1 files changed, 67 insertions, 0 deletions
diff --git a/src/path/filepath/path_windows.go b/src/path/filepath/path_windows.go
index c754301bf4..b26658a937 100644
--- a/src/path/filepath/path_windows.go
+++ b/src/path/filepath/path_windows.go
@@ -20,6 +20,73 @@ func toUpper(c byte) byte {
 	return c
 }
 
+// isReservedName reports if name is a Windows reserved device name.
+// It does not detect names with an extension, which are also reserved on some Windows versions.
+//
+// For details, search for PRN in
+// https://docs.microsoft.com/en-us/windows/desktop/fileio/naming-a-file.
+func isReservedName(name string) bool {
+	if 3 <= len(name) && len(name) <= 4 {
+		switch string([]byte{toUpper(name[0]), toUpper(name[1]), toUpper(name[2])}) {
+		case "CON", "PRN", "AUX", "NUL":
+			return len(name) == 3
+		case "COM", "LPT":
+			return len(name) == 4 && '1' <= name[3] && name[3] <= '9'
+		}
+	}
+	return false
+}
+
+func isLocal(path string) bool {
+	if path == "" {
+		return false
+	}
+	if isSlash(path[0]) {
+		// Path rooted in the current drive.
+		return false
+	}
+	if strings.IndexByte(path, ':') >= 0 {
+		// Colons are only valid when marking a drive letter ("C:foo").
+		// Rejecting any path with a colon is conservative but safe.
+		return false
+	}
+	hasDots := false // contains . or .. path elements
+	for p := path; p != ""; {
+		var part string
+		part, p, _ = cutPath(p)
+		if part == "." || part == ".." {
+			hasDots = true
+		}
+		// Trim the extension and look for a reserved name.
+		base, _, hasExt := strings.Cut(part, ".")
+		if isReservedName(base) {
+			if !hasExt {
+				return false
+			}
+			// The path element is a reserved name with an extension. Some Windows
+			// versions consider this a reserved name, while others do not. Use
+			// FullPath to see if the name is reserved.
+			//
+			// FullPath will convert references to reserved device names to their
+			// canonical form: \\.\${DEVICE_NAME}
+			//
+			// FullPath does not perform this conversion for paths which contain
+			// a reserved device name anywhere other than in the last element,
+			// so check the part rather than the full path.
+			if p, _ := syscall.FullPath(part); len(p) >= 4 && p[:4] == `\\.\` {
+				return false
+			}
+		}
+	}
+	if hasDots {
+		path = Clean(path)
+	}
+	if path == ".." || strings.HasPrefix(path, `..\`) {
+		return false
+	}
+	return true
+}
+
 // IsAbs reports whether the path is absolute.
 func IsAbs(path string) (b bool) {
 	l := volumeNameLen(path)