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

On Mon, Sep 9, 2024 at 12:46 AM Jiri Olsa [off-list ref] 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@kernel.org>
---
 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)

[...]