Patch "vt: change SGR 21 to follow the standards" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

vt: change SGR 21 to follow the standards

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
vt-change-sgr-21-to-follow-the-standards.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Mike Frysinger <[email protected]>
Date: Mon, 29 Jan 2018 17:08:21 -0500
Subject: vt: change SGR 21 to follow the standards

From: Mike Frysinger <[email protected]>

commit 65d9982d7e523a1a8e7c9af012da0d166f72fc56 upstream.

ECMA-48 [1] (aka ISO 6429) has defined SGR 21 as "doubly underlined" since at least March 1984. The Linux kernel has treated it as SGR 22 "normal intensity" since it was added in Linux-0.96b in June 1992. Before that, it was simply ignored. Other terminal emulators have either ignored it, or treat it as double underline now. xterm for example added support in its 304 release (May 2014) [2] where it was previously ignoring it.

Changing this behavior shouldn't be an issue:
- It isn't a named capability in ncurses's terminfo database, so no script is using libtinfo/libcurses to look this up, or using tput to query & output the right sequence.
- Any script assuming SGR 21 will reset intensity in all terminals already do not work correctly on non-Linux VTs (including running under screen/tmux/etc...).
- If someone has written a script that only runs in the Linux VT, and they're using SGR 21 (instead of SGR 22), the output should still be readable.

imo it's important to change this as the Linux VT's non-conformance is sometimes used as an argument for other terminal emulators to not implement SGR 21 at all, or do so incorrectly.

[1]: https://www.ecma-international.org/publications/standards/Ecma-048.htm
[2]: https://github.com/ThomasDickey/xterm-snapshots/commit/2fd29cb98d214cb536bcafbee00bc73b3f1eeb9d

---
 drivers/tty/vt/vt.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

@@ -1354,6 +1354,11 @@ static void csi_m(struct vc_data *vc)
 		case 3:
 			vc->vc_italic = 1;
 			break;
+		case 21:
+			/*
+			 * No console drivers support double underline, so
+			 * convert it to a single underline.
+			 */
 		case 4:
 			vc->vc_underline = 1;
 			break;

@@ -1389,7 +1394,6 @@ static void csi_m(struct vc_data *vc)
 			vc->vc_disp_ctrl = 1;
 			vc->vc_toggle_meta = 1;
 			break;
-		case 21:
 		case 22:
 			vc->vc_intensity = 1;
 			break;

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/vt-change-sgr-21-to-follow-the-standards.patch