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Date:   Tue, 24 Sep 2019 13:48:23 -0600
From:   Shuah Khan <>
To:     Daniel Borkmann <>
Cc:     Yonghong Song <>,
        Alexei Starovoitov <>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <>, bpf <>,
        Networking <>,
        "David S. Miller" <>,
        Andrii Nakryiko <>,
        "skh >> Shuah Khan" <>
Subject: Re: Linux 5.4 - bpf test build fails

On 9/24/19 1:19 PM, Daniel Borkmann wrote:
> On Tue, Sep 24, 2019 at 12:56:53PM -0600, Shuah Khan wrote:
>> On 9/24/19 12:49 PM, Daniel Borkmann wrote:
>>> On Tue, Sep 24, 2019 at 09:48:35AM -0600, Shuah Khan wrote:
>>>> On 9/24/19 9:43 AM, Yonghong Song wrote:
>>>>> On 9/24/19 8:26 AM, Shuah Khan wrote:
>>>>>> Hi Alexei and Daniel,
>>>>>> bpf test doesn't build on Linux 5.4 mainline. Do you know what's
>>>>>> happening here.
>>>>>> make -C tools/testing/selftests/bpf/
>>>>>> -c progs/test_core_reloc_ptr_as_arr.c -o - || echo "clang failed") | \
>>>>>> llc -march=bpf -mcpu=generic  -filetype=obj -o
>>>>>> /mnt/data/lkml/linux_5.4/tools/testing/selftests/bpf/test_core_reloc_ptr_as_arr.o
>>>>>> progs/test_core_reloc_ptr_as_arr.c:25:6: error: use of unknown builtin
>>>>>>           '__builtin_preserve_access_index' [-Wimplicit-function-declaration]
>>>>>>             if (BPF_CORE_READ(&out->a, &in[2].a))
>>>>>>                 ^
>>>>>> ./bpf_helpers.h:533:10: note: expanded from macro 'BPF_CORE_READ'
>>>>>>                            __builtin_preserve_access_index(src))
>>>>>>                            ^
>>>>>> progs/test_core_reloc_ptr_as_arr.c:25:6: warning: incompatible integer to
>>>>>>           pointer conversion passing 'int' to parameter of type 'const void *'
>>>>>>           [-Wint-conversion]
>>>>>>             if (BPF_CORE_READ(&out->a, &in[2].a))
>>>>>>                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>>>> ./bpf_helpers.h:533:10: note: expanded from macro 'BPF_CORE_READ'
>>>>>>                            __builtin_preserve_access_index(src))
>>>>>>                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>>>> 1 warning and 1 error generated.
>>>>>> llc: error: llc: <stdin>:1:1: error: expected top-level entity
>>>>>> clang failed
>>>>>> Also
>>>>>> make TARGETS=bpf kselftest fails as well. Dependency between
>>>>>> tools/lib/bpf and the test. How can we avoid this type of
>>>>>> dependency or resolve it in a way it doesn't result in build
>>>>>> failures?
>>>>> Thanks, Shuah.
>>>>> The clang __builtin_preserve_access_index() intrinsic is
>>>>> introduced in LLVM9 (which just released last week) and
>>>>> the builtin and other CO-RE features are only supported
>>>>> in LLVM10 (current development branch) with more bug fixes
>>>>> and added features.
>>>>> I think we should do a feature test for llvm version and only
>>>>> enable these tests when llvm version >= 10.
>>>> Yes. If new tests depend on a particular llvm revision, the failing
>>>> the build is a regression. I would like to see older tests that don't
>>>> have dependency build and run.
>>> So far we haven't made it a requirement as majority of BPF contributors
>>> that would run/add tests in here are also on bleeding edge LLVM anyway
>>> and other CIs like 0-day bot have simply upgraded their LLVM version
>>> from git whenever there was a failure similar to the one here so its
>>> ensured that really /all/ test cases are running and nothing would be
>>> skipped. There is worry to some degree that CIs just keep sticking to
>>> an old compiler since tests "just" pass and regressions wouldn't be
>>> caught on new releases for those that are skipped. >
>> Sure. Bleeding edge is developer mode. We still have to be concerned
>> about users that might not upgrade quickly.
>>> That said, for the C based tests, it should actually be straight forward
>>> to categorize them based on built-in macros like ...
>>> $ echo | clang -dM -E -
>>> [...]
>>> #define __clang_major__ 10
>>> #define __clang_minor__ 0
>>> [...]
>> What would nice running the tests you can run and then say some tests
>> aren't going to run. Is this something you can support?
> Once there is such infra in place, should be possible.

Can't you do it in bpf run-time or during build for dependency?
You should be able to handle this as a dependency and let users
know at least.

>>> ... given there is now also bpf-gcc, the test matrix gets bigger anyway,
>>> so it might be worth rethinking to run the suite multiple times with
>>> different major llvm{,gcc} versions at some point to make sure their
>>> generated BPF bytecode keeps passing the verifier, and yell loudly if
>>> newer features had to be skipped due to lack of recent compiler version.
>>> This would be a super set of /just/ skipping tests and improve coverage
>>> at the same time.
>> Probably. Reality is most users will just quit and add bpf to "hard to
>> run category" of tests.
> I don't really worry too much about such users at this point, more important
> is that we have a way to test bpf-gcc and llvm behavior side by side to
> make sure behavior is consistent and to have some sort of automated CI
> integration that runs BPF kselftests before we even stare at a patch for
> review. These are right now the two highest prio items from BPF testing
> side where we need to get to.

What happens if CI's can't upgrade quickly and newer versions aren't
supported on test machines that are in their test rings?

-- Shuah

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