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

[Andrii Nakryiko]

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
>