proc/gdbserial: do not run rr when perf_event_paranoid > 1

On newer kernels when /proc/sys/kernel/perf_event_paranoid is > 1 rr
will crash with SIGABRT. Return an error instead of letting this
happen.

pkg/proc/gdbserial/rr.go

@@ -8,6 +8,7 @@ import (
 	"io/ioutil"
 	"os"
 	"os/exec"
+	"strconv"
 	"strings"
 	"unicode"
 )
@@ -15,8 +16,8 @@ import (
 // Record uses rr to record the execution of the specified program and
 // returns the trace directory's path.
 func Record(cmd []string, wd string, quiet bool) (tracedir string, err error) {
-	if _, err := exec.LookPath("rr"); err != nil {
-		return "", &ErrBackendUnavailable{}
+	if err := checkRRAvailabe(); err != nil {
+		return "", err
 	}
 
 	rfd, wfd, err := os.Pipe()
@@ -53,8 +54,8 @@ func Record(cmd []string, wd string, quiet bool) (tracedir string, err error) {
 // Replay starts an instance of rr in replay mode, with the specified trace
 // directory, and connects to it.
 func Replay(tracedir string, quiet bool) (*Process, error) {
-	if _, err := exec.LookPath("rr"); err != nil {
-		return nil, &ErrBackendUnavailable{}
+	if err := checkRRAvailabe(); err != nil {
+		return nil, err
 	}
 
 	rrcmd := exec.Command("rr", "replay", "--dbgport=0", tracedir)
@@ -90,6 +91,33 @@ func Replay(tracedir string, quiet bool) (*Process, error) {
 	return p, nil
 }
 
+// ErrPerfEventParanoid is the error returned by Reply and Record if
+// /proc/sys/kernel/perf_event_paranoid is greater than 1.
+type ErrPerfEventParanoid struct {
+	actual int
+}
+
+func (err ErrPerfEventParanoid) Error() string {
+	return fmt.Sprintf("rr needs /proc/sys/kernel/perf_event_paranoid <= 1, but it is %d", err.actual)
+}
+
+func checkRRAvailabe() error {
+	if _, err := exec.LookPath("rr"); err != nil {
+		return &ErrBackendUnavailable{}
+	}
+
+	// Check that /proc/sys/kernel/perf_event_paranoid doesn't exist or is <= 1.
+	buf, err := ioutil.ReadFile("/proc/sys/kernel/perf_event_paranoid")
+	if err == nil {
+		perfEventParanoid, _ := strconv.Atoi(strings.TrimSpace(string(buf)))
+		if perfEventParanoid > 1 {
+			return ErrPerfEventParanoid{perfEventParanoid}
+		}
+	}
+
+	return nil
+}
+
 type rrInit struct {
 	port string
 	exe  string