vt: properly support zero-width Unicode code points

Zero-width Unicode code points are causing misalignment in vertically
aligned content, disrupting the visual layout. Let's handle zero-width
code points more intelligently.

Double-width code points are stored in the screen grid followed by a white
space code point to create the expected screen layout. When a double-width
code point is followed by a zero-width code point in the console incoming
bytestream (e.g., an emoji with a presentation selector) then we may
replace the white space padding by that zero-width code point instead of
dropping it. This maximize screen content information while preserving
proper layout.

If a zero-width code point is preceded by a single-width code point then
the above trick is not possible and such zero-width code point must
be dropped.

VS16 (Variation Selector 16, U+FE0F) is special as it doubles the width
of the preceding single-width code point. We handle that case by giving
VS16 a width of 1 when that happens.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Link: https://lore.kernel.org/r/20250410011839.64418-4-nico@fluxnic.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 44 insertions, 2 deletions

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index bcb508bc15ab..5d53feeb5d2b 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -443,6 +443,15 @@ static void vc_uniscr_scroll(struct vc_data *vc, unsigned int top,
 	}
 }
 
+static u32 vc_uniscr_getc(struct vc_data *vc, int relative_pos)
+{
+	int pos = vc->state.x + vc->vc_need_wrap + relative_pos;
+
+	if (vc->vc_uni_lines && pos >= 0 && pos < vc->vc_cols)
+		return vc->vc_uni_lines[vc->state.y][pos];
+	return 0;
+}
+
 static void vc_uniscr_copy_area(u32 **dst_lines,
 				unsigned int dst_cols,
 				unsigned int dst_rows,
@@ -2905,18 +2914,49 @@ static bool vc_is_control(struct vc_data *vc, int tc, int c)
 	return false;
 }
 
+static void vc_con_rewind(struct vc_data *vc)
+{
+	if (vc->state.x && !vc->vc_need_wrap) {
+		vc->vc_pos -= 2;
+		vc->state.x--;
+	}
+	vc->vc_need_wrap = 0;
+}
+
 static int vc_con_write_normal(struct vc_data *vc, int tc, int c,
 		struct vc_draw_region *draw)
 {
-	int next_c;
+	int next_c, prev_c;
 	unsigned char vc_attr = vc->vc_attr;
 	u16 himask = vc->vc_hi_font_mask, charmask = himask ? 0x1ff : 0xff;
 	u8 width = 1;
 	bool inverse = false;
 
 	if (vc->vc_utf && !vc->vc_disp_ctrl) {
-		if (ucs_is_double_width(c))
+		if (ucs_is_double_width(c)) {
 			width = 2;
+		} else if (ucs_is_zero_width(c)) {
+			prev_c = vc_uniscr_getc(vc, -1);
+			if (prev_c == ' ' &&
+			    ucs_is_double_width(vc_uniscr_getc(vc, -2))) {
+				/*
+				 * Let's merge this zero-width code point with
+				 * the preceding double-width code point by
+				 * replacing the existing whitespace padding.
+				 */
+				vc_con_rewind(vc);
+			} else if (c == 0xfe0f && prev_c != 0) {
+				/*
+				 * VS16 (U+FE0F) is special. Let it have a
+				 * width of 1 when preceded by a single-width
+				 * code point effectively making the later
+				 * double-width.
+				 */
+			} else {
+				/* Otherwise zero-width code points are ignored */
+				goto out;
+			}
+		}
 	}
 
 	/* Now try to find out how to display it */
@@ -2995,6 +3035,8 @@ static int vc_con_write_normal(struct vc_data *vc, int tc, int c,
 			tc = ' ';
 		next_c = ' ';
 	}
+
+out:
 	notify_write(vc, c);
 
 	if (inverse)