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

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Date:   Mon, 1 Mar 2021 16:34:24 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     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>, Yonghong Song <yhs@...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 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 :)

> +{
> +       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
>