netdev - Re: [PATCHv3 bpf-next 09/13] libbpf: Add bpf_program__attach_kprobe_multi

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Date:   Thu, 17 Mar 2022 20:53:15 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, lkml <linux-kernel@...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>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCHv3 bpf-next 09/13] libbpf: Add bpf_program__attach_kprobe_multi_opts
 function

On Wed, Mar 16, 2022 at 5:26 AM Jiri Olsa <jolsa@...nel.org> wrote:
> +
> +struct bpf_link *
> +bpf_program__attach_kprobe_multi_opts(const struct bpf_program *prog,
> +                                     const char *pattern,
> +                                     const struct bpf_kprobe_multi_opts *opts)
> +{
> +       LIBBPF_OPTS(bpf_link_create_opts, lopts);
> +       struct kprobe_multi_resolve res = {
> +               .pattern = pattern,
> +       };
> +       struct bpf_link *link = NULL;
> +       char errmsg[STRERR_BUFSIZE];
> +       const unsigned long *addrs;
> +       int err, link_fd, prog_fd;
> +       const __u64 *cookies;
> +       const char **syms;
> +       bool retprobe;
> +       size_t cnt;
> +
> +       if (!OPTS_VALID(opts, bpf_kprobe_multi_opts))
> +               return libbpf_err_ptr(-EINVAL);
> +
> +       syms    = OPTS_GET(opts, syms, false);
> +       addrs   = OPTS_GET(opts, addrs, false);
> +       cnt     = OPTS_GET(opts, cnt, false);
> +       cookies = OPTS_GET(opts, cookies, false);
> +
> +       if (!pattern && !addrs && !syms)
> +               return libbpf_err_ptr(-EINVAL);
> +       if (pattern && (addrs || syms || cookies || cnt))
> +               return libbpf_err_ptr(-EINVAL);
> +       if (!pattern && !cnt)
> +               return libbpf_err_ptr(-EINVAL);
> +       if (addrs && syms)
> +               return libbpf_err_ptr(-EINVAL);
> +
> +       if (pattern) {
> +               err = libbpf_kallsyms_parse(resolve_kprobe_multi_cb, &res);
> +               if (err)
> +                       goto error;
> +               if (!res.cnt) {
> +                       err = -ENOENT;
> +                       goto error;
> +               }
> +               addrs = res.addrs;
> +               cnt = res.cnt;
> +       }

Thanks Jiri.
Great stuff and a major milestone!
I've applied Masami's and your patches to bpf-next.

But the above needs more work.
Currently test_progs -t kprobe_multi
takes 4 seconds on lockdep+debug kernel.
Mainly because of the above loop.

    18.05%  test_progs       [kernel.kallsyms]   [k]
kallsyms_expand_symbol.constprop.4
    12.53%  test_progs       libc-2.28.so        [.] _IO_vfscanf
     6.31%  test_progs       [kernel.kallsyms]   [k] number
     4.66%  test_progs       [kernel.kallsyms]   [k] format_decode
     4.65%  test_progs       [kernel.kallsyms]   [k] string_nocheck

Single test_skel_api() subtest takes almost a second.

A cache inside libbpf probably won't help.
Maybe introduce a bpf iterator for kallsyms?

On the kernel side kprobe_multi_resolve_syms() looks similarly inefficient.
I'm not sure whether it would be a bottle neck though.

Orthogonal to this issue please add a new stress test
to selftest/bpf that attaches to a lot of functions.