Re: [PATCH v2] libbpf: Cleanup the kprobe_event on failed add_kprobe_event_legacy()

[Chuang W]

Hi, Andrii,

Oh, yes. I verified that when bpf_program__attach_kprobe_opts() fails,
the kprobe_event did not clean up.
I will resubmit V3 soon, and fix it.

Thanks.

On Thu, Jun 23, 2022 at 12:03 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Tue, Jun 21, 2022 at 12:32 AM Chuang W <nashuiliang@xxxxxxxxx> wrote:
> >
> > Before the 0bc11ed5ab60 commit ("kprobes: Allow kprobes coexist with
> > livepatch"), in a scenario where livepatch and kprobe coexist on the
> > same function entry, the creation of kprobe_event using
> > add_kprobe_event_legacy() will be successful, at the same time as a
> > trace event (e.g. /debugfs/tracing/events/kprobe/XX) will exist, but
> > perf_event_open() will return an error because both livepatch and kprobe
> > use FTRACE_OPS_FL_IPMODIFY.
> >
> > With this patch, whenever an error is returned after
> > add_kprobe_event_legacy(), this ensures that the created kprobe_event is
> > cleaned.
> >
> > Signed-off-by: Chuang W <nashuiliang@xxxxxxxxx>
> > Signed-off-by: Jingren Zhou <zhoujingren@xxxxxxxxxxxxxx>
> > ---
>
> This part is good, but I think there are few error paths in
> bpf_program__attach_kprobe_opts() itself that would need to call
> remove_kprobe_event_legacy() explicitly as well, no?
>
> >  tools/lib/bpf/libbpf.c | 12 +++++++++---
> >  1 file changed, 9 insertions(+), 3 deletions(-)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 0781fae58a06..d0a36350e22a 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -10809,10 +10809,11 @@ static int perf_event_kprobe_open_legacy(const char *probe_name, bool retprobe,
> >         }
> >         type = determine_kprobe_perf_type_legacy(probe_name, retprobe);
> >         if (type < 0) {
> > +               err = type;
> >                 pr_warn("failed to determine legacy kprobe event id for '%s+0x%zx': %s\n",
> >                         kfunc_name, offset,
> > -                       libbpf_strerror_r(type, errmsg, sizeof(errmsg)));
> > -               return type;
> > +                       libbpf_strerror_r(err, errmsg, sizeof(errmsg)));
> > +               goto clear_kprobe_event;
> >         }
> >         attr.size = sizeof(attr);
> >         attr.config = type;
> > @@ -10826,9 +10827,14 @@ static int perf_event_kprobe_open_legacy(const char *probe_name, bool retprobe,
> >                 err = -errno;
> >                 pr_warn("legacy kprobe perf_event_open() failed: %s\n",
> >                         libbpf_strerror_r(err, errmsg, sizeof(errmsg)));
> > -               return err;
> > +               goto clear_kprobe_event;
> >         }
> >         return pfd;
> > +
> > +clear_kprobe_event:
> > +       /* Clear the newly added kprobe_event */
> > +       remove_kprobe_event_legacy(probe_name, retprobe);
> > +       return err;
> >  }
> >
> >  struct bpf_link *
> > --
> > 2.34.1
> >