--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -7226,8 +7226,11 @@ A jump table for the options with a shor
 			{not in Vi}
 			{only when compiled with the +persistent_undo feature}
 	List of directory names for undo files, separated with commas.
-	See |'backupdir'| for the format.  Specifically, "." means using the
-	directory of the file.
+	See |'backupdir'| for the format.
+	"." means using the directory of the file.  The undo file name for
+	"file.txt" is ".file.txt.un~".
+	For other directories the file name is the full path of the edited
+	file, with path separators replaced with "%".
 	When writing: The first directory that exists is used. "." always
 	works, no directories after "." will be used for writing.
 	When reading all entries are tried to find an undo file.  The first

--- a/src/undo.c
+++ b/src/undo.c
@@ -1233,6 +1233,7 @@ u_write_undo(name, forceit, buf, hash)
     int		perm;
     int		write_ok = FALSE;
 #ifdef UNIX
+    int		st_old_valid = FALSE;
     struct stat	st_old;
     struct stat	st_new;
 #endif
@@ -1246,16 +1247,25 @@ u_write_undo(name, forceit, buf, hash)
     else
         file_name = name;
 
-#ifdef UNIX
-    if (mch_stat((char *)buf->b_ffname, &st_old) >= 0)
-	perm = st_old.st_mode;
-    else
+    if (buf->b_ffname == NULL)
 	perm = 0600;
+    else
+    {
+#ifdef UNIX
+	if (mch_stat((char *)buf->b_ffname, &st_old) >= 0)
+	{
+	    perm = st_old.st_mode;
+	    st_old_valid = TRUE;
+	}
+	else
+	    perm = 0600;
 #else
-    perm = mch_getperm(buf->b_ffname);
-    if (perm < 0)
-	perm = 0600;
+	perm = mch_getperm(buf->b_ffname);
+	if (perm < 0)
+	    perm = 0600;
 #endif
+    }
+
     /* set file protection same as original file, but strip s-bit */
     perm = perm & 0777;
 
@@ -1309,15 +1319,16 @@ u_write_undo(name, forceit, buf, hash)
      * this fails, set the protection bits for the group same as the
      * protection bits for others.
      */
-    if (mch_stat((char *)file_name, &st_new) >= 0
-	    && st_new.st_gid != st_old.st_gid
+    if (st_old_valid && (mch_stat((char *)file_name, &st_new) >= 0
+		&& st_new.st_gid != st_old.st_gid
 # ifdef HAVE_FCHOWN  /* sequent-ptx lacks fchown() */
-	    && fchown(fd, (uid_t)-1, st_old.st_gid) != 0
+		&& fchown(fd, (uid_t)-1, st_old.st_gid) != 0)
 # endif
        )
 	mch_setperm(file_name, (perm & 0707) | ((perm & 07) << 3));
 # ifdef HAVE_SELINUX
-    mch_copy_sec(buf->b_ffname, file_name);
+    if (buf->b_ffname != NULL)
+	mch_copy_sec(buf->b_ffname, file_name);
 # endif
 #endif
 
@@ -1438,9 +1449,11 @@ write_error:
         EMSG2(_("E829: write error in undo file: %s"), file_name);
 
 #if defined(MACOS_CLASSIC) || defined(WIN3264)
-    (void)mch_copy_file_attribute(buf->b_ffname, file_name);
+    if (buf->b_ffname != NULL)
+	(void)mch_copy_file_attribute(buf->b_ffname, file_name);
 #endif
 #ifdef HAVE_ACL
+    if (buf->b_ffname != NULL)
     {
 	vim_acl_T	    acl;