Re: [PATCH bpf-next v5 1/3] bpf: Show precise link_type for {uprobe,kprobe}_multi fdinfo

[Andrii Nakryiko]

On Mon, Jun 23, 2025 at 10:56 AM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Mon, Jun 23, 2025 at 6:44 AM Tao Chen <chen.dylane@xxxxxxxxx> wrote:
> >
> > Alexei suggested, 'link_type' can be more precise and differentiate
> > for human in fdinfo. In fact BPF_LINK_TYPE_KPROBE_MULTI includes
> > kretprobe_multi type, the same as BPF_LINK_TYPE_UPROBE_MULTI, so we
> > can show it more concretely.
> >
> > link_type:      kprobe_multi
> > link_id:        1
> > prog_tag:       d2b307e915f0dd37
> > ...
> > link_type:      kretprobe_multi
> > link_id:        2
> > prog_tag:       ab9ea0545870781d
> > ...
> > link_type:      uprobe_multi
> > link_id:        9
> > prog_tag:       e729f789e34a8eca
> > ...
> > link_type:      uretprobe_multi
> > link_id:        10
> > prog_tag:       7db356c03e61a4d4
> >
> > Signed-off-by: Tao Chen <chen.dylane@xxxxxxxxx>
> > ---
> >  include/linux/trace_events.h | 10 ++++++++++
> >  kernel/bpf/syscall.c         |  9 ++++++++-
> >  kernel/trace/bpf_trace.c     | 28 ++++++++++++++++++++++++++++
> >  3 files changed, 46 insertions(+), 1 deletion(-)
> >
> > Change list:
> >   v4 -> v5:
> >     - Add patch1 to show precise link_type for
> >       {uprobe,kprobe}_multi.(Alexei)
> >     - patch2,3 just remove type field, which will be showed in
> >       link_type
> >   v4:
> >   https://lore.kernel.org/bpf/20250619034257.70520-1-chen.dylane@xxxxxxxxx
> >
> >   v3 -> v4:
> >     - use %pS to print func info.(Alexei)
> >   v3:
> >   https://lore.kernel.org/bpf/20250616130233.451439-1-chen.dylane@xxxxxxxxx
> >
> >   v2 -> v3:
> >     - show info in one line for multi events.(Jiri)
> >   v2:
> >   https://lore.kernel.org/bpf/20250615150514.418581-1-chen.dylane@xxxxxxxxx
> >
> >   v1 -> v2:
> >     - replace 'func_cnt' with 'uprobe_cnt'.(Andrii)
> >     - print func name is more readable and security for kprobe_multi.(Alexei)
> >   v1:
> >   https://lore.kernel.org/bpf/20250612115556.295103-1-chen.dylane@xxxxxxxxx
> >
> > diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
> > index fa9cf4292df..951c91babbc 100644
> > --- a/include/linux/trace_events.h
> > +++ b/include/linux/trace_events.h
> > @@ -780,6 +780,8 @@ int bpf_get_perf_event_info(const struct perf_event *event, u32 *prog_id,
> >                             unsigned long *missed);
> >  int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *prog);
> >  int bpf_uprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *prog);
> > +void bpf_kprobe_multi_link_type_show(const struct bpf_link *link, char *link_type, int len);
> > +void bpf_uprobe_multi_link_type_show(const struct bpf_link *link, char *link_type, int len);
> >  #else
> >  static inline unsigned int trace_call_bpf(struct trace_event_call *call, void *ctx)
> >  {
> > @@ -832,6 +834,14 @@ bpf_uprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
> >  {
> >         return -EOPNOTSUPP;
> >  }
> > +static inline void
> > +bpf_kprobe_multi_link_type_show(const struct bpf_link *link, char *link_type, int len)
> > +{
> > +}
> > +static inline void
> > +bpf_uprobe_multi_link_type_show(const struct bpf_link *link, char *link_type, int len)
> > +{
> > +}
> >  #endif
> >
> >  enum {
> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> > index 51ba1a7aa43..43b821b37bc 100644
> > --- a/kernel/bpf/syscall.c
> > +++ b/kernel/bpf/syscall.c
> > @@ -3226,9 +3226,16 @@ static void bpf_link_show_fdinfo(struct seq_file *m, struct file *filp)
> >         const struct bpf_prog *prog = link->prog;
> >         enum bpf_link_type type = link->type;
> >         char prog_tag[sizeof(prog->tag) * 2 + 1] = { };
> > +       char link_type[64] = {};
> >
> >         if (type < ARRAY_SIZE(bpf_link_type_strs) && bpf_link_type_strs[type]) {
> > -               seq_printf(m, "link_type:\t%s\n", bpf_link_type_strs[type]);
> > +               if (link->type == BPF_LINK_TYPE_KPROBE_MULTI)
> > +                       bpf_kprobe_multi_link_type_show(link, link_type, sizeof(link_type));
> > +               else if (link->type == BPF_LINK_TYPE_UPROBE_MULTI)
> > +                       bpf_uprobe_multi_link_type_show(link, link_type, sizeof(link_type));
> > +               else
> > +                       strscpy(link_type, bpf_link_type_strs[type], sizeof(link_type));
> > +               seq_printf(m, "link_type:\t%s\n", link_type);
>
> New callbacks just to print a string?
> Let's find a different way.
>
> How about moving 'flags' from bpf_[ku]probe_multi_link into bpf_link ?
> (There is a 7 byte hole there anyway)
> and checking flags inline.
>
> Jiri, Andrii,
>
> better ideas?

We can just remember original attr->link_create.attach_type in
bpf_link itself, and then have a small helper that will accept link
type and attach type, and fill out link type representation based on
those two. Internally we can do the special-casing of  uprobe vs
uretprobe and kprobe vs kretprobe transparently to all the other code.
And use that here in show_fdinfo