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

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Date:   Wed, 1 Jul 2020 10:02:48 -0700
From:   Hao Luo <haoluo@...gle.com>
To:     Yonghong Song <yhs@...com>
Cc:     Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        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>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Bill Wendling <morbo@...gle.com>
Subject: Re: [PATCH bpf-next] selftests/bpf: Switch test_vmlinux to use hrtimer_range_start_ns.

On Tue, Jun 30, 2020 at 7:26 PM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 6/30/20 5:10 PM, Hao Luo wrote:
> > Ok, with the help of my colleague Ian Rogers, I think we solved the
> > mystery. Clang actually inlined hrtimer_nanosleep() inside
> > SyS_nanosleep(), so there is no call to that function throughout the
> > path of the nanosleep syscall. I've been looking at the function body
> > of hrtimer_nanosleep for quite some time, but clearly overlooked the
> > caller of hrtimer_nanosleep. hrtimer_nanosleep is pretty short and
> > there are many constants, inlining would not be too surprising.
>
> Oh thanks for explanation. inlining makes sense. We have many other
> instances like this in the past where kprobe won't work properly.
>
> Could you reword your commit message then?
>
>  > causing fentry and kprobe to not hook on this function properly on a
>  > Clang build kernel.
>
> The above is a little vague on what happens. What really happens is
> fentry/kprobe does hook on this function but has no effect since
> its caller has inlined the function.

Sure, sending a v2 with a more accurate description of the issue.

Hao