link: Add Info interface support for perf event kprobe link

Implementing Info function (Link interface) for perf event
kprobe link, it's now possibe to retrieve link's info with:

  info, err := link.Info()
  if err != nil {
    return err
  }
  pevent := info.PerfEvent()
  kp := pevent.Kprobe()

At the moment we support/return only Addr and Missed fields.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>

internal/sys/syscall.go

@@ -103,6 +103,10 @@ func (i *KprobeMultiLinkInfo) info() (unsafe.Pointer, uint32) {
 	return unsafe.Pointer(i), uint32(unsafe.Sizeof(*i))
 }
 
+func (i *KprobeLinkInfo) info() (unsafe.Pointer, uint32) {
+	return unsafe.Pointer(i), uint32(unsafe.Sizeof(*i))
+}
+
 var _ Info = (*BtfInfo)(nil)
 
 func (i *BtfInfo) info() (unsafe.Pointer, uint32) {

link/link.go

@@ -200,6 +200,16 @@ type KprobeMultiInfo struct {
 	Missed uint64
 }
 
+type PerfEventInfo struct {
+	PerfEventType sys.PerfEventType
+	extra         interface{}
+}
+
+type KprobeInfo struct {
+	Addr   uint64
+	Missed uint64
+}
+
 // Tracing returns tracing type-specific link info.
 //
 // Returns nil if the type-specific link info isn't available.
@@ -264,6 +274,19 @@ func (r Info) KprobeMulti() *KprobeMultiInfo {
 	return e
 }
 
+// PerfEvent returns perf-event type-specific link info.
+//
+// Returns nil if the type-specific link info isn't available.
+func (r Info) PerfEvent() *PerfEventInfo {
+	e, _ := r.extra.(*PerfEventInfo)
+	return e
+}
+
+func (r *PerfEventInfo) Kprobe() *KprobeInfo {
+	e, _ := r.extra.(*KprobeInfo)
+	return e
+}
+
 // RawLink is the low-level API to bpf_link.
 //
 // You should consider using the higher level interfaces in this
@@ -439,8 +462,7 @@ func (l *RawLink) Info() (*Info, error) {
 		extra = &XDPInfo{
 			Ifindex: xdpInfo.Ifindex,
 		}
-	case RawTracepointType, IterType,
-		PerfEventType, UprobeMultiType:
+	case RawTracepointType, IterType, UprobeMultiType:
 		// Extra metadata not supported.
 	case TCXType:
 		var tcxInfo sys.TcxLinkInfo
@@ -487,6 +509,30 @@ func (l *RawLink) Info() (*Info, error) {
 			Flags:  kprobeMultiInfo.Flags,
 			Missed: kprobeMultiInfo.Missed,
 		}
+	case PerfEventType:
+		var kprobeInfo sys.KprobeLinkInfo
+		if err := sys.ObjInfo(l.fd, &kprobeInfo); err != nil {
+			return nil, fmt.Errorf("kprobe multi link info: %s", err)
+		}
+		var extra2 interface{}
+
+		switch kprobeInfo.PerfEventType {
+		case KprobePerfEventInfoType, KretprobePerfEventInfoType:
+			// There's a gap between when perf link got introduced and
+			// when it supported link info interface. We can detect that
+			// with kprobe specific Addr field being zero.
+			if kprobeInfo.Addr == 0 {
+				return nil, fmt.Errorf("perf event link info: %w", ErrNotSupported)
+			}
+			extra2 = &KprobeInfo{
+				Addr:   kprobeInfo.Addr,
+				Missed: kprobeInfo.Missed,
+			}
+		}
+		extra = &PerfEventInfo{
+			PerfEventType: kprobeInfo.PerfEventType,
+			extra:         extra2,
+		}
 	default:
 		return nil, fmt.Errorf("unknown link info type: %d", info.Type)
 	}

link/link_test.go

@@ -281,6 +281,16 @@ func testLink(t *testing.T, link Link, prog *ebpf.Program) {
 			if kmulti.Count == 0 {
 				t.Fatalf("Failed to get link KprobeMulti extra info")
 			}
+		case sys.BPF_LINK_TYPE_PERF_EVENT:
+			// test default Info data
+			pevent := info.PerfEvent()
+			switch pevent.PerfEventType {
+			case KprobePerfEventInfoType, KretprobePerfEventInfoType:
+				kp := pevent.Kprobe()
+				if kp.Addr == 0 {
+					t.Fatalf("Failed to get link Kprobe extra info")
+				}
+			}
 		}
 	})