Re: [PATCHv2 bpf-next 14/24] libbpf: Add uprobe multi link support to bpf_program__attach_usdt

[Jiri Olsa <olsajiri@gmail.com>]

On Fri, Jun 23, 2023 at 01:40:27PM -0700, Andrii Nakryiko wrote:
> On Tue, Jun 20, 2023 at 1:38 AM Jiri Olsa <jolsa@kernel.org> wrote:
> >
> > Adding support for usdt_manager_attach_usdt to use uprobe_multi
> > link to attach to usdt probes.
> >
> > The uprobe_multi support is detected before the usdt program is
> > loaded and its expected_attach_type is set accordingly.
> >
> > If uprobe_multi support is detected the usdt_manager_attach_usdt
> > gathers uprobes info and calls bpf_program__attach_uprobe_opts to
> > create all needed uprobes.
> >
> > If uprobe_multi support is not detected the old behaviour stays.
> >
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >  tools/lib/bpf/libbpf.c |  12 ++++-
> >  tools/lib/bpf/usdt.c   | 120 ++++++++++++++++++++++++++++++-----------
> >  2 files changed, 99 insertions(+), 33 deletions(-)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 3d570898459e..9c7a67c5cbe8 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -363,6 +363,8 @@ enum sec_def_flags {
> >         SEC_SLEEPABLE = 8,
> >         /* BPF program support non-linear XDP buffer */
> >         SEC_XDP_FRAGS = 16,
> > +       /* Setup proper attach type for usdt probes. */
> > +       SEC_USDT = 32,
> >  };
> >
> >  struct bpf_sec_def {
> > @@ -6799,6 +6801,10 @@ static int libbpf_prepare_prog_load(struct bpf_program *prog,
> >         if (prog->type == BPF_PROG_TYPE_XDP && (def & SEC_XDP_FRAGS))
> >                 opts->prog_flags |= BPF_F_XDP_HAS_FRAGS;
> >
> > +       /* special check for usdt to use uprobe_multi link */
> > +       if ((def & SEC_USDT) && kernel_supports(NULL, FEAT_UPROBE_LINK))
> > +               prog->expected_attach_type = BPF_TRACE_UPROBE_MULTI;
> 
> this is quite ugly. I think KPROBE programs do not have enforcement
> for expected_attach_type during BPF_PROG_LOAD, so we can set
> BPF_TRACE_UPROBE_MULTI unconditionally, right?

t doesn't but we are adding one earlier to bpf_prog_attach_check_attach_type
(the one with U/K typo):

	+       case BPF_PROG_TYPE_KPROBE:
	+               if (prog->expected_attach_type == BPF_TRACE_UPROBE_MULTI &&
	+                   attach_type != BPF_TRACE_UPROBE_MULTI)
	+                       return -EINVAL;



for same reasons we did the check for kprobe_multi in:

  db8eae6bc5c7 bpf: Force kprobe multi expected_attach_type for kprobe_multi link

> 
> > +
> >         if ((def & SEC_ATTACH_BTF) && !prog->attach_btf_id) {
> >                 int btf_obj_fd = 0, btf_type_id = 0, err;
> >                 const char *attach_name;
> > @@ -6867,7 +6873,6 @@ static int bpf_object_load_prog(struct bpf_object *obj, struct bpf_program *prog
> >         if (!insns || !insns_cnt)
> >                 return -EINVAL;
> >
> > -       load_attr.expected_attach_type = prog->expected_attach_type;
> >         if (kernel_supports(obj, FEAT_PROG_NAME))
> >                 prog_name = prog->name;
> >         load_attr.attach_prog_fd = prog->attach_prog_fd;
> > @@ -6903,6 +6908,9 @@ static int bpf_object_load_prog(struct bpf_object *obj, struct bpf_program *prog
> >                 insns_cnt = prog->insns_cnt;
> >         }
> >
> > +       /* allow prog_prepare_load_fn to change expected_attach_type */
> > +       load_attr.expected_attach_type = prog->expected_attach_type;
> > +
> >         if (obj->gen_loader) {
> >                 bpf_gen__prog_load(obj->gen_loader, prog->type, prog->name,
> >                                    license, insns, insns_cnt, &load_attr,
> > @@ -8703,7 +8711,7 @@ static const struct bpf_sec_def section_defs[] = {
> >         SEC_DEF("uretprobe.multi.s+",   KPROBE, BPF_TRACE_UPROBE_MULTI, SEC_SLEEPABLE, attach_uprobe_multi),
> >         SEC_DEF("ksyscall+",            KPROBE, 0, SEC_NONE, attach_ksyscall),
> >         SEC_DEF("kretsyscall+",         KPROBE, 0, SEC_NONE, attach_ksyscall),
> > -       SEC_DEF("usdt+",                KPROBE, 0, SEC_NONE, attach_usdt),
> > +       SEC_DEF("usdt+",                KPROBE, 0, SEC_USDT, attach_usdt),
> 
> btw, given you are touching USDT stuff, can you please also add
> sleepable USDT (usdt.s+) ?

