Commit e88391f7 authored by Nicolas Pitre's avatar Nicolas Pitre Committed by Greg Kroah-Hartman
Browse files

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: default avatarNicolas Pitre <npitre@baylibre.com>
Link: https://lore.kernel.org/r/20250410011839.64418-4-nico@fluxnic.net


Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 2acaf27c

drivers/tty/vt/vt.c

+44 −2
Original line number Diff line number Diff line
@@ -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)

include/linux/consolemap.h

+10 −0
Original line number Diff line number Diff line
@@ -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__ */