vt: make VS16 emoji widening controllable

index 49a140b49eff..335253aec549 100644
@@ -3128,6 +3128,15 @@ static int vc_process_ucs(struct vc_data *vc, int *c, int *tc)
 	}
 
 	/* From here the preceding character, if any, must be single-width. */
+
+	/*
+	 * The Unicode screen memory is allocated only when required. VS16
+	 * needs it to find the character it applies to, so it can't depend
+	 * on whether a double-width character was displayed earlier.
+	 */
+	if (curr_c == UCS_VS16)
+		vc_uniscr_check(vc);
+
 	prev_c = vc_uniscr_getc(vc, -1);
 
 	if (curr_c == UCS_VS16 && prev_c != 0) {

index 335253aec549..4d5c9cabd058 100644
@@ -166,6 +166,9 @@ EXPORT_SYMBOL(global_cursor_default);
 static int cur_default = CUR_UNDERLINE;
 module_param(cur_default, int, S_IRUGO | S_IWUSR);
 
+static bool default_vs16_wide = true;
+module_param(default_vs16_wide, bool, 0644);
+
 /*
  * ignore_poke: don't unblank the screen when things are typed.  This is
  * mainly for the privacy of braille terminal users.

@@ -1984,6 +1987,7 @@ enum {
 	CSI_DEC_hl_MOUSE_VT200	= 1000,
 	CSI_DEC_hl_ALT_SCREEN	= 1049,
 	CSI_DEC_hl_BRACKETED_PASTE = 2004,
+	CSI_DEC_hl_GRAPHEME_WIDTH = 2027,	/* VS16 widening */
 };
 
 /* console_lock is held */

@@ -2039,6 +2043,9 @@ static void csi_DEC_hl(struct vc_data *vc, bool on_off)
 		case CSI_DEC_hl_BRACKETED_PASTE:
 			vc->vc_bracketed_paste = on_off;
 			break;
+		case CSI_DEC_hl_GRAPHEME_WIDTH:
+			vc->vc_vs16_wide = on_off;
+			break;
 		case CSI_DEC_hl_ALT_SCREEN:
 			if (on_off)
 				enter_alt_screen(vc);

@@ -2284,6 +2291,7 @@ static void reset_terminal(struct vc_data *vc, int do_clear)
 	vc->vc_decawm		= 1;
 	vc->vc_deccm		= global_cursor_default;
 	vc->vc_decim		= 0;
+	vc->vc_vs16_wide	= default_vs16_wide;
 
 	if (vc->vc_saved_screen != NULL) {
 		kfree(vc->vc_saved_screen);

@@ -3139,12 +3147,17 @@ static int vc_process_ucs(struct vc_data *vc, int *c, int *tc)
 
 	prev_c = vc_uniscr_getc(vc, -1);
 
-	if (curr_c == UCS_VS16 && prev_c != 0) {
+	if (curr_c == UCS_VS16 && prev_c != 0 && vc->vc_vs16_wide) {
 		/*
 		 * VS16 (U+FE0F) is special. It typically turns the preceding
 		 * single-width character into a double-width one. Let it
 		 * have a width of 1 effectively making the combination with
 		 * the preceding character double-width.
+		 *
+		 * This can be disabled with DEC private mode 2027 for
+		 * applications that compute widths per code point with
+		 * wcwidth(), where VS16 is always zero-width. Otherwise
+		 * they lose track of the cursor position.
 		 */
 		*tc = ' ';
 		return 1;

index fe915afdece5..8a1031cd097e 100644
@@ -167,6 +167,7 @@ struct vc_data {
 	unsigned int	vc_decawm	: 1;	/* Autowrap Mode */
 	unsigned int	vc_deccm	: 1;	/* Cursor Visible */
 	unsigned int	vc_decim	: 1;	/* Insert Mode */
+	unsigned int	vc_vs16_wide	: 1;	/* VS16 makes preceding char wide */
 	/* misc */
 	unsigned int	vc_priv		: 3;
 	unsigned int	vc_need_wrap	: 1;

index 4d5c9cabd058..22dfab8de7bc 100644
@@ -2083,6 +2083,82 @@ static void csi_hl(struct vc_data *vc, bool on_off)
 		}
 }
 
