1 files changed, 105 insertions, 0 deletions
diff --git a/src/os/root_test.go b/src/os/root_test.go
index f9fbd11575..71ce5e559a 100644
--- a/src/os/root_test.go
+++ b/src/os/root_test.go
@@ -1952,3 +1952,108 @@ func TestRootName(t *testing.T) {
 		t.Errorf(`root.OpenRoot("dir").Name() = %q, want %q`, got, want)
 	}
 }
+
+// TestRootNoLstat verifies that we do not use lstat (possibly escaping the root)
+// when reading directories in a Root.
+func TestRootNoLstat(t *testing.T) {
+	if runtime.GOARCH == "wasm" {
+		t.Skip("wasm lacks fstatat")
+	}
+
+	dir := makefs(t, []string{
+		"subdir/",
+	})
+	const size = 42
+	contents := strings.Repeat("x", size)
+	if err := os.WriteFile(dir+"/subdir/file", []byte(contents), 0666); err != nil {
+		t.Fatal(err)
+	}
+	root, err := os.OpenRoot(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer root.Close()
+
+	test := func(name string, fn func(t *testing.T, f *os.File)) {
+		t.Run(name, func(t *testing.T) {
+			os.SetStatHook(t, func(f *os.File, name string) (os.FileInfo, error) {
+				if f == nil {
+					t.Errorf("unexpected Lstat(%q)", name)
+				}
+				return nil, nil
+			})
+			f, err := root.Open("subdir")
+			if err != nil {
+				t.Fatal(err)
+			}
+			defer f.Close()
+			fn(t, f)
+		})
+	}
+
+	checkFileInfo := func(t *testing.T, fi fs.FileInfo) {
+		t.Helper()
+		if got, want := fi.Name(), "file"; got != want {
+			t.Errorf("FileInfo.Name() = %q, want %q", got, want)
+		}
+		if got, want := fi.Size(), int64(size); got != want {
+			t.Errorf("FileInfo.Size() = %v, want %v", got, want)
+		}
+	}
+	checkDirEntry := func(t *testing.T, d fs.DirEntry) {
+		t.Helper()
+		if got, want := d.Name(), "file"; got != want {
+			t.Errorf("DirEntry.Name() = %q, want %q", got, want)
+		}
+		if got, want := d.IsDir(), false; got != want {
+			t.Errorf("DirEntry.IsDir() = %v, want %v", got, want)
+		}
+		fi, err := d.Info()
+		if err != nil {
+			t.Fatalf("DirEntry.Info() = _, %v", err)
+		}
+		checkFileInfo(t, fi)
+	}
+
+	test("Stat", func(t *testing.T, subdir *os.File) {
+		fi, err := subdir.Stat()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !fi.IsDir() {
+			t.Fatalf(`Open("subdir").Stat().IsDir() = false, want true`)
+		}
+	})
+	// File.ReadDir, returning []DirEntry
+	test("ReadDirEntry", func(t *testing.T, subdir *os.File) {
+		dirents, err := subdir.ReadDir(-1)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if len(dirents) != 1 {
+			t.Fatalf(`Open("subdir").ReadDir(-1) = {%v}, want {file}`, dirents)
+		}
+		checkDirEntry(t, dirents[0])
+	})
+	// File.Readdir, returning []FileInfo
+	test("ReadFileInfo", func(t *testing.T, subdir *os.File) {
+		fileinfos, err := subdir.Readdir(-1)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if len(fileinfos) != 1 {
+			t.Fatalf(`Open("subdir").Readdir(-1) = {%v}, want {file}`, fileinfos)
+		}
+		checkFileInfo(t, fileinfos[0])
+	})
+	// File.Readdirnames, returning []string
+	test("Readdirnames", func(t *testing.T, subdir *os.File) {
+		names, err := subdir.Readdirnames(-1)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if got, want := names, []string{"file"}; !slices.Equal(got, want) {
+			t.Fatalf(`Open("subdir").Readdirnames(-1) = %q, want %q`, got, want)
+		}
+	})
+}