syz-fuzzer: use of monotonic time for latency measurement

time.Now/Since may reject to use monotonic time if the fuzzer
messes with system time badly enough. Enforce use of monotonic time.

2 files changed, 22 insertions, 0 deletions

diff --git a/pkg/osutil/osutil.go b/pkg/osutil/osutil.go
index 478c091dc..a619f9169 100644
--- a/pkg/osutil/osutil.go
+++ b/pkg/osutil/osutil.go
@@ -17,6 +17,7 @@ import (
 	"sync"
 	"syscall"
 	"time"
+	_ "unsafe" // required to use go:linkname
 )
 
 const (
@@ -337,3 +338,14 @@ func Abs(path string) string {
 	}
 	return filepath.Join(wd, path)
 }
+
+// MonotonicNano returns monotonic time in nanoseconds from some unspecified point in time.
+// Useful mostly to measure time intervals.
+// This function should be used inside of tested VMs b/c time.Now may reject to use monotonic time
+// if the fuzzer messes with system time (sets time past Y2157, see comments in time/time.go).
+// This is a hacky way to use the private runtime function.
+// If this ever breaks, we can either provide specializations for different Go versions
+// using build tags, or fall back to time.Now.
+//
+//go:linkname MonotonicNano runtime.nanotime
+func MonotonicNano() time.Duration
diff --git a/pkg/osutil/osutil_test.go b/pkg/osutil/osutil_test.go
index 713a62c14..7d5913b4a 100644
--- a/pkg/osutil/osutil_test.go
+++ b/pkg/osutil/osutil_test.go
@@ -9,6 +9,7 @@ import (
 	"path/filepath"
 	"strings"
 	"testing"
+	"time"
 )
 
 func TestIsExist(t *testing.T) {
@@ -119,3 +120,12 @@ func TestCopyFiles(t *testing.T) {
 		})
 	}
 }
+
+func TestMonotonicNano(t *testing.T) {
+	start := MonotonicNano()
+	time.Sleep(100 * time.Millisecond)
+	diff := MonotonicNano() - start
+	if diff <= 0 || diff > 10*time.Second {
+		t.Fatalf("diff %v", diff)
+	}
+}