Re: [PATCHv3 bpf-next 09/13] libbpf: Add bpf_program__attach_kprobe_multi_opts function

Alexei Starovoitov
2022-03-18T03:53:31Z

On Wed, Mar 16, 2022 at 5:26 AM Jiri Olsa <jolsa@kernel.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.