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

On Mon, Sep 09, 2024 at 04:44:44PM -0700, Andrii Nakryiko wrote:

On Mon, Sep 9, 2024 at 12:46 AM Jiri Olsa [off-list ref] wrote:

quoted

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(-)

[...]

quoted

+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.

ok, that's better

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)

hum, right.. I wonder why the test is passing, will send a fix

thanks,
jirka

quoted

+
 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;

[...]