Re: [PATCHv3 bpf-next 7/7] selftests/bpf: Add test for bpf_get_func_ip in kprobe+offset probe

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Jiri Olsa <jolsa@redhat.com>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andriin@fb.com>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@chromium.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Alan Maguire <alan.maguire@oracle.com>
Subject: Re: [PATCHv3 bpf-next 7/7] selftests/bpf: Add test for bpf_get_func_ip in kprobe+offset probe
Date: Mon, 12 Jul 2021 16:32:25 -0700	[thread overview]
Message-ID: <CAEf4BzYQfe6-UngVn=kTE9gg6Gc7HFdDQ2NGX7p0+uuO27RETA@mail.gmail.com> (raw)
In-Reply-To: <YOsEsb1sMasi1WyR@krava>

On Sun, Jul 11, 2021 at 7:48 AM Jiri Olsa <jolsa@redhat.com> wrote:
>
> On Wed, Jul 07, 2021 at 05:18:49PM -0700, Andrii Nakryiko wrote:
> > On Wed, Jul 7, 2021 at 2:54 PM Jiri Olsa <jolsa@redhat.com> wrote:
> > >
> > > Adding test for bpf_get_func_ip in kprobe+ofset probe.
> >
> > typo: offset
> >
> > > Because of the offset value it's arch specific, adding
> > > it only for x86_64 architecture.
> >
> > I'm not following, you specified +0x5 offset explicitly, why is this
> > arch-specific?
>
> I need some instruction offset != 0 in the traced function,
> x86_64's fentry jump is 5 bytes, other archs will be different

Right, ok. I don't see an easy way to detect this offset, but the
#ifdef __x86_64__ detection doesn't work because we are compiling with
-target bpf. Please double-check that it actually worked in the first
place.

I think a better way would be to have test6 defined unconditionally in
BPF code, but then disable loading test6 program on anything but
x86_64 platform at runtime with bpf_program__set_autoload(false).

>
> >
> > >
> > > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > > ---
> > >  .../testing/selftests/bpf/progs/get_func_ip_test.c  | 13 +++++++++++++
> > >  1 file changed, 13 insertions(+)
> > >
> > > diff --git a/tools/testing/selftests/bpf/progs/get_func_ip_test.c b/tools/testing/selftests/bpf/progs/get_func_ip_test.c
> > > index 8ca54390d2b1..e8a9428a0ea3 100644
> > > --- a/tools/testing/selftests/bpf/progs/get_func_ip_test.c
> > > +++ b/tools/testing/selftests/bpf/progs/get_func_ip_test.c
> > > @@ -10,6 +10,7 @@ extern const void bpf_fentry_test2 __ksym;
> > >  extern const void bpf_fentry_test3 __ksym;
> > >  extern const void bpf_fentry_test4 __ksym;
> > >  extern const void bpf_modify_return_test __ksym;
> > > +extern const void bpf_fentry_test6 __ksym;
> > >
> > >  __u64 test1_result = 0;
> > >  SEC("fentry/bpf_fentry_test1")
> > > @@ -60,3 +61,15 @@ int BPF_PROG(fmod_ret_test, int a, int *b, int ret)
> > >         test5_result = (const void *) addr == &bpf_modify_return_test;
> > >         return ret;
> > >  }
> > > +
> > > +#ifdef __x86_64__
> > > +__u64 test6_result = 0;
> >
> > see, and you just forgot to update the user-space part of the test to
> > even check test6_result...
> >
> > please group variables together and do explicit ASSERT_EQ
>
> right.. will change
>
> thanks,
> jirka
>
> >
> > > +SEC("kprobe/bpf_fentry_test6+0x5")
> > > +int test6(struct pt_regs *ctx)
> > > +{
> > > +       __u64 addr = bpf_get_func_ip(ctx);
> > > +
> > > +       test6_result = (const void *) addr == &bpf_fentry_test6 + 5;
> > > +       return 0;
> > > +}
> > > +#endif
> > > --
> > > 2.31.1
> > >
> >
>