Re: [PATCH v2] arm64: Add support for relocating the kernel with RELR relocations

[Will Deacon <will@kernel.org>]

On Fri, Jul 12, 2019 at 12:38:46PM -0700, Peter Collingbourne wrote:
> RELR is a relocation packing format for relative relocations.
> The format is described in a generic-abi proposal:
> https://groups.google.com/d/topic/generic-abi/bX460iggiKg/discussion
> 
> The LLD linker can be instructed to pack relocations in the RELR
> format by passing the flag --pack-dyn-relocs=relr.
> 
> This patch adds a new config option, CONFIG_RELR. Enabling this option
> instructs the linker to pack vmlinux's relative relocations in the RELR
> format, and causes the kernel to apply the relocations at startup along
> with the RELA relocations. RELA relocations still need to be applied
> because the linker will emit RELA relative relocations if they are
> unrepresentable in the RELR format (i.e. address not a multiple of 2).
> 
> Enabling CONFIG_RELR reduces the size of a defconfig kernel image
> with CONFIG_RANDOMIZE_BASE by 3.5MB/16% uncompressed, or 550KB/5%
> compressed (lz4).
> 
> Signed-off-by: Peter Collingbourne <pcc@google.com>
> Tested-by: Nick Desaulniers <ndesaulniers@google.com>
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
> Changes in v2:
> - Reverted change to RELA processing
> - Added more comments, as requested by Nick and Will
> - Added a feature test for NM and OBJCOPY
> - Made CONFIG_RELR=y the default if the tools support it
> 
>  arch/arm64/Kconfig              | 10 ++++
>  arch/arm64/Makefile             |  4 ++
>  arch/arm64/kernel/head.S        | 96 ++++++++++++++++++++++++++++++---
>  arch/arm64/kernel/vmlinux.lds.S |  9 ++++
>  init/Kconfig                    |  3 ++
>  scripts/tools-support-relr.sh   | 16 ++++++
>  6 files changed, 132 insertions(+), 6 deletions(-)
>  create mode 100755 scripts/tools-support-relr.sh
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 7442edbcabfc3..cf3907d21d097 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1476,6 +1476,16 @@ config RELOCATABLE
>  	  relocation pass at runtime even if the kernel is loaded at the
>  	  same address it was linked at.
>  
> +config RELR
> +	bool "Use RELR relocation packing"
> +	depends on RELOCATABLE && TOOLS_SUPPORT_RELR
> +	default y
> +	help
> +	  Store the kernel's dynamic relocations in the RELR relocation packing
> +	  format. Requires a compatible linker (currently only LLD supports

Drop "currently" because it will just rot

> +	  this feature), as well as compatible NM and OBJCOPY utilities
> +	  (llvm-nm and llvm-objcopy are compatible).
> +
>  config RANDOMIZE_BASE
>  	bool "Randomize the address of the kernel image"
>  	select ARM64_MODULE_PLTS if MODULES
> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index bb1f1dbb34e8f..11f84450c7784 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -22,6 +22,10 @@ LDFLAGS_vmlinux		+= -shared -Bsymbolic -z notext -z norelro \
>  			$(call ld-option, --no-apply-dynamic-relocs)
>  endif
>  
> +ifeq ($(CONFIG_RELR),y)
> +  LDFLAGS_vmlinux += --pack-dyn-relocs=relr
> +endif

RELR isn't arm64-specific, right? So we could put this in the top-level
Makefile and have arm64 select ARCH_HAS_RELR if relocatable, so that other
architecture can easily support this in future.

>  ifeq ($(CONFIG_ARM64_ERRATUM_843419),y)
>    ifeq ($(call ld-option, --fix-cortex-a53-843419),)
>  $(warning ld does not support --fix-cortex-a53-843419; kernel may be susceptible to erratum)
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index 2cdacd1c141b9..cc23302e9d95e 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -102,6 +102,8 @@ pe_header:
>  	 *  x23        stext() .. start_kernel()  physical misalignment/KASLR offset
>  	 *  x28        __create_page_tables()     callee preserved temp register
>  	 *  x19/x20    __primary_switch()         callee preserved temp registers
> +	 *  x24        __primary_switch() .. relocate_kernel()
> +	 *                                        current RELR displacement
>  	 */
>  ENTRY(stext)
>  	bl	preserve_boot_args
> @@ -834,14 +836,93 @@ __relocate_kernel:
>  
>  0:	cmp	x9, x10
>  	b.hs	1f
> -	ldp	x11, x12, [x9], #24
> -	ldr	x13, [x9, #-8]
> -	cmp	w12, #R_AARCH64_RELATIVE
> +	ldp	x12, x13, [x9], #24
> +	ldr	x14, [x9, #-8]
> +	cmp	w13, #R_AARCH64_RELATIVE
>  	b.ne	0b
> -	add	x13, x13, x23			// relocate
> -	str	x13, [x11, x23]
> +	add	x14, x14, x23			// relocate
> +	str	x14, [x12, x23]
>  	b	0b
> -1:	ret
> +
> +1:
> +#ifdef CONFIG_RELR
> +	/*
> +	 * Apply RELR relocations.
> +	 *
> +	 * RELR is a compressed format for storing relative relocations. The
> +	 * encoded sequence of entries looks like:
> +	 * [ AAAAAAAA BBBBBBB1 BBBBBBB1 ... AAAAAAAA BBBBBB1 ... ]

I assume these are treated as an array of u64 types for the purposes of
endianness? (have you tested with a big-endian kernel?).

Will

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