netdev - Re: [PATCH bpf-next] selftests/bpf: Fix test_attach

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Message-ID: <c18d94d3-3e5d-08f7-a8ba-f13bfa7eec05@fb.com>
Date:   Tue, 2 Mar 2021 09:19:49 -0800
From:   Yonghong Song <yhs@...com>
To:     Jiri Olsa <jolsa@...hat.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
CC:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Yauheni Kaliuta <ykaliuta@...hat.com>
Subject: Re: [PATCH bpf-next] selftests/bpf: Fix test_attach_probe for powerpc
 uprobes



On 3/2/21 3:14 AM, Jiri Olsa wrote:
> On Mon, Mar 01, 2021 at 04:34:24PM -0800, Andrii Nakryiko wrote:
>> On Mon, Mar 1, 2021 at 11:11 AM Jiri Olsa <jolsa@...nel.org> wrote:
>>>
>>> When testing uprobes we the test gets GEP (Global Entry Point)
>>> address from kallsyms, but then the function is called locally
>>> so the uprobe is not triggered.
>>>
>>> Fixing this by adjusting the address to LEP (Local Entry Point)
>>> for powerpc arch.
>>>
>>> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
>>> ---
>>>   .../selftests/bpf/prog_tests/attach_probe.c    | 18 +++++++++++++++++-
>>>   1 file changed, 17 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/tools/testing/selftests/bpf/prog_tests/attach_probe.c b/tools/testing/selftests/bpf/prog_tests/attach_probe.c
>>> index a0ee87c8e1ea..c3cfb48d3ed0 100644
>>> --- a/tools/testing/selftests/bpf/prog_tests/attach_probe.c
>>> +++ b/tools/testing/selftests/bpf/prog_tests/attach_probe.c
>>> @@ -2,6 +2,22 @@
>>>   #include <test_progs.h>
>>>   #include "test_attach_probe.skel.h"
>>>
>>> +#if defined(__powerpc64__)
>>> +/*
>>> + * We get the GEP (Global Entry Point) address from kallsyms,
>>> + * but then the function is called locally, so we need to adjust
>>> + * the address to get LEP (Local Entry Point).
>>> + */
>>> +#define LEP_OFFSET 8
>>> +
>>> +static ssize_t get_offset(ssize_t offset)
>>
>> if we mark this function __weak global, would it work as is? Would it
>> get an address of a global entry point? I know nothing about this GEP
>> vs LEP stuff, interesting :)
> 
> you mean get_base_addr? it's already global
> 
> all the calls to get_base_addr within the object are made
> to get_base_addr+0x8
> 
> 00000000100350c0 <test_attach_probe>:
>      ...
>      100350e0:   59 fd ff 4b     bl      10034e38 <get_base_addr+0x8>
>      ...
>      100358a8:   91 f5 ff 4b     bl      10034e38 <get_base_addr+0x8>
> 
> 
> I'm following perf fix we had for similar issue:
>    7b6ff0bdbf4f perf probe ppc64le: Fixup function entry if using kallsyms lookup
> 
> I'll get more info on that

Thanks. The patch
    7b6ff0bdbf4f perf probe ppc64le: Fixup function entry if using 
kallsyms lookup
talked about offset + 8 for kernel symbols.
I guess uprobe symbol might be in the same situation if using the
same compilation mechanism as kernel. But it would be good
to get confirmation from ppc people.

> 
> jirka
> 
>>
>>> +{
>>> +       return offset + LEP_OFFSET;
>>> +}
>>> +#else
>>> +#define get_offset(offset) (offset)
>>> +#endif
>>> +
>>>   ssize_t get_base_addr() {
>>>          size_t start, offset;
>>>          char buf[256];
>>> @@ -36,7 +52,7 @@ void test_attach_probe(void)
>>>          if (CHECK(base_addr < 0, "get_base_addr",
>>>                    "failed to find base addr: %zd", base_addr))
>>>                  return;
>>> -       uprobe_offset = (size_t)&get_base_addr - base_addr;
>>> +       uprobe_offset = get_offset((size_t)&get_base_addr - base_addr);
>>>
>>>          skel = test_attach_probe__open_and_load();
>>>          if (CHECK(!skel, "skel_open", "failed to open skeleton\n"))
>>> --
>>> 2.29.2
>>>
>>
>