Re: [PATCHv2 bpf-next 13/24] libbpf: Add uprobe multi link detection

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, Jun 26, 2023 at 11:21:40AM -0700, Andrii Nakryiko wrote:

SNIP

> > >
> > > > +       if (prog_fd < 0)
> > > > +               return -errno;
> > > > +
> > > > +       /* No need to specify attach function. If the link is not supported
> > > > +        * we will get -EOPNOTSUPP error before any other check is performed.
> > >
> > > what will actually return this -EOPNOTSUPP? I couldn't find this in
> > > the code quickly, can you please point me where?
> >
> >         #else /* !CONFIG_UPROBES */
> >         int bpf_uprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
> >         {
> >                 return -EOPNOTSUPP;
> >         }
> 
> that's a new code in new kernel that doesn't have CONFIG_UPROBES. What
> about old kernels? They will return -EINVAL, no? Which we will assume
> means "yep, multi-uprobe BPF link is supported", which would be wrong.
> Or am I missing something?

ah nope, I'm missing old kernel case.. will check and fix that

jirka

> 
> >
> > >
> > > > +        */
> > > > +       link_fd = bpf_link_create(prog_fd, -1, BPF_TRACE_UPROBE_MULTI, NULL);
> > > > +       err = -errno; /* close() can clobber errno */
> > > > +
> > > > +       if (link_fd >= 0)
> > > > +               close(link_fd);
> > > > +       close(prog_fd);
> > > > +
> > > > +       return link_fd < 0 && err != -EOPNOTSUPP;
> > > > +}
> > > > +
> > > >  static int probe_kern_bpf_cookie(void)
> > > >  {
> > > >         struct bpf_insn insns[] = {
> > > > @@ -4911,6 +4937,9 @@ static struct kern_feature_desc {
> > > >         [FEAT_SYSCALL_WRAPPER] = {
> > > >                 "Kernel using syscall wrapper", probe_kern_syscall_wrapper,
> > > >         },
> > > > +       [FEAT_UPROBE_LINK] = {
> > > > +               "BPF uprobe multi link support", probe_uprobe_multi_link,
> > > > +       },
> > > >  };
> > > >
> > > >  bool kernel_supports(const struct bpf_object *obj, enum kern_feature_id feat_id)
> > > > diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
> > > > index 22b0834e7fe1..a257eb81af25 100644
> > > > --- a/tools/lib/bpf/libbpf_internal.h
> > > > +++ b/tools/lib/bpf/libbpf_internal.h
> > > > @@ -354,6 +354,8 @@ enum kern_feature_id {
> > > >         FEAT_BTF_ENUM64,
> > > >         /* Kernel uses syscall wrapper (CONFIG_ARCH_HAS_SYSCALL_WRAPPER) */
> > > >         FEAT_SYSCALL_WRAPPER,
> > > > +       /* BPF uprobe_multi link support */
> > > > +       FEAT_UPROBE_LINK,
> > >
> > > UPROBE_MULTI_LINK, we might have non-multi link in the future as well
> >
> > ok
> >
> > thanks,
> > jirka