ok

> 
> 
> >         SEC_DEF("tc",                   SCHED_CLS, 0, SEC_NONE),
> >         SEC_DEF("classifier",           SCHED_CLS, 0, SEC_NONE),
> >         SEC_DEF("action",               SCHED_ACT, 0, SEC_NONE),
> > diff --git a/tools/lib/bpf/usdt.c b/tools/lib/bpf/usdt.c
> > index f1a141555f08..33f0a2b4cc1c 100644
> > --- a/tools/lib/bpf/usdt.c
> > +++ b/tools/lib/bpf/usdt.c
> > @@ -808,6 +808,16 @@ struct bpf_link_usdt {
> >                 long abs_ip;
> >                 struct bpf_link *link;
> >         } *uprobes;
> > +
> > +       bool has_uprobe_multi;
> > +
> > +       struct {
> > +               char *path;
> > +               unsigned long *offsets;
> > +               unsigned long *ref_ctr_offsets;
> > +               __u64 *cookies;
> 
> you shouldn't need to persist this, this can be allocated and freed
> inside usdt_manager_attach_usdt(), you only need the link pointer

aah right, that can go away, nice

> 
> 
> > +               struct bpf_link *link;
> > +       } uprobe_multi;
> >  };
> >
> >  static int bpf_link_usdt_detach(struct bpf_link *link)
> > @@ -816,19 +826,23 @@ static int bpf_link_usdt_detach(struct bpf_link *link)
> >         struct usdt_manager *man = usdt_link->usdt_man;
> >         int i;
> >
> > -       for (i = 0; i < usdt_link->uprobe_cnt; i++) {
> > -               /* detach underlying uprobe link */
> > -               bpf_link__destroy(usdt_link->uprobes[i].link);
> > -               /* there is no need to update specs map because it will be
> > -                * unconditionally overwritten on subsequent USDT attaches,
> > -                * but if BPF cookies are not used we need to remove entry
> > -                * from ip_to_spec_id map, otherwise we'll run into false
> > -                * conflicting IP errors
> > -                */
> > -               if (!man->has_bpf_cookie) {
> > -                       /* not much we can do about errors here */
> > -                       (void)bpf_map_delete_elem(bpf_map__fd(man->ip_to_spec_id_map),
> > -                                                 &usdt_link->uprobes[i].abs_ip);
> > +       if (usdt_link->has_uprobe_multi) {
> > +               bpf_link__destroy(usdt_link->uprobe_multi.link);
> > +       } else {
> > +               for (i = 0; i < usdt_link->uprobe_cnt; i++) {
> > +                       /* detach underlying uprobe link */
> > +                       bpf_link__destroy(usdt_link->uprobes[i].link);
> > +                       /* there is no need to update specs map because it will be
> > +                        * unconditionally overwritten on subsequent USDT attaches,
> > +                        * but if BPF cookies are not used we need to remove entry
> > +                        * from ip_to_spec_id map, otherwise we'll run into false
> > +                        * conflicting IP errors
> > +                        */
> > +                       if (!man->has_bpf_cookie) {
> > +                               /* not much we can do about errors here */
> > +                               (void)bpf_map_delete_elem(bpf_map__fd(man->ip_to_spec_id_map),
> > +                                                         &usdt_link->uprobes[i].abs_ip);
> > +                       }
> >                 }
> 
> you can avoid shifting all this by keeping uprobe_cnt to zero
> 
> bpf_link__destory(usdt_link->uprobe_multi.link) will work fine for NULL
> 
> so just do both clean ups sequentially, knowing that only one of them
> will actually do anything

ok, good idea

> 
> >         }
> >
> > @@ -868,9 +882,15 @@ static void bpf_link_usdt_dealloc(struct bpf_link *link)
> >  {
> >         struct bpf_link_usdt *usdt_link = container_of(link, struct bpf_link_usdt, link);
> >
> > -       free(usdt_link->spec_ids);
> > -       free(usdt_link->uprobes);
> > -       free(usdt_link);
> > +       if (usdt_link->has_uprobe_multi) {
> > +               free(usdt_link->uprobe_multi.offsets);
> > +               free(usdt_link->uprobe_multi.ref_ctr_offsets);
> > +               free(usdt_link->uprobe_multi.cookies);
> > +       } else {
> > +               free(usdt_link->spec_ids);
> > +               free(usdt_link->uprobes);
> > +               free(usdt_link);
> > +       }
> 
> similar to the above, just do *all* the clean up unconditionally and
> rely on free() handling NULLs just fine

ok

> 
> >  }
> >
> >  static size_t specs_hash_fn(long key, void *ctx)
> > @@ -943,11 +963,13 @@ struct bpf_link *usdt_manager_attach_usdt(struct usdt_manager *man, const struct
> >                                           const char *usdt_provider, const char *usdt_name,
> >                                           __u64 usdt_cookie)
> >  {
> > +       LIBBPF_OPTS(bpf_uprobe_multi_opts, opts_multi);
> >         int i, fd, err, spec_map_fd, ip_map_fd;
> >         LIBBPF_OPTS(bpf_uprobe_opts, opts);
> >         struct hashmap *specs_hash = NULL;
> >         struct bpf_link_usdt *link = NULL;
> >         struct usdt_target *targets = NULL;
> > +       struct bpf_link *uprobe_link;
> >         size_t target_cnt;
> >         Elf *elf;
> >
> > @@ -1003,16 +1025,29 @@ struct bpf_link *usdt_manager_attach_usdt(struct usdt_manager *man, const struct
> >         link->usdt_man = man;
> >         link->link.detach = &bpf_link_usdt_detach;
> >         link->link.dealloc = &bpf_link_usdt_dealloc;
> > +       link->has_uprobe_multi = bpf_program__expected_attach_type(prog) == BPF_TRACE_UPROBE_MULTI;
> 
> just use kernel_supports(), it's cleaner (and result is cached, so
> it's not less efficient)

ok

thanks,
jirka