Re: [PATCH bpf-next v3 4/5] selftests/bpf: silence clang compilation warnings

[Yonghong Song <yhs@fb.com>]

On 4/13/21 3:17 PM, Nick Desaulniers wrote:
> On Tue, Apr 13, 2021 at 3:08 PM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
>>
>> On Tue, Apr 13, 2021 at 8:34 AM Yonghong Song <yhs@fb.com> wrote:
>>>
>>> With clang compiler:
>>>    make -j60 LLVM=1 LLVM_IAS=1  <=== compile kernel
>>>    make -j60 -C tools/testing/selftests/bpf LLVM=1 LLVM_IAS=1
>>> Some linker flags are not used/effective for some binaries and
>>> we have warnings like:
>>>    warning: -lelf: 'linker' input unused [-Wunused-command-line-argument]
>>>
>>> We also have warnings like:
>>>    .../selftests/bpf/prog_tests/ns_current_pid_tgid.c:74:57: note: treat the string as an argument to avoid this
>>>          if (CHECK(waitpid(cpid, &wstatus, 0) == -1, "waitpid", strerror(errno)))
>>>                                                                 ^
>>>                                                                 "%s",
>>>    .../selftests/bpf/test_progs.h:129:35: note: expanded from macro 'CHECK'
>>>          _CHECK(condition, tag, duration, format)
>>>                                           ^
>>>    .../selftests/bpf/test_progs.h:108:21: note: expanded from macro '_CHECK'
>>>                  fprintf(stdout, ##format);                              \
>>>                                    ^
>>> The first warning can be silenced with clang option -Wno-unused-command-line-argument.
>>> For the second warning, source codes are modified as suggested by the compiler
>>> to silence the warning. Since gcc does not support the option
>>> -Wno-unused-command-line-argument and the warning only happens with clang
>>> compiler, the option -Wno-unused-command-line-argument is enabled only when
>>> clang compiler is used.
>>>
>>> Signed-off-by: Yonghong Song <yhs@fb.com>
>>> ---
>>
>> LGTM, please see the question below.
>>
>> Acked-by: Andrii Nakryiko <andrii@kernel.org>
>>
>>>   tools/testing/selftests/bpf/Makefile                         | 5 +++++
>>>   tools/testing/selftests/bpf/prog_tests/fexit_sleep.c         | 4 ++--
>>>   tools/testing/selftests/bpf/prog_tests/ns_current_pid_tgid.c | 4 ++--
>>>   3 files changed, 9 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
>>> index dcc2dc1f2a86..c45ae13b88a0 100644
>>> --- a/tools/testing/selftests/bpf/Makefile
>>> +++ b/tools/testing/selftests/bpf/Makefile
>>> @@ -28,6 +28,11 @@ CFLAGS += -g -Og -rdynamic -Wall $(GENFLAGS) $(SAN_CFLAGS)           \
>>>            -Dbpf_load_program=bpf_test_load_program
>>>   LDLIBS += -lcap -lelf -lz -lrt -lpthread
>>>
>>> +# Silence some warnings when compiled with clang
>>> +ifneq ($(LLVM),)
>>
>> This won't handle the case where someone does `make CC=clang`, right?
>> Do we care at all?
> 
> Right; it would be better to check CC=clang and have LLVM=1 enable
> CC=clang from a higher level Makefile (since tools/build/Makefile
> seems to override the top level $CC). See the top level Makefile
> L448-456.  Then it should work for CC=clang or LLVM=1.

I would consider LLVM=1 is recommended and should be supported
first. Support for CC=clang CXX=clang++ etc. can be done later
if people find cases where LLVM=1 does not fit their need.

> 
>>
>>
>>> +CFLAGS += -Wno-unused-command-line-argument
>>> +endif
>>> +
>>
>> [...]
> 
> 
>