Re: [PATCHv3 4/7] libbpf: Add support for uprobe multi session attach

[Andrii Nakryiko]

On Mon, Sep 9, 2024 at 12:46 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
>
> Adding support to attach program in uprobe session mode
> with bpf_program__attach_uprobe_multi function.
>
> Adding session bool to bpf_uprobe_multi_opts struct that allows
> to load and attach the bpf program via uprobe session.
> the attachment to create uprobe multi session.
>
> Also adding new program loader section that allows:
>   SEC("uprobe.session/bpf_fentry_test*")
>
> and loads/attaches uprobe program as uprobe session.
>
> Adding sleepable hook (uprobe.session.s) as well.
>
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> ---
>  tools/lib/bpf/bpf.c    |  1 +
>  tools/lib/bpf/libbpf.c | 50 ++++++++++++++++++++++++++++++++++++++++--
>  tools/lib/bpf/libbpf.h |  4 +++-
>  3 files changed, 52 insertions(+), 3 deletions(-)
>

[...]

> +static int attach_uprobe_session(const struct bpf_program *prog, long cookie, struct bpf_link **link)
> +{
> +       LIBBPF_OPTS(bpf_uprobe_multi_opts, opts, .session = true);
> +       char *binary_path = NULL, *func_name = NULL;
> +       int n, ret = -EINVAL;
> +       const char *spec;
> +
> +       *link = NULL;
> +
> +       spec = prog->sec_name + sizeof("uprobe.session/") - 1;
> +       if (cookie & SEC_SLEEPABLE)
> +               spec += 2; /* extra '.s' */
> +       n = sscanf(spec, "%m[^:]:%m[^\n]", &binary_path, &func_name);
> +
> +       switch (n) {
> +       case 1:
> +               /* but auto-attach is impossible. */
> +               ret = 0;
> +               break;
> +       case 2:
> +               *link = bpf_program__attach_uprobe_multi(prog, -1, binary_path, func_name, &opts);
> +               ret = *link ? 0 : -errno;
> +               break;
> +       default:
> +               pr_warn("prog '%s': invalid format of section definition '%s'\n", prog->name,
> +                       prog->sec_name);
> +               break;
> +       }
> +       free(binary_path);
> +       free(func_name);
> +       return ret;
> +}

why adding this whole attach_uprobe_session if attach_uprobe_multi()
is almost exactly what you need. We just need to teach
attach_uprobe_multi to recognize uprobe.session prefix with strncmp(),
no? The rest of the logic is exactly the same.

BTW, maybe you can fix attach_uprobe_multi() while at it:

opts.retprobe = strcmp(probe_type, "uretprobe.multi") == 0;

that should be strncmp() to accommodate uretprobe.multi.s, no?
Original author (wink-wink) didn't account for that ".s", it seems...

(actually please send a small fix to bpf-next separately, so we can
apply it sooner)

> +
>  static void gen_uprobe_legacy_event_name(char *buf, size_t buf_sz,
>                                          const char *binary_path, uint64_t offset)
>  {
> @@ -11933,10 +11969,12 @@ bpf_program__attach_uprobe_multi(const struct bpf_program *prog,
>         const unsigned long *ref_ctr_offsets = NULL, *offsets = NULL;
>         LIBBPF_OPTS(bpf_link_create_opts, lopts);
>         unsigned long *resolved_offsets = NULL;
> +       enum bpf_attach_type attach_type;
>         int err = 0, link_fd, prog_fd;
>         struct bpf_link *link = NULL;
>         char errmsg[STRERR_BUFSIZE];
>         char full_path[PATH_MAX];
> +       bool retprobe, session;
>         const __u64 *cookies;
>         const char **syms;
>         size_t cnt;

[...]