Re: [PATCH bpf-next v2 2/3] bpftool: Prevent setting duplicate _GNU_SOURCE in Makefile

[Jiri Olsa <olsajiri@gmail.com>]

On Fri, Oct 18, 2024 at 08:49:00AM +0200, Viktor Malik wrote:
> When building selftests with CFLAGS set via env variable, the value of
> CFLAGS is propagated into bpftool Makefile (called from selftests
> Makefile). This makes the compilation fail as _GNU_SOURCE is defined two
> times - once from selftests Makefile (by including lib.mk) and once from
> bpftool Makefile (by calling `llvm-config --cflags`):
> 
>     $ CFLAGS="" make -C tools/testing/selftests/bpf
>     [...]
>     CC      /bpf-next/tools/testing/selftests/bpf/tools/build/bpftool/btf.o
>     <command-line>: error: "_GNU_SOURCE" redefined [-Werror]
>     <command-line>: note: this is the location of the previous definition
>     cc1: all warnings being treated as errors
>     [...]
> 
> Filter out -D_GNU_SOURCE from the result of `llvm-config --cflags` in
> bpftool Makefile to prevent this error.
> 
> Signed-off-by: Viktor Malik <vmalik@redhat.com>

Acked-by: Jiri Olsa <jolsa@kernel.org>

jirka

> ---
>  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 ba927379eb20..a4263dfb5e03 100644
> --- a/tools/bpf/bpftool/Makefile
> +++ b/tools/bpf/bpftool/Makefile
> @@ -147,7 +147,11 @@ ifeq ($(feature-llvm),1)
>    # If LLVM is available, use it for JIT disassembly
>    CFLAGS  += -DHAVE_LLVM_SUPPORT
>    LLVM_CONFIG_LIB_COMPONENTS := mcdisassembler all-targets
> -  CFLAGS  += $(shell $(LLVM_CONFIG) --cflags)
> +  # llvm-config always adds -D_GNU_SOURCE, however, it may already be in CFLAGS
> +  # (e.g. when bpftool build is called from selftests build as selftests
> +  # Makefile includes lib.mk which sets -D_GNU_SOURCE) which would cause
> +  # compilation error due to redefinition. Let's filter it out here.
> +  CFLAGS  += $(filter-out -D_GNU_SOURCE,$(shell $(LLVM_CONFIG) --cflags))
>    LIBS    += $(shell $(LLVM_CONFIG) --libs $(LLVM_CONFIG_LIB_COMPONENTS))
>    ifeq ($(shell $(LLVM_CONFIG) --shared-mode),static)
>      LIBS += $(shell $(LLVM_CONFIG) --system-libs $(LLVM_CONFIG_LIB_COMPONENTS))
> -- 
> 2.47.0
>