From: Karel Zak <kzak@redhat.com>
Date: Wed, 17 Sep 2025 10:58:59 +0000 (+0200)
Subject: agetty: fix erasure of escape sequences and tab characters
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=b462fdefe74836c22cb02039dca419a512dc6c88;p=thirdparty%2Futil-linux.git

agetty: fix erasure of escape sequences and tab characters

When escape sequences (like arrow keys) or tab characters are entered
at the login prompt, they are properly visualized but only partially
erasable with backspace. This is because the erase logic assumes each
stored character corresponds to one visual character, but escape
sequences display as "^[" (2 chars) and tabs expand to multiple spaces.

Track visual character width for each stored byte in a parallel array.
When erasing, use the stored visual width to properly erase all
displayed characters for that input byte.

Fixes: https://github.com/util-linux/util-linux/issues/3624
Signed-off-by: Karel Zak <kzak@redhat.com>
---

diff --git a/term-utils/agetty.c b/term-utils/agetty.c
index 5e564c4f0..c6cb48081 100644
--- a/term-utils/agetty.c
+++ b/term-utils/agetty.c
@@ -2131,20 +2131,30 @@ static void next_speed(struct options *op, struct termios *tp)
 	tcsetattr(STDIN_FILENO, TCSANOW, tp);
 }
 
+/* Erase visual characters for one stored character */
+static void erase_char(int visual_count, struct chardata *cp)
+{
+	static const char *const erase[] = {	/* backspace-space-backspace */
+		"\010\040\010",		/* space parity */
+		"\010\040\010",		/* odd parity */
+		"\210\240\210",		/* even parity */
+		"\210\240\210",		/* no parity */
+	};
+	int i;
+	for (i = 0; i < visual_count; i++)
+		write_all(1, erase[cp->parity], 3);
+}
+
 /* Get user name, establish parity, speed, erase, kill & eol. */
 static char *get_logname(struct issue *ie, struct options *op, struct termios *tp, struct chardata *cp)
 {
 	static char logname[BUFSIZ];
+	static int visual_widths[BUFSIZ];	/* visual char count for each stored byte */
 	char *bp;
+	int *visual_bp;
 	char c;			/* input character, full eight bits */
 	char ascval;		/* low 7 bits of input character */
 	int eightbit;
-	static const char *const erase[] = {	/* backspace-space-backspace */
-		"\010\040\010",		/* space parity */
-		"\010\040\010",		/* odd parity */
-		"\210\240\210",		/* even parity */
-		"\210\240\210",		/* no parity */
-	};
 
 	/* Initialize kill, erase, parity etc. (also after switching speeds). */
 	INIT_CHARDATA(cp);
@@ -2158,7 +2168,11 @@ static char *get_logname(struct issue *ie, struct options *op, struct termios *t
 	tcflush(STDIN_FILENO, TCIFLUSH);
 
 	eightbit = (op->flags & (F_EIGHTBITS|F_UTF8));
+
+	/* Initialize buffer pointers. visual_widths tracks how many visual
+	 * characters each stored byte represents (e.g. ESC = 2 for "^[", tab = 1-8 spaces) */
 	bp = logname;
+	visual_bp = visual_widths;
 	*bp = '\0';
 
 	eval_issue_file(ie, op, tp);
@@ -2182,6 +2196,7 @@ static char *get_logname(struct issue *ie, struct options *op, struct termios *t
 			    && (op->flags & F_NOCLEAR) == 0)
 				termio_clear(STDOUT_FILENO);
 			bp = logname;
+			visual_bp = visual_widths;
 			*bp = '\0';
 			continue;
 		}
@@ -2259,8 +2274,9 @@ static char *get_logname(struct issue *ie, struct options *op, struct termios *t
 				cp->erase = ascval; /* set erase character */
 				if (bp > logname) {
 					if ((tp->c_lflag & ECHO) == 0)
-						write_all(1, erase[cp->parity], 3);
+						erase_char(*(visual_bp - 1), cp);
 					bp--;
+					visual_bp--;
 				}
 				break;
 			case CTL('U'):
@@ -2272,8 +2288,9 @@ static char *get_logname(struct issue *ie, struct options *op, struct termios *t
 					break;
 				while (bp > logname) {
 					if ((tp->c_lflag & ECHO) == 0)
-						write_all(1, erase[cp->parity], 3);
+						erase_char(*(visual_bp - 1), cp);
 					bp--;
+					visual_bp--;
 				}
 				break;
 			case CTL('D'):
@@ -2283,15 +2300,29 @@ static char *get_logname(struct issue *ie, struct options *op, struct termios *t
 					log_err(_("%s: input overrun"), op->tty);
 				if ((tp->c_lflag & ECHO) == 0) {
 					/* Visualize escape sequence instead of its execution */
-					if (ascval == CTL('['))
+					if (ascval == CTL('[')) {
 						/* Ideally it should be "\xe2\x90\x9b"
 						 * if (op->flags & (F_UTF8)),
 						 * but only some fonts contain it */
 						write_all(1, "^[", 2);
-					else
+						*visual_bp = 2;		/* ESC shows as ^[ (2 chars) */
+					} else if (ascval == '\t') {
+						/* Tab expands to spaces */
+						int pos = bp - logname;
+						int spaces = 8 - (pos % 8);
+						int i;
+						for (i = 0; i < spaces; i++)
+							write_all(1, " ", 1);
+						*visual_bp = spaces;
+					} else {
 						write_all(1, &c, 1);	/* echo the character */
+						*visual_bp = 1;		/* normal char shows as 1 */
+					}
+				} else {
+					*visual_bp = 1;		/* when echo is on, assume 1 char */
 				}
 				*bp++ = ascval;			/* and store it */
+				visual_bp++;
 				break;
 			}
 			/* Everything was erased. */