Re: [PATCH bpf-next v3 0/5] bpf: tools: support build selftests/bpf with clang

[Yonghong Song <yhs@fb.com>]

On 4/15/21 5:21 PM, Alexei Starovoitov wrote:
> On 4/13/21 8:34 AM, Yonghong Song 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
>>
>> I am using latest bpf-next kernel code base and
>> latest clang built from source from
>>    https://github.com/llvm/llvm-project.git
>> Using earlier version of llvm may have compilation errors, see
>>    tools/testing/selftests/bpf
>> due to continuous development in llvm bpf features and selftests
>> to use these features.
>>
>> To run bpf selftest properly, you need have certain necessary
>> kernel configs like at:
>>    bpf-next:tools/testing/selftests/bpf/config
>> (not that this is not a complete .config file and some other configs
>>   might still be needed.)
>>
>> 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:
>>    v2 -> v3:
>>      . more test environment description in cover letter. (Sedat)
>>      . use a different fix, but similar to other use in selftests/bpf
>>        Makefile, to exclude header files from CXX compilation command
>>        line. (Andrii)
>>      . fix codes instead of adding -Wno-format-security. (Andrii)
> 
> I struggled to tweak my llvm setup, but at the end it compiled and
> selftests/bpf/test_progs passed compiled by clang,
> so I've applied to bpf-next.
> 
> The things I've seen:
> 1.
> include <iostream> not found due to my setup quirks.

This header file is included by test_cpp.cpp. Probably
some clang stdc++ setup issue.

> 2.
> diff selftests/bpf/tools/build/libbpf/libbpf_global_syms.tmp
> diff selftests/bpf/tools/build/libbpf/libbpf_versioned_syms.tmp
> btf__set_pointer_size
> btf__str_by_offset
> btf__type_by_id
> +LIBBPF_0.0.1
> +LIBBPF_0.0.2
> and this was happening with packaged llvm builds,
> but my own llvm build was fine, so I didn't debug further.

I am also use latest llvm as I can easily verify all selftests passed.

> 
> 3.
> clang-12: error: unsupported option '-mrecord-mcount' for target 
> 'x86_64-unknown-linux-gnu'
> due to kernel not built with clang.

Top level Makefile has
Makefile:  CC_FLAGS_FTRACE      += -mrecord-mcount
tools Makefile has some dependency on top level Makefile.

> 
> I suspect followups will be needed to make it bulletproof.

Definitely, Nick has libz issues, cross compile is not supported,
tools build depends on kernel build, both or neither with LLVM=1, ...
clang build got some traction recently and hopefully more people
can start to contribute to make the build process more robust.