	Peter Collingbourne <pcc@google.com>,
	James Morse <james.morse@arm.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Nathan Chancellor <natechancellor@gmail.com>,
	clang-built-linux@googlegroups.com,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 2/2] arm64/build: Warn on orphan section placement
Date: Tue, 23 Jun 2020 15:52:18 +0100	[thread overview]
Message-ID: <20200623145218.GC4336@willie-the-truck> (raw)
In-Reply-To: <20200622205815.2988115-3-keescook@chromium.org>

On Mon, Jun 22, 2020 at 01:58:15PM -0700, Kees Cook 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.
> 
> Explicitly include debug sections when they're present. Add .eh_frame*
> to discard as it seems that these are still generated even though
> -fno-asynchronous-unwind-tables is being specified. Add .plt and
> .data.rel.ro to discards as they are not actually used. Add .got.plt
> to the image as it does appear to be mapped near .data. Finally enable
> orphan section warnings.

Can you elaborate a bit on what .got.plt is being used for, please? I
wonder if there's an interaction with an erratum workaround in the linker
or something.

> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index a0d94d063fa8..3e628983445a 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -29,6 +29,10 @@ LDFLAGS_vmlinux	+= --fix-cortex-a53-843419
>    endif
>  endif
>  
> +# We never want expected sections to be placed heuristically by the
> +# linker. All sections should be explicitly named in the linker script.
> +LDFLAGS_vmlinux += --orphan-handling=warn
> +
>  ifeq ($(CONFIG_ARM64_USE_LSE_ATOMICS), y)
>    ifneq ($(CONFIG_ARM64_LSE_ATOMICS), y)
>  $(warning LSE atomics not supported by binutils)
> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> index 5427f502c3a6..c9ecb3b2007d 100644
> --- a/arch/arm64/kernel/vmlinux.lds.S
> +++ b/arch/arm64/kernel/vmlinux.lds.S
> @@ -94,7 +94,8 @@ SECTIONS
>  	/DISCARD/ : {
>  		*(.interp .dynamic)
>  		*(.dynsym .dynstr .hash .gnu.hash)
> -		*(.eh_frame)
> +		*(.plt) *(.data.rel.ro)
> +		*(.eh_frame) *(.init.eh_frame)

Do we need to include .eh_frame_hdr here too?

>  	}
>  
>  	. = KIMAGE_VADDR + TEXT_OFFSET;
> @@ -209,6 +210,7 @@ SECTIONS
>  	_data = .;
>  	_sdata = .;
>  	RW_DATA(L1_CACHE_BYTES, PAGE_SIZE, THREAD_ALIGN)
> +	.got.plt : ALIGN(8) { *(.got.plt) }
>  
>  	/*
>  	 * Data written with the MMU off but read with the MMU on requires
> @@ -244,6 +246,7 @@ SECTIONS
>  	_end = .;
>  
>  	STABS_DEBUG
> +	DWARF_DEBUG

I know you didn't add it, but do we _really_ care about stabs debug? Who
generates that for arm64?

Will