Re: [PATCH bpf-next v2 13/18] libbpf: support tracing_multi

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Jul 3, 2025 at 5:24 AM Menglong Dong <menglong8.dong@gmail.com> wrote:
>
> Add supporting for the attach types of:
>
> BPF_TRACE_FENTRY_MULTI
> BPF_TRACE_FEXIT_MULTI
> BPF_MODIFY_RETURN_MULTI
>
> Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
> ---
>  tools/bpf/bpftool/common.c |   3 +
>  tools/lib/bpf/bpf.c        |  10 +++
>  tools/lib/bpf/bpf.h        |   6 ++
>  tools/lib/bpf/libbpf.c     | 168 ++++++++++++++++++++++++++++++++++++-
>  tools/lib/bpf/libbpf.h     |  19 +++++
>  tools/lib/bpf/libbpf.map   |   1 +
>  6 files changed, 204 insertions(+), 3 deletions(-)
>

[...]

> diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
> index 1342564214c8..5c97acec643d 100644
> --- a/tools/lib/bpf/bpf.h
> +++ b/tools/lib/bpf/bpf.h
> @@ -422,6 +422,12 @@ struct bpf_link_create_opts {
>                 struct {
>                         __u64 cookie;
>                 } tracing;
> +               struct {
> +                       __u32 cnt;
> +                       const __u32 *btf_ids;
> +                       const __u32 *tgt_fds;

tgt_fds are always BTF FDs, right? Do we intend to support
freplace-style multi attachment at all? If not, I'd name them btf_fds,
and btf_ids -> btf_type_ids (because BTF ID can also refer to kernel
ID of BTF object, so ambiguous and somewhat confusing)

> +                       const __u64 *cookies;
> +               } tracing_multi;
>                 struct {
>                         __u32 pf;
>                         __u32 hooknum;
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 530c29f2f5fc..ae38b3ab84c7 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -136,6 +136,9 @@ static const char * const attach_type_name[] = {
>         [BPF_NETKIT_PEER]               = "netkit_peer",
>         [BPF_TRACE_KPROBE_SESSION]      = "trace_kprobe_session",
>         [BPF_TRACE_UPROBE_SESSION]      = "trace_uprobe_session",
> +       [BPF_TRACE_FENTRY_MULTI]        = "trace_fentry_multi",
> +       [BPF_TRACE_FEXIT_MULTI]         = "trace_fexit_multi",
> +       [BPF_MODIFY_RETURN_MULTI]       = "modify_return_multi",
>  };
>
>  static const char * const link_type_name[] = {
> @@ -410,6 +413,8 @@ enum sec_def_flags {
>         SEC_XDP_FRAGS = 16,
>         /* Setup proper attach type for usdt probes. */
>         SEC_USDT = 32,
> +       /* attachment target is multi-link */
> +       SEC_ATTACH_BTF_MULTI = 64,
>  };
>
>  struct bpf_sec_def {
> @@ -7419,9 +7424,9 @@ static int libbpf_prepare_prog_load(struct bpf_program *prog,
>                 opts->expected_attach_type = BPF_TRACE_UPROBE_MULTI;
>         }
>
> -       if ((def & SEC_ATTACH_BTF) && !prog->attach_btf_id) {
> +       if ((def & (SEC_ATTACH_BTF | SEC_ATTACH_BTF_MULTI)) && !prog->attach_btf_id) {
>                 int btf_obj_fd = 0, btf_type_id = 0, err;
> -               const char *attach_name;
> +               const char *attach_name, *name_end;
>
>                 attach_name = strchr(prog->sec_name, '/');
>                 if (!attach_name) {
> @@ -7440,7 +7445,27 @@ static int libbpf_prepare_prog_load(struct bpf_program *prog,
>                 }
>                 attach_name++; /* skip over / */
>
> -               err = libbpf_find_attach_btf_id(prog, attach_name, &btf_obj_fd, &btf_type_id);
> +               name_end = strchr(attach_name, ',');
> +               /* for multi-link tracing, use the first target symbol during
> +                * loading.
> +                */
> +               if ((def & SEC_ATTACH_BTF_MULTI) && name_end) {
> +                       int len = name_end - attach_name + 1;

for multi-kprobe we decided to only support a single glob  as a target
in declarative SEC() definition. If a user needs more control, they
can always fallback to the programmatic bpf_program__attach_..._opts()
variant. Let's do the same here, glob is good enough for declarative
use cases, and for complicated cases programmatic is the way to go
anyways. You'll avoid unnecessary complications like this one then.

