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

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Date:   Tue, 16 Jun 2020 16:14:39 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
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 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.

> Thanks,
> Daniel