[PATCH bpf-next v2 1/8] bpftool: Pass host flags to bootstrap libbpf

[Leo Yan]

bpftool builds a bootstrap libbpf with HOSTCC, but the libbpf submake can
still inherit target build flags through CFLAGS. This can break cross
builds when host objects are compiled with target-only options.

Since HOST_CFLAGS contains warning options that are not suitable for
building libbpf, use LIBBPF_BOOTSTRAP_CFLAGS with the warning options
removed to build the bootstrap libbpf. Clear EXTRA_CFLAGS so target
extra flags are not mixed into the host bootstrap libbpf build.

Signed-off-by: Leo Yan <leo.yan@xxxxxxx>
---
 tools/bpf/bpftool/Makefile | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/tools/bpf/bpftool/Makefile b/tools/bpf/bpftool/Makefile
index 0febf60e1b64656035dfe93cd15cba090015587a..1f033e578f90a2d6b93168ab41399de758bdf659 100644
--- a/tools/bpf/bpftool/Makefile
+++ b/tools/bpf/bpftool/Makefile
@@ -47,7 +47,8 @@ $(LIBBPF_INTERNAL_HDRS): $(LIBBPF_HDRS_DIR)/%.h: $(BPF_DIR)/%.h | $(LIBBPF_HDRS_
 $(LIBBPF_BOOTSTRAP): $(wildcard $(BPF_DIR)/*.[ch] $(BPF_DIR)/Makefile) | $(LIBBPF_BOOTSTRAP_OUTPUT)
 	$(Q)$(MAKE) -C $(BPF_DIR) OUTPUT=$(LIBBPF_BOOTSTRAP_OUTPUT) \
 		DESTDIR=$(LIBBPF_BOOTSTRAP_DESTDIR:/=) prefix= \
-		ARCH= CROSS_COMPILE= CC="$(HOSTCC)" LD="$(HOSTLD)" AR="$(HOSTAR)" $@ install_headers
+		ARCH= CROSS_COMPILE= CC="$(HOSTCC)" LD="$(HOSTLD)" AR="$(HOSTAR)" \
+		CFLAGS="$(LIBBPF_BOOTSTRAP_CFLAGS)" EXTRA_CFLAGS= $@ install_headers
 
 $(LIBBPF_BOOTSTRAP_INTERNAL_HDRS): $(LIBBPF_BOOTSTRAP_HDRS_DIR)/%.h: $(BPF_DIR)/%.h | $(LIBBPF_BOOTSTRAP_HDRS_DIR)
 	$(call QUIET_INSTALL, $@)
@@ -92,6 +93,9 @@ HOST_CFLAGS := $(subst -I$(LIBBPF_INCLUDE),-I$(LIBBPF_BOOTSTRAP_INCLUDE),\
 		$(subst $(CLANG_CROSS_FLAGS),,$(CFLAGS)))
 HOST_LDFLAGS := $(LDFLAGS)
 
+# Remove warnings for libbpf bootstrap build
+LIBBPF_BOOTSTRAP_CFLAGS := $(filter-out -W -Wall -Wextra -Wformat -Wformat-signedness,$(HOST_CFLAGS))
+
 INSTALL ?= install
 RM ?= rm -f
 

-- 
2.34.1