Re: [PATCH bpf-next v2 0/5] bpf: tools: support build selftests/bpf with clang

[Yonghong Song <yhs@fb.com>]

On 4/12/21 4:58 PM, Nick Desaulniers wrote:
> On Mon, Apr 12, 2021 at 7:29 AM Yonghong Song <yhs@fb.com> wrote:
>>
>> To build kernel with clang, people typically use
>>    make -j60 LLVM=1 LLVM_IAS=1
>> LLVM_IAS=1 is not required for non-LTO build but
>> is required for LTO build. In my environment,
>> I am always having LLVM_IAS=1 regardless of
>> whether LTO is enabled or not.
>>
>> After kernel is build with clang, the following command
>> can be used to build selftests with clang:
>>    make -j60 -C tools/testing/selftests/bpf LLVM=1 LLVM_IAS=1
> 
> Thank you for the series Yonghong.  When I test the above command with
> your series applied, I observe:
> /usr/bin/ld: cannot find -lcap
> clang-13: error: linker command failed with exit code 1 (use -v to see
> invocation)
> 
> I need to install libcap-dev, but this also seems to imply that BFD is
> being used as the linker, not LLD.  Perhaps if the compiler is being
> used as the "driver" to also link executables, `-fuse-ld=lld` is
> needed for the compiler flags.

Yes, bfd is needed to build selftests/bpf. This is due to a dependency
on bpftool which needs uses libbfd to disassemble the jited code.

> 
> Then there's:
> tools/include/tools/libc_compat.h:11:21: error: static declaration of
> 'reallocarray' follows non-static declaration
> static inline void *reallocarray(void *ptr, size_t nmemb, size_t size)
>                      ^
> /usr/include/stdlib.h:559:14: note: previous declaration is here
> extern void *reallocarray (void *__ptr, size_t __nmemb, size_t __size)
>               ^
> so perhaps the detection of
> COMPAT_NEED_REALLOCARRAY/feature-reallocarray is incorrect?

libbpf already stopped to use system reallocarray(), but
bpftool still uses it.

In bpftool makefile, we have

ifeq ($(feature-reallocarray), 0)
CFLAGS += -DCOMPAT_NEED_REALLOCARRAY
endif

I guess probably detection of feature-reallocarray is
not correct? Could you take a look at your system?
My system supports reallocarray, so the above
-DCOMPAT_NEED_REALLOCARRAY is not added to
compilation flags.

> 
> 
>>
>> But currently, using the above command, some compilations
>> still use gcc and there are also compilation errors and warnings.
>> This patch set intends to fix these issues.
>> Patch #1 and #2 fixed the issue so clang/clang++ is
>> used instead of gcc/g++. Patch #3 fixed a compilation
>> failure. Patch #4 and #5 fixed various compiler warnings.
>>
>> Changelog:
>>    v1 -> v2:
>>      . add -Wno-unused-command-line-argument and -Wno-format-security
>>        for clang only as (1). gcc does not exhibit those
>>        warnings, and (2). -Wno-unused-command-line-argument is
>>        only supported by clang. (Sedat)
>>
>> Yonghong Song (5):
>>    selftests: set CC to clang in lib.mk if LLVM is set
>>    tools: allow proper CC/CXX/... override with LLVM=1 in
>>      Makefile.include
>>    selftests/bpf: fix test_cpp compilation failure with clang
>>    selftests/bpf: silence clang compilation warnings
>>    bpftool: fix a clang compilation warning
>>
>>   tools/bpf/bpftool/net.c              |  2 +-
>>   tools/scripts/Makefile.include       | 12 ++++++++++--
>>   tools/testing/selftests/bpf/Makefile |  7 ++++++-
>>   tools/testing/selftests/lib.mk       |  4 ++++
>>   4 files changed, 21 insertions(+), 4 deletions(-)
>>
>> --
>> 2.30.2
>>
> 
> 
> --
> Thanks,
> ~Nick Desaulniers
>