Re: [PATCH v4] MIPS: Truncate link address into 32bit for 32bit kernel

From: Kees Cook <keescook@chromium.org>
To: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: linux-mips@vger.kernel.org, macro@linux-mips.org,
	clang-built-linux@googlegroups.com,
	Fangrui Song <maskray@google.com>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	Borislav Petkov <bp@suse.de>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4] MIPS: Truncate link address into 32bit for 32bit kernel
Date: Mon, 13 Apr 2020 09:25:31 -0700	[thread overview]
Message-ID: <202004130925.F1B57BC7@keescook> (raw)
In-Reply-To: <20200413062651.3992652-1-jiaxun.yang@flygoat.com>

On Mon, Apr 13, 2020 at 02:26:49PM +0800, Jiaxun Yang wrote:
> LLD failed to link vmlinux with 64bit load address for 32bit ELF
> while bfd will strip 64bit address into 32bit silently.
> To fix LLD build, we should truncate load address provided by platform
> into 32bit for 32bit kernel.
> 
> Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> Link: https://github.com/ClangBuiltLinux/linux/issues/786
> Link: https://sourceware.org/bugzilla/show_bug.cgi?id=25784
> Cc: Fangrui Song <maskray@google.com>
> Cc: Nathan Chancellor <natechancellor@gmail.com>
> --
> V2: Take MaskRay's shell magic.
> 
> V3: After spent an hour on dealing with special character issue in
> Makefile, I gave up to do shell hacks and write a util in C instead.
> Thanks Maciej for pointing out Makefile variable problem.
> 
> v4: Finally we managed to find a Makefile method to do it properly
> thanks to Kees. As it's too far from the initial version, I removed
> Review & Test tag from Nick and Fangrui and Cc instead.
> ---
>  arch/mips/Makefile             | 12 +++++++++++-
>  arch/mips/kernel/vmlinux.lds.S |  2 +-
>  2 files changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/mips/Makefile b/arch/mips/Makefile
> index e1c44aed8156..18495568f03e 100644
> --- a/arch/mips/Makefile
> +++ b/arch/mips/Makefile
> @@ -288,9 +288,19 @@ ifdef CONFIG_64BIT
>    endif
>  endif
>  
> +# When linking a 32-bit executable the LLVM linker cannot cope with a
> +# 32-bit load address that has been sign-extended to 64 bits.  Simply
> +# remove the upper 32 bits then, as it is safe to do so with other
> +# linkers.
> +ifdef CONFIG_64BIT
> +	load-ld			= $(load-y)
> +else
> +	load-ld			= $(subst 0xffffffff,0x,$(load-y))
> +endif
> +
>  KBUILD_AFLAGS	+= $(cflags-y)
>  KBUILD_CFLAGS	+= $(cflags-y)
> -KBUILD_CPPFLAGS += -DVMLINUX_LOAD_ADDRESS=$(load-y)
> +KBUILD_CPPFLAGS += -DVMLINUX_LOAD_ADDRESS=$(load-y) -DVMLINUX_LINK_ADDRESS=$(load-ld)
>  KBUILD_CPPFLAGS += -DDATAOFFSET=$(if $(dataoffset-y),$(dataoffset-y),0)
>  
>  bootvars-y	= VMLINUX_LOAD_ADDRESS=$(load-y) \
> diff --git a/arch/mips/kernel/vmlinux.lds.S b/arch/mips/kernel/vmlinux.lds.S
> index a5f00ec73ea6..5226cd8e4bee 100644
> --- a/arch/mips/kernel/vmlinux.lds.S
> +++ b/arch/mips/kernel/vmlinux.lds.S
> @@ -55,7 +55,7 @@ SECTIONS
>  	/* . = 0xa800000000300000; */
>  	. = 0xffffffff80300000;
>  #endif
> -	. = VMLINUX_LOAD_ADDRESS;
> +	. = VMLINUX_LINK_ADDRESS;
>  	/* read-only */
>  	_text = .;	/* Text and read-only data */
>  	.text : {
> -- 
> 2.26.0.rc2
> 

-- 
Kees Cook