linux-kernel - [PATCH 11/11] vt: pad double-width code points with a zero-white-space

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Message-ID: <20250410011839.64418-12-nico@fluxnic.net>
Date: Wed,  9 Apr 2025 21:14:03 -0400
From: Nicolas Pitre <nico@...xnic.net>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>
Cc: Nicolas Pitre <npitre@...libre.com>,
	Dave Mielke <Dave@...lke.cc>,
	linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 11/11] vt: pad double-width code points with a zero-white-space

From: Nicolas Pitre <npitre@...libre.com>

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 or selection.

Let's use a better code point for the column padding's purpose i.e. a
zero-white-space rather than a full space.

Signed-off-by: Nicolas Pitre <npitre@...libre.com>
---
 drivers/tty/vt/vt.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index e3d35c4f92..dc84f9c6b7 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -2937,12 +2937,13 @@ static int vc_con_write_normal(struct vc_data *vc, int tc, int c,
 			width = 2;
 		} else if (ucs_is_zero_width(c)) {
 			prev_c = vc_uniscr_getc(vc, -1);
-			if (prev_c == ' ' &&
+			if (prev_c == 0x200B &&
 			    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.
+				 * replacing the existing zero-white-space
+				 * padding.
 				 */
 				vc_con_rewind(vc);
 			} else if (c == 0xfe0f && prev_c != 0) {
@@ -3040,7 +3041,11 @@ 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-white-space in the Unicode screen given that
+		 * the previous code point is semantically double-width.
+		 */
+		next_c = 0x200B;
 	}
 
 out:
-- 
2.49.0