internal/fastwalk: avoid slice bounds out of range for long file names

Fixes golang/go#28715

Change-Id: Ibed78d2376f4ec010ee9e5e772069afd14fd5552
Reviewed-on: https://go-review.googlesource.com/c/148884
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

internal/fastwalk/fastwalk_dirent_namlen_linux.go

@@ -16,7 +16,12 @@ import (
 func direntNamlen(dirent *syscall.Dirent) uint64 {
 	const fixedHdr = uint16(unsafe.Offsetof(syscall.Dirent{}.Name))
 	nameBuf := (*[unsafe.Sizeof(dirent.Name)]byte)(unsafe.Pointer(&dirent.Name[0]))
-	nameLen := bytes.IndexByte(nameBuf[:dirent.Reclen-fixedHdr], 0)
+	const nameBufLen = uint16(len(nameBuf))
+	limit := dirent.Reclen - fixedHdr
+	if limit > nameBufLen {
+		limit = nameBufLen
+	}
+	nameLen := bytes.IndexByte(nameBuf[:limit], 0)
 	if nameLen < 0 {
 		panic("failed to find terminating 0 byte in dirent")
 	}

internal/fastwalk/fastwalk_test.go

@@ -98,6 +98,23 @@ func TestFastWalk_Basic(t *testing.T) {
 		})
 }
 
+func TestFastWalk_LongFileName(t *testing.T) {
+	longFileName := strings.Repeat("x", 255)
+
+	testFastWalk(t, map[string]string{
+		longFileName: "one",
+	},
+		func(path string, typ os.FileMode) error {
+			return nil
+		},
+		map[string]os.FileMode{
+			"":                     os.ModeDir,
+			"/src":                 os.ModeDir,
+			"/src/" + longFileName: 0,
+		},
+	)
+}
+
 func TestFastWalk_Symlink(t *testing.T) {
 	switch runtime.GOOS {
 	case "windows", "plan9":