+/* DECRPM mode states */
+enum {
+	DECRPM_NOT_RECOGNIZED	= 0,
+	DECRPM_SET		= 1,
+	DECRPM_RESET		= 2,
+	DECRPM_PERMANENTLY_RESET = 4,
+};
+
+static int decrpm_state(bool set)
+{
+	return set ? DECRPM_SET : DECRPM_RESET;
+}
+
+/* State of a DEC private mode as handled by csi_DEC_hl() */
+static int csi_DEC_mode(struct vc_data *vc, unsigned int mode)
+{
+	switch (mode) {
+	case CSI_DEC_hl_CURSOR_KEYS:
+		return decrpm_state(is_kbd(vc, decckm));
+	case CSI_DEC_hl_132_COLUMNS:
+		return DECRPM_PERMANENTLY_RESET;
+	case CSI_DEC_hl_REVERSE_VIDEO:
+		return decrpm_state(vc->vc_decscnm);
+	case CSI_DEC_hl_ORIGIN_MODE:
+		return decrpm_state(vc->vc_decom);
+	case CSI_DEC_hl_AUTOWRAP:
+		return decrpm_state(vc->vc_decawm);
+	case CSI_DEC_hl_AUTOREPEAT:
+		return decrpm_state(is_kbd(vc, decarm));
+	case CSI_DEC_hl_MOUSE_X10:
+		return decrpm_state(vc->vc_report_mouse == 1);
+	case CSI_DEC_hl_SHOW_CURSOR:
+		return decrpm_state(vc->vc_deccm);
+	case CSI_DEC_hl_MOUSE_VT200:
+		return decrpm_state(vc->vc_report_mouse == 2);
+	case CSI_DEC_hl_ALT_SCREEN:
+		return decrpm_state(vc->vc_saved_screen != NULL);
+	case CSI_DEC_hl_BRACKETED_PASTE:
+		return decrpm_state(vc->vc_bracketed_paste);
+	case CSI_DEC_hl_GRAPHEME_WIDTH:
+		return decrpm_state(vc->vc_vs16_wide);
+	}
+	return DECRPM_NOT_RECOGNIZED;
+}
+
+/* State of an ANSI mode as handled by csi_hl() */
+static int csi_ANSI_mode(struct vc_data *vc, unsigned int mode)
+{
+	switch (mode) {
+	case CSI_hl_DISPLAY_CTRL:
+		return decrpm_state(vc->vc_disp_ctrl);
+	case CSI_hl_INSERT:
+		return decrpm_state(vc->vc_decim);
+	case CSI_hl_AUTO_NL:
+		return decrpm_state(is_kbd(vc, lnm));
+	}
+	return DECRPM_NOT_RECOGNIZED;
+}
+
+/*
+ * DECRQM: request mode, "CSI Ps $ p" for ANSI modes and "CSI ? Ps $ p" for
+ * DEC private modes. Reply with DECRPM: "CSI [?] Ps ; Pm $ y".
+ */
+/* console_lock is held */
+static void csi_RQM(struct tty_struct *tty, struct vc_data *vc)
+{
+	unsigned int mode = vc->vc_par[0];
+	bool dec = vc->vc_priv == EPdec;
+	char buf[32];
+	int len;
+
+	len = snprintf(buf, sizeof(buf), "\033[%s%u;%d$y", dec ? "?" : "",
+		       mode, dec ? csi_DEC_mode(vc, mode) : csi_ANSI_mode(vc, mode));
+	respond_string(buf, len, tty->port);
+}
+
 enum CSI_right_square_bracket {
 	CSI_RSB_COLOR_FOR_UNDERLINE		= 1,
 	CSI_RSB_COLOR_FOR_HALF_BRIGHT		= 2,

@@ -2233,6 +2309,7 @@ static void restore_cur(struct vc_data *vc)
  * @ESsetG1:		ESC ) parsed
  * @ESpercent:		ESC % parsed
  * @EScsiignore:	CSI [0x20-0x3f] parsed
+ * @ESdollar:		CSI parameters $ parsed -- final char expected
  * @ESnonstd:		OSC parsed
  * @ESpalette:		OSC P parsed
  * @ESosc:		OSC [0-9] parsed

@@ -2254,6 +2331,7 @@ enum vc_ctl_state {
 	ESsetG1,
 	ESpercent,
 	EScsiignore,
+	ESdollar,
 	ESnonstd,
 	ESpalette,
 	ESosc,

@@ -2788,6 +2866,9 @@ static void do_con_trol(struct tty_struct *tty, struct vc_data *vc, u8 c)
 			vc->vc_par[vc->vc_npar] *= 10;
 			vc->vc_par[vc->vc_npar] += c - '0';
 			return;
+		case '$':
+			vc->vc_state = ESdollar;
+			return;
 		}
 		if (c >= ASCII_CSI_IGNORE_FIRST && c <= ASCII_CSI_IGNORE_LAST) {
 			vc->vc_state = EScsiignore;

@@ -2846,6 +2927,15 @@ static void do_con_trol(struct tty_struct *tty, struct vc_data *vc, u8 c)
 			return;
 		vc->vc_state = ESnormal;
 		return;
+	case ESdollar:	/* CSI parameters $, final char expected */
+		if (c >= ASCII_CSI_IGNORE_FIRST && c <= ASCII_CSI_IGNORE_LAST) {
+			vc->vc_state = EScsiignore;
+			return;
+		}
+		vc->vc_state = ESnormal;
+		if (c == 'p' && (vc->vc_priv == EPdec || vc->vc_priv == EPecma))
+			csi_RQM(tty, vc);
+		return;
 	case ESpercent:	/* ESC % */
 		vc->vc_state = ESnormal;
 		switch (c) {