Re: [PATCH bpf-next v2 1/6] tools: Help cross-building with clang
From: Nick Desaulniers <ndesaulniers@google.com>
Date: 2021-12-16 19:17:12
Also in:
bpf, linux-kselftest
On Thu, Dec 16, 2021 at 8:50 AM Jean-Philippe Brucker [off-list ref] wrote:
quoted hunk ↗ jump to hunk
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 <redacted> Signed-off-by: Jean-Philippe Brucker <redacted> --- 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