Cygwin: path: Add fallback for DFS mounted drive.

- If UNC path for DFS is mounted to a drive with drive letter, the
  error "Too many levels of symbolic links" occurs when accessing
  to that drive. This is because GetDosDeviceW() returns unexpected
  string such as "\Device\Mup\DfsClient\;Z:000000000003fb89\dfsserver
  \dfs\linkname" for the mounted UNC path "\??\UNC\fileserver\share".
  This patch adds a workaround for this issue.

  Addresses: https://cygwin.com/pipermail/cygwin/2022-March/250979.html

diff --git a/winsup/cygwin/path.cc b/winsup/cygwin/path.cc
index 4ad4e0821d9467e246a9652ed67bcbabbf9d2388..eceafbbcf0b237832af6c42bcc64598f1131e4f0 100644 (file)
--- a/winsup/cygwin/path.cc
+++ b/winsup/cygwin/path.cc
@@ -3527,7 +3527,7 @@ restart:
                      int remlen = QueryDosDeviceW (drive, remote, MAX_PATH);
                      if (remlen < 3)
                        goto file_not_symlink; /* fallback */
-                     remlen -= 2;
+                     remlen -= 2; /* Two L'\0' */
 
                      if (remote[remlen - 1] == L'\\')
                        remlen--;
@@ -3535,20 +3535,27 @@ restart:
                      UNICODE_STRING rpath;
                      RtlInitCountedUnicodeString (&rpath, remote,
                                                   remlen * sizeof (WCHAR));
+                     const USHORT uncp_len =
+                       ro_u_uncp.Length / sizeof (WCHAR) - 1;
                      if (RtlEqualUnicodePathPrefix (&rpath, &ro_u_uncp, TRUE))
-                       remlen -= 6;
+                       {
+                         remlen -= uncp_len;
+                         p = remote + uncp_len;
+                       }
                      else if ((p = wcschr (remote, L';'))
                               && p + 3 < remote + remlen
                               && wcsncmp (p + 1, drive, 2) == 0
                               && (p = wcschr (p + 3, L'\\')))
-                       remlen -= p - remote - 1;
+                       remlen -= p - remote;
                      else
                        goto file_not_symlink; /* fallback */
+                     if (wcsncasecmp (fpath.Buffer + uncp_len, p, remlen))
+                       goto file_not_symlink; /* fallback (not expected) */
                      /* Hackfest */
                      fpath.Buffer[4] = drive[0]; /* Drive letter */
                      fpath.Buffer[5] = L':';
-                     WCHAR *to = fpath.Buffer + 6;
-                     WCHAR *from = to + remlen;
+                     WCHAR *to = fpath.Buffer + 6; /* Next to L':' */
+                     WCHAR *from = fpath.Buffer + uncp_len + remlen;
                      memmove (to, from,
                               (wcslen (from) + 1) * sizeof (WCHAR));
                      fpath.Length -= (from - to) * sizeof (WCHAR);