netdev - Re: [PATCH bpf 2/2] selftests/bpf: add variable-length data concatenation pattern test

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Date:   Wed, 17 Jun 2020 17:51:49 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Kernel Team <kernel-team@...com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH bpf 2/2] selftests/bpf: add variable-length data
 concatenation pattern test

On 6/17/20 1:14 AM, Andrii Nakryiko wrote:
> On Tue, Jun 16, 2020 at 3:23 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>> On 6/16/20 11:27 PM, Andrii Nakryiko wrote:
>>> On Tue, Jun 16, 2020 at 1:21 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>>>> On 6/16/20 7:04 AM, Andrii Nakryiko wrote:
>>>>> Add selftest that validates variable-length data reading and concatentation
>>>>> with one big shared data array. This is a common pattern in production use for
>>>>> monitoring and tracing applications, that potentially can read a lot of data,
>>>>> but usually reads much less. Such pattern allows to determine precisely what
>>>>> amount of data needs to be sent over perfbuf/ringbuf and maximize efficiency.
>>>>>
>>>>> This is the first BPF selftest that at all looks at and tests
>>>>> bpf_probe_read_str()-like helper's return value, closing a major gap in BPF
>>>>> testing. It surfaced the problem with bpf_probe_read_kernel_str() returning
>>>>> 0 on success, instead of amount of bytes successfully read.
>>>>>
>>>>> Signed-off-by: Andrii Nakryiko <andriin@...com>
>>>>
>>>> Fix looks good, but I'm seeing an issue in the selftest on my side. With latest
>>>> Clang/LLVM I'm getting:
>>>>
>>>> # ./test_progs -t varlen
>>>> #86 varlen:OK
>>>> Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED
>>>>
>>>> All good, however, the test_progs-no_alu32 fails for me with:
>>>
>>> Yeah, same here. It's due to Clang emitting unnecessary bit shifts
>>> because bpf_probe_read_kernel_str() is defined as returning 32-bit
>>> int. I have a patch ready locally, just waiting for bpf-next to open,
>>> which switches those helpers to return long, which auto-matically
>>> fixes this test.
>>>
>>> If it's not a problem, I'd just wait for that patch to go into
>>> bpf-next. If not, I can sprinkle bits of assembly magic around to
>>> force the kernel to do those bitshifts earlier. But I figured having
>>> test_progs-no_alu32 failing one selftest temporarily wasn't too bad.
>>
>> Given {net,bpf}-next will open up soon, another option could be to take in the fix
>> itself to bpf and selftest would be submitted together with your other improvement;
>> any objections?
> 
> Yeah, no objections.

Sounds good, done.