Re: [PATCH] arm64: relocatable: fix inconsistencies in linker script and options

[Will Deacon <will.deacon@arm.com>]

Hey Ard,

On Mon, Dec 03, 2018 at 08:58:05PM +0100, Ard Biesheuvel wrote:
> readelf complains about the section layout of vmlinux when building
> with CONFIG_RELOCATABLE=y (for KASLR):
> 
>   readelf: Warning: [21]: Link field (0) should index a symtab section.
>   readelf: Warning: [21]: Info field (0) should index a relocatable section.
> 
> Also, it seems that our use of '-pie -shared' is contradictory, and
> thus ambiguous. In general, the way KASLR is wired up at the moment
> is highly tailored to how ld.bfd happens to implement (and conflate)
> PIE executables and shared libraries, so given the current effort to
> support other toolchains, let's fix some of these issues as well.
> 
> - Drop the -pie linker argument and just leave -shared. In ld.bfd,
>   the differences between them are unclear (except for the ELF type
>   of the produced image [0]) but lld chokes on seeing both at the
>   same time.
> 
> - Rename the .rela output section to .rela.dyn, as is customary for
>   shared libraries and PIE executables, so that it is not misidentified
>   by readelf as a static relocation section (producing the warnings
>   above).
> 
> - Pass the -z notext and -z norelro options to explicitly instruct the
>   linker to permit text relocations, and to omit the RELRO program
>   header (which requires a certain section layout that we don't adhere
>   to in the kernel). These are the defaults for current versions of
>   ld.bfd.
> 
> - Discard .eh_frame and .gnu.hash sections to avoid them from being
>   emitted between .head.text and .text, screwing up the section layout.
> 
> These changes only affect the ELF image, and produce the same binary
> image.

Thanks, I've queued this up on for-next/core.

> [0] b9dce7f1ba01 ("arm64: kernel: force ET_DYN ELF type for ...")
> 
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Peter Smith <peter.smith@linaro.org>
> Tested-by: Nick Desaulniers <ndesaulniers@google.com>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
> This supersedes
> 
> [RFT PATCH] arm64: relocatable: build the kernel as a proper shared library
> [RFT PATCH] arm64: add support for building the KASLR kernel with LLVM lld

Also, this ^^ is /really/ helpful :)

Thanks,

Will

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