linux-kernel - Re: [PATCH bpf-next] selftests/bpf: Switch test_vmlinux to use hrtimer_range_start

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Date:   Tue, 30 Jun 2020 14:26:21 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Hao Luo <haoluo@...gle.com>
Cc:     Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Shuah Khan <shuah@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andriin@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [PATCH bpf-next] selftests/bpf: Switch test_vmlinux to use hrtimer_range_start_ns.

On Tue, Jun 30, 2020 at 1:47 PM Hao Luo <haoluo@...gle.com> wrote:
>
> The test_vmlinux test uses hrtimer_nanosleep as hook to test tracing
> programs. But it seems Clang may have done an aggressive optimization,
> causing fentry and kprobe to not hook on this function properly on a
> Clang build kernel.
>
> A possible fix is switching to use a more reliable function, e.g. the
> ones exported to kernel modules such as hrtimer_range_start_ns. After
> we switch to using hrtimer_range_start_ns, the test passes again even
> on a clang build kernel.
>
> Tested:
>  In a clang build kernel, the test fail even when the flags
>  {fentry, kprobe}_called are set unconditionally in handle__kprobe()
>  and handle__fentry(), which implies the programs do not hook on
>  hrtimer_nanosleep() properly. This could be because clang's code
>  transformation is too aggressive.
>
>  test_vmlinux:PASS:skel_open 0 nsec
>  test_vmlinux:PASS:skel_attach 0 nsec
>  test_vmlinux:PASS:tp 0 nsec
>  test_vmlinux:PASS:raw_tp 0 nsec
>  test_vmlinux:PASS:tp_btf 0 nsec
>  test_vmlinux:FAIL:kprobe not called
>  test_vmlinux:FAIL:fentry not called
>
>  After we switch to hrtimer_range_start_ns, the test passes.
>
>  test_vmlinux:PASS:skel_open 0 nsec
>  test_vmlinux:PASS:skel_attach 0 nsec
>  test_vmlinux:PASS:tp 0 nsec
>  test_vmlinux:PASS:raw_tp 0 nsec
>  test_vmlinux:PASS:tp_btf 0 nsec
>  test_vmlinux:PASS:kprobe 0 nsec
>  test_vmlinux:PASS:fentry 0 nsec
>
> Signed-off-by: Hao Luo <haoluo@...gle.com>
> ---

Took me a bit of jumping around to find how it is related to nanosleep
call :) But seems like it's unconditionally called, so should be fine.

Acked-by: Andrii Nakryiko <andriin@...com>


>  tools/testing/selftests/bpf/progs/test_vmlinux.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
>

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