link: Add PerfFd interface function

Adding PerfFd interface function that returns perf event
file descriptor for perf event based links.

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

link/cgroup.go

@@ -168,6 +168,10 @@ func (cg *progAttachCgroup) Info() (*Info, error) {
 	return nil, fmt.Errorf("can't get cgroup info: %w", ErrNotSupported)
 }
 
+func (cg *progAttachCgroup) PerfFd() (int, error) {
+	return -1, fmt.Errorf("perf event fd: %w", ErrNotSupported)
+}
+
 type linkCgroup struct {
 	RawLink
 }

link/link.go

@@ -44,6 +44,11 @@ type Link interface {
 
 	// Prevent external users from implementing this interface.
 	isLink()
+
+	// PerfFd returns perf event file descriptor for perf event based links.
+	//
+	// May return an error wrapping ErrNotSupported
+	PerfFd() (int, error)
 }
 
 // NewLinkFromFD creates a link from a raw fd.
@@ -362,3 +367,7 @@ func (l *RawLink) Info() (*Info, error) {
 		extra,
 	}, nil
 }
+
+func (l *RawLink) PerfFd() (int, error) {
+	return -1, fmt.Errorf("perf event fd: %w", ErrNotSupported)
+}

link/perf_event.go

@@ -156,6 +156,14 @@ func (pi *perfEventIoctl) Info() (*Info, error) {
 	return nil, fmt.Errorf("perf event ioctl info: %w", ErrNotSupported)
 }
 
+func (pi *perfEventIoctl) PerfFd() (int, error) {
+	dup, err := pi.fd.Dup()
+	if err != nil {
+		return -1, fmt.Errorf("can't clone fd: %w", err)
+	}
+	return dup.Int(), nil
+}
+
 type PerfEventInfo struct {
 	Type  PerfEventInfoType
 	extra interface{}
@@ -244,6 +252,14 @@ func (pl *perfEventLink) Info() (*Info, error) {
 	}, nil
 }
 
+func (pl *perfEventLink) PerfFd() (int, error) {
+	dup, err := pl.fd.Dup()
+	if err != nil {
+		return -1, fmt.Errorf("can't clone fd: %w", err)
+	}
+	return dup.Int(), nil
+}
+
 // attach the given eBPF prog to the perf event stored in pe.
 // pe must contain a valid perf event fd.
 // prog's type must match the program type stored in pe.

link/raw_tracepoint.go

@@ -76,6 +76,10 @@ func (frt *simpleRawTracepoint) Info() (*Info, error) {
 	return nil, fmt.Errorf("can't get raw_tracepoint info: %w", ErrNotSupported)
 }
 
+func (frt *simpleRawTracepoint) PerfFd() (int, error) {
+	return -1, fmt.Errorf("perf event fd: %w", ErrNotSupported)
+}
+
 type rawTracepoint struct {
 	RawLink
 }