io/fs: allow backslash in ValidPath, reject in os.DirFS.Open

Rejecting backslash introduces problems with presenting
underlying OS file systems that contain names with backslash.
Rejecting backslash also does not Windows-proof the syntax,
because colon can also be a path separator. And we are not
going to reject colon from all names. So don't reject backslash
either.

There is a similar problem on Windows with names containing
slashes, but those are more difficult (though not impossible)
to create.

Also document and enforce that paths must be UTF-8.

Fixes #44166.

Change-Id: Iac7a9a268025c1fd31010dbaf3f51e1660c7ae2a
Reviewed-on: https://go-review.googlesource.com/c/go/+/290709
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>

src/io/fs/fs.go

@@ -10,6 +10,7 @@ package fs
 import (
 	"internal/oserror"
 	"time"
+	"unicode/utf8"
 )
 
 // An FS provides access to a hierarchical file system.
@@ -32,15 +33,22 @@ type FS interface {
 
 // ValidPath reports whether the given path name
 // is valid for use in a call to Open.
-// Path names passed to open are unrooted, slash-separated
-// sequences of path elements, like “x/y/z”.
-// Path names must not contain a “.” or “..” or empty element,
-// except for the special case that the root directory is named “.”.
-// Leading and trailing slashes (like “/x” or “x/”) are not allowed.
 //
-// Paths are slash-separated on all systems, even Windows.
-// Backslashes must not appear in path names.
+// Path names passed to open are UTF-8-encoded,
+// unrooted, slash-separated sequences of path elements, like “x/y/z”.
+// Path names must not contain an element that is “.” or “..” or the empty string,
+// except for the special case that the root directory is named “.”.
+// Paths must not start or end with a slash: “/x” and “x/” are invalid.
+//
+// Note that paths are slash-separated on all systems, even Windows.
+// Paths containing other characters such as backslash and colon
+// are accepted as valid, but those characters must never be
+// interpreted by an FS implementation as path element separators.
 func ValidPath(name string) bool {
+	if !utf8.ValidString(name) {
+		return false
+	}
+
 	if name == "." {
 		// special case
 		return true
@@ -50,9 +58,6 @@ func ValidPath(name string) bool {
 	for {
 		i := 0
 		for i < len(name) && name[i] != '/' {
-			if name[i] == '\\' {
-				return false
-			}
 			i++
 		}
 		elem := name[:i]

src/io/fs/fs_test.go

@@ -33,9 +33,10 @@ var isValidPathTests = []struct {
 	{"x/..", false},
 	{"x/../y", false},
 	{"x//y", false},
-	{`x\`, false},
-	{`x\y`, false},
-	{`\x`, false},
+	{`x\`, true},
+	{`x\y`, true},
+	{`x:y`, true},
+	{`\x`, true},
 }
 
 func TestValidPath(t *testing.T) {

src/os/file.go

@@ -620,10 +620,21 @@ func DirFS(dir string) fs.FS {
 	return dirFS(dir)
 }
 
+func containsAny(s, chars string) bool {
+	for i := 0; i < len(s); i++ {
+		for j := 0; j < len(chars); j++ {
+			if s[i] == chars[j] {
+				return true
+			}
+		}
+	}
+	return false
+}
+
 type dirFS string
 
 func (dir dirFS) Open(name string) (fs.File, error) {
-	if !fs.ValidPath(name) {
+	if !fs.ValidPath(name) || runtime.GOOS == "windows" && containsAny(name, `\:`) {
 		return nil, &PathError{Op: "open", Path: name, Err: ErrInvalid}
 	}
 	f, err := Open(string(dir) + "/" + name)

src/os/os_test.go

@@ -2719,6 +2719,40 @@ func TestDirFS(t *testing.T) {
 	if err := fstest.TestFS(DirFS("./testdata/dirfs"), "a", "b", "dir/x"); err != nil {
 		t.Fatal(err)
 	}
+
+	// Test that Open does not accept backslash as separator.
+	d := DirFS(".")
+	_, err := d.Open(`testdata\dirfs`)
+	if err == nil {
+		t.Fatalf(`Open testdata\dirfs succeeded`)
+	}
+}
+
+func TestDirFSPathsValid(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skipf("skipping on Windows")
+	}
+
+	d := t.TempDir()
+	if err := os.WriteFile(filepath.Join(d, "control.txt"), []byte(string("Hello, world!")), 0644); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(d, `e:xperi\ment.txt`), []byte(string("Hello, colon and backslash!")), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	fsys := os.DirFS(d)
+	err := fs.WalkDir(fsys, ".", func(path string, e fs.DirEntry, err error) error {
+		if fs.ValidPath(e.Name()) {
+			t.Logf("%q ok", e.Name())
+		} else {
+			t.Errorf("%q INVALID", e.Name())
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
 }
 
 func TestReadFileProc(t *testing.T) {