Re: [PATCH bpf v2] samples/bpf: remove unnecessary -I flags from libbpf EXTRA_CFLAGS

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2024-12-02 at 16:23 -0800, Stanislav Fomichev wrote:

[...[

> > diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
> > index bcf103a4c14f..44f7e05973de 100644
> > --- a/samples/bpf/Makefile
> > +++ b/samples/bpf/Makefile
> > @@ -146,13 +146,14 @@ ifeq ($(ARCH), x86)
> >  BPF_EXTRA_CFLAGS += -fcf-protection
> >  endif
> >  
> > -TPROGS_CFLAGS += -Wall -O2
> > -TPROGS_CFLAGS += -Wmissing-prototypes
> > -TPROGS_CFLAGS += -Wstrict-prototypes
> > -TPROGS_CFLAGS += $(call try-run,\
> > +COMMON_CFLAGS += -Wall -O2
> > +COMMON_CFLAGS += -Wmissing-prototypes
> > +COMMON_CFLAGS += -Wstrict-prototypes
> > +COMMON_CFLAGS += $(call try-run,\
> >  	printf "int main() { return 0; }" |\
> >  	$(CC) -Werror -fsanitize=bounds -x c - -o "$$TMP",-fsanitize=bounds,)
> >  
> > +TPROGS_CFLAGS += $(COMMON_CFLAGS)
> >  TPROGS_CFLAGS += -I$(objtree)/usr/include
> >  TPROGS_CFLAGS += -I$(srctree)/tools/testing/selftests/bpf/
> >  TPROGS_CFLAGS += -I$(LIBBPF_INCLUDE)
> > @@ -229,7 +230,7 @@ clean:
> >  
> >  $(LIBBPF): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(LIBBPF_OUTPUT)
> >  # Fix up variables inherited from Kbuild that tools/ build system won't like
> > -	$(MAKE) -C $(LIBBPF_SRC) RM='rm -rf' EXTRA_CFLAGS="$(TPROGS_CFLAGS)" \
> > +	$(MAKE) -C $(LIBBPF_SRC) RM='rm -rf' EXTRA_CFLAGS="$(COMMON_CFLAGS)" \
> >  		LDFLAGS="$(TPROGS_LDFLAGS)" srctree=$(BPF_SAMPLES_PATH)/../../ \
> >  		O= OUTPUT=$(LIBBPF_OUTPUT)/ DESTDIR=$(LIBBPF_DESTDIR) prefix= \
> >  		$@ install_headers
> > -- 
> > 2.47.0
> > 
> 
> Naive question: why pass EXTRA_CFLAGS to libbpf at all? Can we drop it?

This was added by the commit [0].
As far as I understand, the idea is to pass the following flags:

    ifeq ($(ARCH), arm)
    # Strip all except -D__LINUX_ARM_ARCH__ option needed to handle linux
    # headers when arm instruction set identification is requested.
    ARM_ARCH_SELECTOR := $(filter -D__LINUX_ARM_ARCH__%, $(KBUILD_CFLAGS))
    ...
    TPROGS_CFLAGS += $(ARM_ARCH_SELECTOR)
    endif

    ifeq ($(ARCH), mips)
    TPROGS_CFLAGS += -D__SANE_USERSPACE_TYPES__
    ...
    endif

Not sure if these are still necessary.

[0] commit d8ceae91e9f0 ("samples/bpf: Provide C/LDFLAGS to libbpf")