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

[Viktor Malik <vmalik@redhat.com>]

On 10/15/24 12:03, Quentin Monnet wrote:
> 2024-10-15 08:54 UTC+0200 ~ Viktor Malik <vmalik@redhat.com>
>> 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
>>      [...]
>>
>> Let bpftool Makefile check if _GNU_SOURCE is already defined and if so,
>> do not let llvm-config add it again.
>>
>> Signed-off-by: Viktor Malik <vmalik@redhat.com>
>> ---
>>   tools/bpf/bpftool/Makefile | 8 +++++++-
>>   1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/tools/bpf/bpftool/Makefile b/tools/bpf/bpftool/Makefile
>> index ba927379eb20..2b5a713d71d8 100644
>> --- a/tools/bpf/bpftool/Makefile
>> +++ b/tools/bpf/bpftool/Makefile
>> @@ -147,7 +147,13 @@ 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)
>> +  # When bpftool build is called from another Makefile which already sets
>> +  # -D_GNU_SOURCE, do not let llvm-config add it again as it will cause conflict.
> 
> 
> Thanks! Can you please make your comment more explicit and mention the 
> file tools/testing/selftests/lib.mk and/or the use case addressed 
> (building bpftool from selftests), given that you match on the exact 
> string "-D_GNU_SOURCE="? Your check won't skip adding the duplicate 
> definition if someone passes "-D_GNU_SOURCE", without the "=", by 
> calling the Makefile from another path; that's fine, but I don't want 
> users to read the Makefile and expect it to remove the second definition 
> in such a case.

Right, that's a good point, I'll expand the comment.

Thanks!

> 
> 
>> +  ifneq ($(filter -D_GNU_SOURCE=,$(CFLAGS)),)
>> +    CFLAGS += $(filter-out -D_GNU_SOURCE,$(shell $(LLVM_CONFIG) --cflags))
>> +  else
>> +    CFLAGS += $(shell $(LLVM_CONFIG) --cflags)
>> +  endif
> 
> Looks good otherwise:
> 
> Reviewed-by: Quentin Monnet <qmo@kernel.org>
>