linux-kernel - Re: [PATCH] x86/boot: allow a relocatable kernel to be linked with lld

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Message-ID: <20200502034307.GA2971661@ubuntu-s3-xlarge-x86>
Date:   Fri, 1 May 2020 20:43:07 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Dmitry Golovin <dima@...ovin.in>
Cc:     clang-built-linux@...glegroups.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/boot: allow a relocatable kernel to be linked with
 lld

On Fri, May 01, 2020 at 08:42:13AM +0000, Dmitry Golovin wrote:
> LLD by default disallows relocations in read-only segments. For a
> relocatable kernel, we pass -z notext to the linker to explicitly
> allow relocations. This behavior is the default for BFD.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/579
> Signed-off-by: Dmitry Golovin <dima@...ovin.in>

I was able to link a Clang built i386_defconfig kernel with ld.lld and
boot it in QEMU 5.0 after this change. A GCC built kernel links still
with ld.bfd and also boots in QEMU successfully. x86_64_defconfig with
both compilers and their respective linkers did not regress.

Tested-by: Nathan Chancellor <natechancellor@...il.com>

> ---
>  arch/x86/boot/compressed/Makefile | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 5f7c262bcc99..7214751e1671 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -57,6 +57,9 @@ else
>  KBUILD_LDFLAGS += $(shell $(LD) --help 2>&1 | grep -q "\-z noreloc-overflow" \
>  	&& echo "-z noreloc-overflow -pie --no-dynamic-linker")
>  endif
> +ifeq ($(CONFIG_RELOCATABLE), y)
> +KBUILD_LDFLAGS += -z notext
> +endif
>  LDFLAGS_vmlinux := -T
>  
>  hostprogs	:= mkpiggy
> -- 
> 2.25.1
>