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>

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index bcb508bc15ab970e036fab03df13a50b785eede7..5d53feeb5d2bf872b51ef857416a7f761b0524f5 100644 (file)
--- 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 caf079bcb8c993cdeaa527f2d86505491f9caed3..7d778752dcefba74ccdc244cb2351ceca699d67e 100644 (file)
--- 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__ */