vt: pad double-width code points with a zero-width space

In the Unicode screen buffer, we follow double-width code points with a
space to maintain proper column alignment. This, however, creates
semantic problems when e.g. using cut and paste.

Let's use a better code point for the column padding's purpose i.e. a
zero-width space rather than a full space. This way the combination
retains a width of 2.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20250417184849.475581-11-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 76554c2040bfc590d373615240a503f9d6ee2ec2..1bd1878094a0bdc75f65d62f8b55757229061339 100644 (file)
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -2923,6 +2923,7 @@ static void vc_con_rewind(struct vc_data *vc)
        vc->vc_need_wrap = 0;
 }
 
+#define UCS_ZWS                0x200b  /* Zero Width Space */
 #define UCS_VS16       0xfe0f  /* Variation Selector 16 */
 
 static int vc_process_ucs(struct vc_data *vc, int *c, int *tc)
@@ -2941,8 +2942,8 @@ static int vc_process_ucs(struct vc_data *vc, int *c, int *tc)
                /*
                 * Let's merge this zero-width code point with the preceding
                 * double-width code point by replacing the existing
-                * whitespace padding. To do so we rewind one column and
-                * pretend this has a width of 1.
+                * zero-width space padding. To do so we rewind one column
+                * and pretend this has a width of 1.
                 * We give the legacy display the same initial space padding.
                 */
                vc_con_rewind(vc);
@@ -3065,7 +3066,11 @@ static int vc_con_write_normal(struct vc_data *vc, int tc, int c,
                tc = conv_uni_to_pc(vc, ' ');
                if (tc < 0)
                        tc = ' ';
-               next_c = ' ';
+               /*
+                * Store a zero-width space in the Unicode screen given that
+                * the previous code point is semantically double width.
+                */
+               next_c = UCS_ZWS;
        }
 
 out: