Re: [PATCH bpf] selftests/bpf: Filter out _GNU_SOURCE when compiling test_cpp

[Jiri Olsa <olsajiri@gmail.com>]

On Thu, Jul 25, 2024 at 02:40:29PM -0700, Stanislav Fomichev wrote:
> Jakub reports build failures when merging linux/master with net tree:
> 
> CXX      test_cpp
> In file included from <built-in>:454:
> <command line>:2:9: error: '_GNU_SOURCE' macro redefined [-Werror,-Wmacro-redefined]
>     2 | #define _GNU_SOURCE
>       |         ^
> <built-in>:445:9: note: previous definition is here
>   445 | #define _GNU_SOURCE 1
> 
> The culprit is commit cc937dad85ae ("selftests: centralize -D_GNU_SOURCE= to
> CFLAGS in lib.mk") which unconditionally added -D_GNU_SOUCE to CLFAGS.
> Apparently clang++ also unconditionally adds it for the C++ targets [0]
> which causes a conflict. Add small change in the selftests makefile
> to filter it out for test_cpp.
> 
> Not sure which tree it should go via, targeting bpf for now, but net
> might be better?
> 
> 0: https://stackoverflow.com/questions/11670581/why-is-gnu-source-defined-by-default-and-how-to-turn-it-off
> 
> Cc: Jakub Kicinski <kuba@kernel.org>
> Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
> ---
>  tools/testing/selftests/bpf/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
> index dd49c1d23a60..81d4757ecd4c 100644
> --- a/tools/testing/selftests/bpf/Makefile
> +++ b/tools/testing/selftests/bpf/Makefile
> @@ -713,7 +713,7 @@ $(OUTPUT)/xdp_features: xdp_features.c $(OUTPUT)/network_helpers.o $(OUTPUT)/xdp
>  # Make sure we are able to include and link libbpf against c++.
>  $(OUTPUT)/test_cpp: test_cpp.cpp $(OUTPUT)/test_core_extern.skel.h $(BPFOBJ)
>  	$(call msg,CXX,,$@)
> -	$(Q)$(CXX) $(CFLAGS) $(filter %.a %.o %.cpp,$^) $(LDLIBS) -o $@
> +	$(Q)$(CXX) $(subst -D_GNU_SOURCE=,,$(CFLAGS)) $(filter %.a %.o %.cpp,$^) $(LDLIBS) -o $@

nit, seems like we use filter-out for cases like that (but just one instance of -static option)
anyway the fix works for me

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

jirka

>  
>  # Benchmark runner
>  $(OUTPUT)/bench_%.o: benchs/bench_%.c bench.h $(BPFOBJ)
> -- 
> 2.45.2
>