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.

4 files changed, 25 insertions, 3 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)
+	}
+}
diff --git a/syz-fuzzer/fuzzer.go b/syz-fuzzer/fuzzer.go
index f539de6f6..fb7f57b30 100644
--- a/syz-fuzzer/fuzzer.go
+++ b/syz-fuzzer/fuzzer.go
@@ -347,11 +347,11 @@ func (tool *FuzzerTool) exchangeDataCall(needProgs int, results []executionResul
 		a.Results = append(a.Results, tool.convertExecutionResult(result))
 	}
 	r := &rpctype.ExchangeInfoReply{}
-	start := time.Now()
+	start := osutil.MonotonicNano()
 	if err := tool.manager.Call("Manager.ExchangeInfo", a, r); err != nil {
 		log.SyzFatalf("Manager.ExchangeInfo call failed: %v", err)
 	}
-	latency = time.Since(start)
+	latency = osutil.MonotonicNano() - start
 	if needProgs != len(r.Requests) {
 		log.SyzFatalf("manager returned wrong number of requests: %v/%v", needProgs, len(r.Requests))
 	}
diff --git a/tools/syz-linter/linter.go b/tools/syz-linter/linter.go
index 76168c7c9..542d040e6 100644
--- a/tools/syz-linter/linter.go
+++ b/tools/syz-linter/linter.go
@@ -139,7 +139,7 @@ var (
 	noPeriodComment   = regexp.MustCompile(`^// [A-Z][a-z].+[a-z]$`)
 	lowerCaseComment  = regexp.MustCompile(`^// [a-z]+ `)
 	onelineExceptions = regexp.MustCompile(`// want \"|http:|https:`)
-	specialComment    = regexp.MustCompile(`//go:generate|//go:build|//go:embed|// nolint:`)
+	specialComment    = regexp.MustCompile(`//go:generate|//go:build|//go:embed|//go:linkname|// nolint:`)
 )
 
 // checkStringLenCompare checks for string len comparisons with 0.