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>

2 files changed, 18 insertions, 12 deletions

diff --git a/src/io/fs/fs.go b/src/io/fs/fs.go
index c330f123ad..3d2e2ee2ac 100644
--- a/src/io/fs/fs.go
+++ b/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,
+//
+// 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 “.”.
-// Leading and trailing slashes (like “/x” or “x/”) are not allowed.
+// Paths must not start or end with a slash: “/x” and “x/” are invalid.
 //
-// Paths are slash-separated on all systems, even Windows.
-// Backslashes must not appear in path names.
+// 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]
diff --git a/src/io/fs/fs_test.go b/src/io/fs/fs_test.go
index 8d395fc0db..aae1a7606f 100644
--- a/src/io/fs/fs_test.go
+++ b/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) {