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

[Stanislav Fomichev <stfomichev@gmail.com>]

On 12/02, Eduard Zingerman wrote:
> Commit [0] breaks samples/bpf build:
> 
>     $ make M=samples/bpf
>     ...
>     make -C /path/to/kernel/samples/bpf/../../tools/lib/bpf \
>      ...
>      EXTRA_CFLAGS=" \
>      ...
>      -fsanitize=bounds \
>      -I/path/to/kernel/usr/include \
>      ...
>     	/path/to/kernel/samples/bpf/libbpf/libbpf.a install_headers
>       CC      /path/to/kernel/samples/bpf/libbpf/staticobjs/libbpf.o
>     In file included from libbpf.c:29:
>     /path/to/kernel/tools/include/linux/err.h:35:8: error: 'inline' can only appear on functions
>        35 | static inline void * __must_check ERR_PTR(long error_)
>           |        ^
> 
> The error is caused by `objtree` variable changing definition from `.`
> (dot) to an absolute path:
> - The variable TPROGS_CFLAGS is constructed as follows:
>   ...
>   TPROGS_CFLAGS += -I$(objtree)/usr/include
> - It is passed as EXTRA_CFLAGS for libbpf compilation:
>   $(LIBBPF): ...
>     ...
> 	$(MAKE) -C $(LIBBPF_SRC) RM='rm -rf' EXTRA_CFLAGS="$(TPROGS_CFLAGS)"
> - Before commit [0], the line passed to libbpf makefile was
>   '-I./usr/include', where '.' referred to LIBBPF_SRC due to -C flag.
>   The directory $(LIBBPF_SRC)/usr/include does not exist and thus
>   was never resolved by C compiler.
> - After commit [0], the line passed to libbpf makefile became:
>   '<output-dir>/usr/include', this directory exists and is resolved by
>   C compiler.
> - Both 'tools/include' and 'usr/include' define files err.h and types.h.
> - libbpf expects headers like 'linux/err.h' and 'linux/types.h'
>   defined in 'tools/include', not 'usr/include', hence the compilation
>   error.
> 
> This commit removes unnecessary -I flags from libbpf compilation.
> (libbpf sets up the necessary includes at lib/bpf/Makefile:63).
> 
> Changes v1 [1] -> v2:
> - dropped unnecessary replacement of KBUILD_OUTPUT with $(objtree)
> 
> [0] commit 13b25489b6f8 ("kbuild: change working directory to external module directory with M=")
> [1] https://lore.kernel.org/bpf/20241202212154.3174402-1-eddyz87@gmail.com/
> 
> Fixes: 13b25489b6f8 ("kbuild: change working directory to external module directory with M=")
> 
> Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
> ---
>  samples/bpf/Makefile | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
> index bcf103a4c14f..44f7e05973de 100644
> --- a/samples/bpf/Makefile
> +++ b/samples/bpf/Makefile
> @@ -146,13 +146,14 @@ ifeq ($(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?