Re: [PATCH bpf-next v2 1/6] tools: Help cross-building with clang

[Nick Desaulniers <ndesaulniers@google.com>]

On Thu, Dec 16, 2021 at 8:50 AM Jean-Philippe Brucker
<jean-philippe@linaro.org> wrote:
>
> Cross-compilation with clang uses the -target parameter rather than a
> toolchain prefix. Just like the kernel Makefile, add that parameter to
> CFLAGS when CROSS_COMPILE is set.
>
> Unlike the kernel Makefile, we use the --sysroot and --gcc-toolchain
> options because unlike the kernel, tools require standard libraries.
> Commit c91d4e47e10e ("Makefile: Remove '--gcc-toolchain' flag") provides
> some background about --gcc-toolchain. Normally clang finds on its own
> the additional utilities and libraries that it needs (for example GNU ld
> or glibc). On some systems however, this autodetection doesn't work.
> There, our only recourse is asking GCC directly, and pass the result to
> --sysroot and --gcc-toolchain. Of course that only works when a cross
> GCC is available.
>
> Autodetection worked fine on Debian, but to use the aarch64-linux-gnu
> toolchain from Archlinux I needed both --sysroot (for crt1.o) and
> --gcc-toolchain (for crtbegin.o, -lgcc). The --prefix parameter wasn't
> needed there, but it might be useful on other distributions.
>
> Use the CLANG_CROSS_FLAGS variable instead of CLANG_FLAGS because it
> allows tools such as bpftool, that need to build both host and target
> binaries, to easily filter out the cross-build flags from CFLAGS.
>
> Acked-by: Quentin Monnet <quentin@isovalent.com>
> Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> ---
> Most tools I looked at needed additional changes to support cross-build
> with clang. I've only done the work for bpf tools.
> ---
>  tools/scripts/Makefile.include | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/tools/scripts/Makefile.include b/tools/scripts/Makefile.include
> index 071312f5eb92..b0be5f40a3f1 100644
> --- a/tools/scripts/Makefile.include
> +++ b/tools/scripts/Makefile.include
> @@ -87,7 +87,18 @@ LLVM_STRIP   ?= llvm-strip
>
>  ifeq ($(CC_NO_CLANG), 1)
>  EXTRA_WARNINGS += -Wstrict-aliasing=3
> -endif
> +
> +else ifneq ($(CROSS_COMPILE),)
> +CLANG_CROSS_FLAGS := --target=$(notdir $(CROSS_COMPILE:%-=%))
> +GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)gcc))
> +ifneq ($(GCC_TOOLCHAIN_DIR),)
> +CLANG_CROSS_FLAGS += --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
> +CLANG_CROSS_FLAGS += --sysroot=$(shell $(CROSS_COMPILE)gcc -print-sysroot)

^ this is a neat trick; it does rely on having $(CROSS_COMPILE)gcc
installed though. Technically only the header include path is needed
and the path to the cross libc shared object.  I think we should take
this patch&series as a starting point, but there might be future
revisions here to support environments that don't have
$(CROSS_COMPILE)gcc (such as Android's build env).

Acked-by: Nick Desaulniers <ndesaulniers@google.com>

Thanks for the series!

> +CLANG_CROSS_FLAGS += --gcc-toolchain=$(realpath $(GCC_TOOLCHAIN_DIR)/..)
> +endif # GCC_TOOLCHAIN_DIR
> +CFLAGS += $(CLANG_CROSS_FLAGS)
> +AFLAGS += $(CLANG_CROSS_FLAGS)
> +endif # CROSS_COMPILE
>
>  # Hack to avoid type-punned warnings on old systems such as RHEL5:
>  # We should be changing CFLAGS and checking gcc version, but this
> --
> 2.34.1
>


-- 
Thanks,
~Nick Desaulniers