agetty: fix characters reorder in login prompt

The current agetty uses TIOCSTI ioctl to return already read chars
from login name back to the terminal (without read() before
tcsetattr() we will lost data already written by user). The ioctl
based solution is fragile due to race -- we can return chars when
terminal already contains another new chars. The result is reordered
chars in login name.

The solution is to use extra buffer for already read data.

Reported-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Karel Zak <kzak@redhat.com>

1 files changed, 34 insertions, 12 deletions

diff --git a/term-utils/agetty.c b/term-utils/agetty.c
index 449dfef3c..0b22104de 100644
--- a/term-utils/agetty.c
+++ b/term-utils/agetty.c
@@ -1581,12 +1581,12 @@ static int process_netlink(void)
 	return changed;
 }
 
-static int wait_for_term_input(int fd)
+static int wait_for_term_input(int fd, char *prebuf, size_t prebufsz, size_t *readsz)
 {
 	char buffer[sizeof(struct inotify_event) + NAME_MAX + 1];
 	struct termios orig, nonc;
 	fd_set rfds;
-	int count, i;
+	int count;
 
 	/* Our aim here is to fall through if something fails
          * and not be stuck waiting. On failure assume we have input */
@@ -1645,11 +1645,11 @@ static int wait_for_term_input(int fd)
 
 			tcsetattr(fd, TCSANOW, &orig);
 
-			/* Reinject the bytes we read back into the buffer, usually just one byte */
-			for (i = 0; i < count; i++)
-				ioctl(fd, TIOCSTI, buffer + i);
+			if (count > 0 && prebuf && prebufsz) {
+				memcpy(prebuf, buffer, min(sizeof(buffer), prebufsz));
+				*readsz = count;
+			}
 
-			/* Have terminal input */
 			return 1;
 
 		} else if (netlink_fd >= 0 && FD_ISSET(netlink_fd, &rfds)) {
@@ -1718,14 +1718,15 @@ again:
 	if (op->flags & F_LOGINPAUSE) {
 		puts(_("[press ENTER to login]"));
 #ifdef AGETTY_RELOAD
-		if (!wait_for_term_input(STDIN_FILENO)) {
+		if (!wait_for_term_input(STDIN_FILENO, NULL, 0, NULL)) {
 			/* reload issue */
 			if (op->flags & F_VCONSOLE)
 				termio_clear(STDOUT_FILENO);
 			goto again;
 		}
-#endif
+#else
 		getc(stdin);
+#endif
 	}
 #ifdef KDGKBLED
 	if (!(op->flags & F_NOHINTS) && !op->autolog &&
@@ -1844,13 +1845,26 @@ static char *get_logname(struct options *op, struct termios *tp, struct chardata
 	*bp = '\0';
 
 	while (*logname == '\0') {
+		char prebuf[LOGIN_NAME_MAX] = { 0 };
+		size_t presz = 0, precur = 0;
 
 		/* Write issue file and prompt */
 		do_prompt(op, tp);
 
 #ifdef AGETTY_RELOAD
-		/* If asked to reprompt *before* terminal input arrives, then do so */
-		if (!wait_for_term_input(STDIN_FILENO)) {
+		/* If asked to reprompt *before* terminal input arrives, then do so.
+		 *
+		 * Note that wait_for_term_input() calls read() and the result
+		 * is stored to the 'prebuf'. We need this to avoid data lost
+		 * by terminal attributes reset (and return chars back to the
+		 * terminal by TIOCSTI is fragile (chars reorder)).
+		 *
+		 * The data from 'prebuf' are not printed to the terminal yet
+		 * (disabled ECHO in wait_for_term_input()).
+		 */
+		if (!wait_for_term_input(STDIN_FILENO,
+					 prebuf, sizeof(prebuf), &readsz)) {
+
 			if (op->flags & F_VCONSOLE)
 				termio_clear(STDOUT_FILENO);
 			continue;
@@ -1862,8 +1876,15 @@ static char *get_logname(struct options *op, struct termios *tp, struct chardata
 		while (cp->eol == '\0') {
 
 			char key;
+			int force_echo = 0;
 
-			if (read(STDIN_FILENO, &c, 1) < 1) {
+			/* use already read data from buffer */
+			if (presz && precur < presz) {
+				c = prebuf[precur++];
+				force_echo = 1;
+
+			/* read from terminal */
+			} else if (read(STDIN_FILENO, &c, 1) < 1) {
 
 				/* The terminal could be open with O_NONBLOCK when
 				 * -L (force CLOCAL) is specified...  */
@@ -1939,13 +1960,14 @@ static char *get_logname(struct options *op, struct termios *tp, struct chardata
 					break;
 				if ((size_t)(bp - logname) >= sizeof(logname) - 1)
 					log_err(_("%s: input overrun"), op->tty);
-				if ((tp->c_lflag & ECHO) == 0)
+				if ((tp->c_lflag & ECHO) == 0 || force_echo)
 					write_all(1, &c, 1);	/* echo the character */
 				*bp++ = ascval;			/* and store it */
 				break;
 			}
 		}
 	}
+
 #ifdef HAVE_WIDECHAR
 	if ((op->flags & (F_EIGHTBITS|F_UTF8)) == (F_EIGHTBITS|F_UTF8)) {
 		/* Check out UTF-8 multibyte characters */