[PATCH 03/11] vt: properly support zero-width Unicode code points

Nicolas Pitre
2025-04-10T01:19:03Z

From: Nicolas Pitre <npitre@baylibre.com>

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>
---
drivers/tty/vt/vt.c | 46 ++++++++++++++++++++++++++++++++++++--
include/linux/consolemap.h | 10 +++++++++
2 files changed, 54 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index bcb508bc15..5d53feeb5d 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)
diff --git a/include/linux/consolemap.h b/include/linux/consolemap.h
index caf079bcb8..7d778752dc 100644
--- a/include/linux/consolemap.h
+++ b/include/linux/consolemap.h
@@ -29,6 +29,11 @@ u32 conv_8bit_to_uni(unsigned char c);
int conv_uni_to_8bit(u32 uni);
void console_map_init(void);
bool ucs_is_double_width(uint32_t cp);
+static inline bool ucs_is_zero_width(uint32_t cp)
+{
+ /* coming soon */
+ return false;
+}
#else
static inline u16 inverse_translate(const struct vc_data *conp, u16 glyph,
bool use_unicode)
@@ -63,6 +68,11 @@ static inline bool ucs_is_double_width(uint32_t cp)
{
return false;
}
+
+static inline bool ucs_is_zero_width(uint32_t cp)
+{
+ return false;
+}
#endif /* CONFIG_CONSOLE_TRANSLATIONS */

#endif /* __LINUX_CONSOLEMAP_H__ */
--
2.49.0