Cygwin: mount: allow any valid character in UNC paths

The current code only allows server and share names to
start with ASCII chars [a-zA-Z0-9],, which is not correct.
Rather, check for a valid share character.

Fixes: 1fd5e000ac ("import winsup-2000-02-17 snapshot")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/mount.cc

@@ -42,11 +42,20 @@ bool NO_COPY mount_info::got_usr_bin;
 bool NO_COPY mount_info::got_usr_lib;
 int NO_COPY mount_info::root_idx = -1;
 
-/* is_unc_share: Return non-zero if PATH begins with //server/share
+/* is_native_path: Return non-zero if PATH starts with \??\[a-zA-Z] or
+		   with \\?\[a-zA-Z] or with \\.\[a-zA-Z].
+
+   is_unc_share: Return non-zero if PATH begins with //server/share
 		 or with one of the native prefixes //./ or //?/
+
    This function is only used to test for valid input strings.
    The later normalization drops the native prefixes. */
 
+/* list of invalid chars in UNC filenames.  These are a few more than
+   for "normal" filenames. */
+const char _invalid_char[] = "\"/\\[]:|<>+=;,?*";
+#define valid_share_char(_c)	(strchr (_invalid_char, (_c)) == NULL)
+
 static inline bool
 is_native_path (const char *path)
 {
@@ -63,9 +72,9 @@ is_unc_share (const char *path)
   const char *p;
   return (isdirsep (path[0])
 	 && isdirsep (path[1])
-	 && isalnum (path[2])
+	 && valid_share_char (path[2])
 	 && ((p = strpbrk (path + 3, "\\/")) != NULL)
-	 && isalnum (p[1]));
+	 && valid_share_char (p[1]));
 }
 
 /* Return true if src_path is a valid, internally supported device name.

winsup/cygwin/release/3.4.6

@@ -3,3 +3,6 @@ Bug Fixes
 
 Fix a problem that fsync returns EINVAL for block device.
 Addresses: https://cygwin.com/pipermail/cygwin/2023-January/252916.html
+
+Don't reject valid server and share names when mounting.
+Addresses: https://cygwin.com/pipermail/cygwin/2023-January/252928.html