Use a more robust way to get dco-win version

The current way doesn't work if the device is already in use.

Starting from 1.3.0, dco-win creates a non-exclusive
control device \\.\ovpn-dco-ver which can be opened by
multiple apps and supports a single IOCTL to get
a version number.

OpenVPN/ovpn-dco-win#76

This will be expecially handy later when checking which
features driver supports.

Change-Id: Ieb6f3a9d14d76000c1caf8ee1e959c6d0de832bf
Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20240809192257.24208-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg29009.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit e5a8ea3)

src/openvpn/dco_win.c

@@ -389,9 +389,16 @@ dco_version_string(struct gc_arena *gc)
     OVPN_VERSION version;
     ZeroMemory(&version, sizeof(OVPN_VERSION));
 
-    /* try to open device by symbolic name */
-    HANDLE h = CreateFile("\\\\.\\ovpn-dco", GENERIC_READ | GENERIC_WRITE,
-                          0, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_SYSTEM | FILE_FLAG_OVERLAPPED, NULL);
+    /* first, try a non-exclusive control device, available from 1.3.0 */
+    HANDLE h = CreateFile("\\\\.\\ovpn-dco-ver", GENERIC_READ,
+                          0, NULL, OPEN_EXISTING, 0, NULL);
+
+    if (h == INVALID_HANDLE_VALUE)
+    {
+        /* fallback to a "normal" device, this will fail if device is already in use */
+        h = CreateFile("\\\\.\\ovpn-dco", GENERIC_READ,
+                       0, NULL, OPEN_EXISTING, 0, NULL);
+    }
 
     if (h == INVALID_HANDLE_VALUE)
     {