[PATCH] kbuild: Always link with '-z norelro'

[Nick Desaulniers]

On Thu, Nov 12, 2020 at 10:41 AM Nathan Chancellor
<natechancellor at gmail.com> wrote:
>
> Commit 3bbd3db86470 ("arm64: relocatable: fix inconsistencies in linker
> script and options") added '-z norelro' to the arm64 Makefile when
> CONFIG_RELOCATABLE was set to help support ld.lld because ld.lld
> defaults to '-z relro' but the kernel does not use program headers or
> adhere to the section layout that is required for RELRO to work.
>
> Commit 3b92fa7485eb ("arm64: link with -z norelro regardless of
> CONFIG_RELOCATABLE") unconditionally added it to LDFLAGS_vmlinux because
> an error occurs with CONFIG_KASAN set even when CONFIG_RELOCATABLE is
> unset.
>
> As it turns out, ARM experiences the same error after CONFIG_KASAN was
> implemented, meaning that '-z norelro' needs to be added to that
> Makefile as well (multi_v7_defconfig + CONFIG_KASAN=y + LD=ld.lld):
>
> $ make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- LLVM=1 zImage
> ld.lld: error: section: .exit.data is not contiguous with other relro sections
>
> To avoid playing whack-a-mole with different architectures over time,
> hoist '-z norelro' into the main Makefile. This does not affect ld.bfd
> because '-z norelro' is the default for it.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1189
> Suggested-by: Nick Desaulniers <ndesaulniers at google.com>
> Signed-off-by: Nathan Chancellor <natechancellor at gmail.com>

Why not add it additionally to KBUILD_LDFLAGS_MODULE a la
`--build-id=sha1` a few lines above? (or `LDFLAGS_MODULE`, but that
looks unused?)  We probably don't want this for modules either.  In
that case, you could add -z norelo to the two existing lines with
`--build-id=sha1` above?

> ---
>
> Hi all,
>
> This should probably go into Russell's tree with acks from the arm64 and
> kbuild maintainers.
>
> Cheers,
> Nathan
>
>  Makefile            | 2 ++
>  arch/arm64/Makefile | 2 +-
>  2 files changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/Makefile b/Makefile
> index 008aba5f1a20..648bfb486244 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -984,6 +984,8 @@ ifeq ($(CONFIG_RELR),y)
>  LDFLAGS_vmlinux        += --pack-dyn-relocs=relr
>  endif
>
> +LDFLAGS_vmlinux += -z norelro
> +
>  # Align the bit size of userspace programs with the kernel
>  KBUILD_USERCFLAGS  += $(filter -m32 -m64 --target=%, $(KBUILD_CFLAGS))
>  KBUILD_USERLDFLAGS += $(filter -m32 -m64 --target=%, $(KBUILD_CFLAGS))
> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index 5789c2d18d43..85495ff8f0fd 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -10,7 +10,7 @@
>  #
>  # Copyright (C) 1995-2001 by Russell King
>
> -LDFLAGS_vmlinux        :=--no-undefined -X -z norelro
> +LDFLAGS_vmlinux        :=--no-undefined -X
>
>  ifeq ($(CONFIG_RELOCATABLE), y)
>  # Pass --no-apply-dynamic-relocs to restore pre-binutils-2.27 behaviour
>
> base-commit: f8394f232b1eab649ce2df5c5f15b0e528c92091
> --
> 2.29.2
>


-- 
Thanks,
~Nick Desaulniers