linux-kernel - Re: [PATCH v2] arm64: vdso32: Allow ld.lld to properly link the VDSO

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Date:   Tue, 20 Oct 2020 12:37:38 +0100
From:   Will Deacon <will@...nel.org>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>
Subject: Re: [PATCH v2] arm64: vdso32: Allow ld.lld to properly link the VDSO

On Mon, Oct 19, 2020 at 06:14:07PM -0700, Nathan Chancellor wrote:
> As it stands now, the vdso32 Makefile hardcodes the linker to ld.bfd
> using -fuse-ld=bfd with $(CC). This was taken from the arm vDSO
> Makefile, as the comment notes, done in commit d2b30cd4b722 ("ARM:
> 8384/1: VDSO: force use of BFD linker").
> 
> Commit fe00e50b2db8 ("ARM: 8858/1: vdso: use $(LD) instead of $(CC) to
> link VDSO") changed that Makefile to use $(LD) directly instead of
> through $(CC), which matches how the rest of the kernel operates. Since
> then, LD=ld.lld means that the arm vDSO will be linked with ld.lld,
> which has shown no problems so far.
> 
> Allow ld.lld to link this vDSO as we do the regular arm vDSO. To do
> this, we need to do a few things:
> 
> * Add a LD_COMPAT variable, which defaults to $(CROSS_COMPILE_COMPAT)ld
>   with gcc and $(LD) if LLVM is 1, which will be ld.lld, or
>   $(CROSS_COMPILE_COMPAT)ld if not, which matches the logic of the main
>   Makefile. It is overrideable for further customization and avoiding
>   breakage.
> 
> * Eliminate cc32-ldoption, which matches commit 055efab3120b ("kbuild:
>   drop support for cc-ldoption").
> 
> With those, we can use $(LD_COMPAT) in cmd_ldvdso and change the flags
> from compiler linker flags to linker flags directly. We eliminate
> -mfloat-abi=soft because it is not handled by the linker.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/1033
> Reported-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Reviewed-by: Vincenzo Frascino <vincenzo.frascino@....com>
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> ---

Cheers, I'll try to remember to queue this at -rc1 but please feel free to
resend then to remind me :)

Will