Re: [PATCH v3] libbpf: Cleanup the legacy kprobe_event on failed add/attach_event()

[Andrii Nakryiko]

On Mon, Jun 27, 2022 at 6:51 PM Chuang W <nashuiliang@xxxxxxxxx> wrote:
>
> Hi Andrii,
>
> On Tue, Jun 28, 2022 at 5:27 AM Andrii Nakryiko
> <andrii.nakryiko@xxxxxxxxx> wrote:
> >
> > On Sat, Jun 25, 2022 at 8:13 PM 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/XXX) will exist, but
> > > perf_event_open() will return an error because both livepatch and kprobe
> > > use FTRACE_OPS_FL_IPMODIFY. As follows:
> > >
> > > 1) add a livepatch
> > >
> > > $ insmod livepatch-XXX.ko
> > >
> > > 2) add a kprobe using tracefs API (i.e. add_kprobe_event_legacy)
> > >
> > > $ echo 'p:mykprobe XXX' > /sys/kernel/debug/tracing/kprobe_events
> > >
> > > 3) enable this kprobe (i.e. sys_perf_event_open)
> > >
> > > This will return an error, -EBUSY.
> > >
> > > On Andrii Nakryiko's comment, few error paths in
> > > bpf_program__attach_kprobe_opts() which should need to call
> > > remove_kprobe_event_legacy().
> > >
> > > With this patch, whenever an error is returned after
> > > add_kprobe_event_legacy() or bpf_program__attach_perf_event_opts(), this
> > > ensures that the created kprobe_event is cleaned.
> > >
> > > Signed-off-by: Chuang W <nashuiliang@xxxxxxxxx>
> >
> > Is this your full name? Signed-off-by is required to have a full name
> > of a person, please update if it's not
> >
> > > Signed-off-by: Jingren Zhou <zhoujingren@xxxxxxxxxxxxxx>
> > > ---
> > > V2->v3:
> > > - add detail commits
> > > - call remove_kprobe_event_legacy() on failed bpf_program__attach_perf_event_opts()
> > >
> > >  tools/lib/bpf/libbpf.c | 15 ++++++++++++---
> > >  1 file changed, 12 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > > index 49e359cd34df..038b0cb3313f 100644
> > > --- a/tools/lib/bpf/libbpf.c
> > > +++ b/tools/lib/bpf/libbpf.c
> > > @@ -10811,10 +10811,11 @@ 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;
> > > @@ -10828,9 +10829,14 @@ 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 legacy kprobe_event */
> > > +       remove_kprobe_event_legacy(probe_name, retprobe);
> > > +       return err;
> > >  }
> > >
> >
> > this part looks good
> >
> >
> > >  struct bpf_link *
> > > @@ -10899,6 +10905,9 @@ bpf_program__attach_kprobe_opts(const struct bpf_program *prog,
> > >
> > >         return link;
> > >  err_out:
> > > +       /* Clear the newly added legacy kprobe_event */
> > > +       if (legacy)
> > > +               remove_kprobe_event_legacy(legacy_probe, retprobe);
> >
> > this one will call remove_kprobe_event_legacy() even if we failed to
> > create that kprobe_event in the first place. So let's maybe add
> >
> > err_clean_legacy:
> >     if (legacy)
> >          remove_kprobe_event_legacy(legacy_probe, retprobe);
> >
> > before err_out: and goto there if we fail to attach (but not if we
> > fail to create pfd)?
> >
>
> Nice, I will modify it.
>
> >
> > Also, looking through libbpf code, I realized that we have exactly the
> > same problem for uprobes, so please add same fixed to
> > perf_event_uprobe_open_legacy and attach_uprobe_opts. Thanks!
> >
>
> Oh, yes. I also noticed this problem for uprobes, I was planning to
> submit a patch for uprobes.
> Do you think I should submit another patch for uprobes or combine
> kprobes and uprobes into one?
>

two separate patches make more sense, but send them as a patch series?

> Thanks,
> >
> >
> > >         free(legacy_probe);
> > >         return libbpf_err_ptr(err);
> > >  }
> > > --
> > > 2.34.1
> > >