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

[Chuang W]

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