BTW, it's not trivial to figure this out from earlier patches, but
does BPF verifier need to know all these BTF type IDs during program
verification time? If yes, why and then why do we need to specify them
during LINK_CREATE time. And if not, then great, and we don't need to
parse all this during load time.

> +                       char *first_tgt;
> +
> +                       first_tgt = malloc(len);
> +                       if (!first_tgt)
> +                               return -ENOMEM;
> +                       libbpf_strlcpy(first_tgt, attach_name, len);
> +                       first_tgt[len - 1] = '\0';
> +                       err = libbpf_find_attach_btf_id(prog, first_tgt, &btf_obj_fd,
> +                                                       &btf_type_id);
> +                       free(first_tgt);
> +               } else {
> +                       err = libbpf_find_attach_btf_id(prog, attach_name, &btf_obj_fd,
> +                                                       &btf_type_id);
> +               }
> +
>                 if (err)
>                         return err;
>
> @@ -9519,6 +9544,7 @@ static int attach_kprobe_session(const struct bpf_program *prog, long cookie, st
>  static int attach_uprobe_multi(const struct bpf_program *prog, long cookie, struct bpf_link **link);
>  static int attach_lsm(const struct bpf_program *prog, long cookie, struct bpf_link **link);
>  static int attach_iter(const struct bpf_program *prog, long cookie, struct bpf_link **link);
> +static int attach_trace_multi(const struct bpf_program *prog, long cookie, struct bpf_link **link);
>
>  static const struct bpf_sec_def section_defs[] = {
>         SEC_DEF("socket",               SOCKET_FILTER, 0, SEC_NONE),
> @@ -9565,6 +9591,13 @@ static const struct bpf_sec_def section_defs[] = {
>         SEC_DEF("fentry.s+",            TRACING, BPF_TRACE_FENTRY, SEC_ATTACH_BTF | SEC_SLEEPABLE, attach_trace),
>         SEC_DEF("fmod_ret.s+",          TRACING, BPF_MODIFY_RETURN, SEC_ATTACH_BTF | SEC_SLEEPABLE, attach_trace),
>         SEC_DEF("fexit.s+",             TRACING, BPF_TRACE_FEXIT, SEC_ATTACH_BTF | SEC_SLEEPABLE, attach_trace),
> +       SEC_DEF("tp_btf+",              TRACING, BPF_TRACE_RAW_TP, SEC_ATTACH_BTF, attach_trace),

duplicate


> +       SEC_DEF("fentry.multi+",        TRACING, BPF_TRACE_FENTRY_MULTI, SEC_ATTACH_BTF_MULTI, attach_trace_multi),
> +       SEC_DEF("fmod_ret.multi+",      TRACING, BPF_MODIFY_RETURN_MULTI, SEC_ATTACH_BTF_MULTI, attach_trace_multi),
> +       SEC_DEF("fexit.multi+",         TRACING, BPF_TRACE_FEXIT_MULTI, SEC_ATTACH_BTF_MULTI, attach_trace_multi),
> +       SEC_DEF("fentry.multi.s+",      TRACING, BPF_TRACE_FENTRY_MULTI, SEC_ATTACH_BTF_MULTI | SEC_SLEEPABLE, attach_trace_multi),
> +       SEC_DEF("fmod_ret.multi.s+",    TRACING, BPF_MODIFY_RETURN_MULTI, SEC_ATTACH_BTF_MULTI | SEC_SLEEPABLE, attach_trace_multi),
> +       SEC_DEF("fexit.multi.s+",       TRACING, BPF_TRACE_FEXIT_MULTI, SEC_ATTACH_BTF_MULTI | SEC_SLEEPABLE, attach_trace_multi),
>         SEC_DEF("freplace+",            EXT, 0, SEC_ATTACH_BTF, attach_trace),
>         SEC_DEF("lsm+",                 LSM, BPF_LSM_MAC, SEC_ATTACH_BTF, attach_lsm),
>         SEC_DEF("lsm.s+",               LSM, BPF_LSM_MAC, SEC_ATTACH_BTF | SEC_SLEEPABLE, attach_lsm),
> @@ -12799,6 +12832,135 @@ static int attach_trace(const struct bpf_program *prog, long cookie, struct bpf_
>         return libbpf_get_error(*link);
>  }
>

[...]