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

[Quentin Monnet]

2026-06-02 15:47 UTC+0100 ~ Leo Yan <leo.yan@xxxxxxx>
> 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

Nitpick: We could expand this comment to briefly explain why. Not sure
it's worth a respin, though.

> +LIBBPF_BOOTSTRAP_CFLAGS := $(filter-out -W -Wall -Wextra -Wformat -Wformat-signedness,$(HOST_CFLAGS))

Do we need to remove all warnings, or would it be sufficient to take out
-Wformat and -Wformat-signedness? I don't mind removing them all if
necessary, just asking.

Acked-by: Quentin Monnet <qmo@xxxxxxxxxx>

Thanks!