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

[Daniel Borkmann <daniel@iogearbox.net>]

On 6/16/20 11:27 PM, Andrii Nakryiko wrote:
> On Tue, Jun 16, 2020 at 1:21 PM Daniel Borkmann <daniel@iogearbox.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@fb.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?

Thanks,
Daniel