fixup! Instead of creating Cygwin symlinks, use deep copy by default

avoid recursing into dst as well as src.  Given we only ever append to
either, their original lengths should be sufficient, rather than copies
of them.

Author: jeremyd2019

winsup/cygwin/path.cc

@@ -1706,7 +1706,8 @@ recursiveCopyCheckSymlink(PUNICODE_STRING src, bool& isdirlink)
   Create a deep copy of src as dst, while avoiding descending in origpath.
 */
 static int
-recursiveCopy (PUNICODE_STRING src, PUNICODE_STRING dst, PCWSTR origpath, PWIN32_FIND_DATAW dHfile = NULL)
+recursiveCopy (PUNICODE_STRING src, PUNICODE_STRING dst, USHORT origsrclen,
+	       USHORT origdstlen, PWIN32_FIND_DATAW dHfile = NULL)
 {
   HANDLE dH = INVALID_HANDLE_VALUE;
   NTSTATUS status;
@@ -1801,11 +1802,15 @@ recursiveCopy (PUNICODE_STRING src, PUNICODE_STRING dst, PCWSTR origpath, PWIN32
       if ((dHfile->dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY) && !isdirlink)
 	{
 	  /* Recurse into the child directory */
-	  debug_printf ("%S <-> %W", src, origpath);
-	  // avoids endless recursion
-	  if (wcsncmp (src->Buffer, origpath, src->Length / sizeof (WCHAR)))
-	    if (recursiveCopy (src, dst, origpath, dHfile))
+	  /* avoids endless recursion */
+	  if (src->Length <= origsrclen ||
+	      !wcsncmp (src->Buffer, dst->Buffer, origdstlen / sizeof (WCHAR)))
+	    {
+	      set_errno (ELOOP);
 	      goto done;
+	    }
+	  if (recursiveCopy (src, dst, origsrclen, origdstlen, dHfile))
+	    goto done;
 	}
       else
 	{
@@ -2419,14 +2424,13 @@ symlink_worker (const char *oldpath, path_conv &win32_newpath, bool isdevice)
 		    w_newpath->Buffer[1] = L'\\';
 		  if (win32_oldpath.isdir ())
 		    {
-		      PWCHAR origpath =
-			win32_oldpath.get_wide_win32_path (tp.w_get ());
 		      /* we need a larger UNICODE_STRING MaximumLength than
 			 get_nt_native_path allocates for the recursive copy */
 		      UNICODE_STRING u_oldpath, u_newpath;
 		      RtlCopyUnicodeString (tp.u_get (&u_oldpath), w_oldpath);
 		      RtlCopyUnicodeString (tp.u_get (&u_newpath), w_newpath);
-		      res = recursiveCopy (&u_oldpath, &u_newpath, origpath);
+		      res = recursiveCopy (&u_oldpath, &u_newpath,
+					   u_oldpath.Length, u_newpath.Length);
 		    }
 		  else
 		    {