agetty: fix login name DEL/CTRL^U issue

agetty refresh prompt (/etc/issue file etc.) when requested by inotify
or netlink. For this purpose we monitor some file descriptors by
select().

The terminal input file descriptor is switched to non-canonical mode before
select(). The goal is to be informed about user activity before
new-line. The FD is immediately switched back to canonical mode when
activity is detected. The side effect is that all not-read-yet chars in
the input buffer are lost ... so we need to call read() before switch
to canonical mode to save the chars.

The original implementation has been based on TIOCSTI ioctl. It
returns already read chars back to the terminal input buffer to make
them useful for canonical mode. The problem was race (agetty writes to
input buffer in the same time as user) and result was reordered chars
in login name... so useless.

This issue has been later fixed by extra buffer (commit
790119b8850ae13bb4254c5096a54b2aeb355b20) for already read data.  And
TIOCSTI ioctl has been removed. Unfortunately this solution is also
wrong, because the buffer is maintained only by agetty and
inaccessible for terminal when user edit (by DEL/CTRL^U) login name in
canonical mode.

The solution is simple -- just don't try to be smart and keep terminal
in canonical mode all time (so terminal controls DEL, CTRL^U, etc) and
flush input buffer (=discard unread data) and ask user for login name
again after prompt reload.

The agetty reload is very rarely situation and for user it's pretty
obvious that he has to type login name again (as all terminal has been
clear+redraw).

Addresses: https://github.com/karelzak/util-linux/issues/454
Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1464148
Signed-off-by: Karel Zak <kzak@redhat.com>

1 files changed, 15 insertions, 55 deletions

diff --git a/term-utils/agetty.c b/term-utils/agetty.c
index 938fa2f97..5b910011c 100644
--- a/term-utils/agetty.c
+++ b/term-utils/agetty.c
@@ -1579,26 +1579,10 @@ static int process_netlink(void)
 	return changed;
 }
 
-static int wait_for_term_input(int fd, char *prebuf, size_t prebufsz, size_t *readsz)
+static int wait_for_term_input(int fd)
 {
 	char buffer[sizeof(struct inotify_event) + NAME_MAX + 1];
-	struct termios orig, nonc;
 	fd_set rfds;
-	int count;
-
-	/* Our aim here is to fall through if something fails
-         * and not be stuck waiting. On failure assume we have input */
-
-	if (tcgetattr(fd, &orig) != 0)
-		return 1;
-
-	memcpy(&nonc, &orig, sizeof (nonc));
-	nonc.c_lflag &= ~(ICANON | ECHO | ECHOE | ECHOK | ECHOKE);
-	nonc.c_cc[VMIN] = 1;
-	nonc.c_cc[VTIME] = 0;
-
-	if (tcsetattr(fd, TCSANOW, &nonc) != 0)
-		return 1;
 
 	if (inotify_fd == AGETTY_RELOAD_FDNONE) {
 		/* make sure the reload trigger file exists */
@@ -1639,15 +1623,6 @@ static int wait_for_term_input(int fd, char *prebuf, size_t prebufsz, size_t *re
 			return 1;
 
 		if (FD_ISSET(fd, &rfds)) {
-			count = read(fd, buffer, sizeof (buffer));
-
-			tcsetattr(fd, TCSANOW, &orig);
-
-			if (count > 0 && prebuf && prebufsz) {
-				memcpy(prebuf, buffer, min(sizeof(buffer), prebufsz));
-				*readsz = count;
-			}
-
 			return 1;
 
 		} else if (netlink_fd >= 0 && FD_ISSET(netlink_fd, &rfds)) {
@@ -1659,9 +1634,6 @@ static int wait_for_term_input(int fd, char *prebuf, size_t prebufsz, size_t *re
 			while (read(inotify_fd, buffer, sizeof (buffer)) > 0);
 		}
 
-		tcsetattr(fd, TCSANOW, &orig);
-
-		/* Need to reprompt */
 		return 0;
 	}
 }
@@ -1716,7 +1688,7 @@ again:
 	if (op->flags & F_LOGINPAUSE) {
 		puts(_("[press ENTER to login]"));
 #ifdef AGETTY_RELOAD
-		if (!wait_for_term_input(STDIN_FILENO, NULL, 0, NULL)) {
+		if (!wait_for_term_input(STDIN_FILENO)) {
 			/* reload issue */
 			if (op->flags & F_VCONSOLE)
 				termio_clear(STDOUT_FILENO);
@@ -1843,28 +1815,21 @@ 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.
-		 *
-		 * 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), &presz)) {
-
+		if (!wait_for_term_input(STDIN_FILENO)) {
+			/* refresh prompt -- discard input data, clear terminal
+			 * and call do_prompt() again
+			 */
+			if ((op->flags & F_VCONSOLE) == 0)
+				sleep(1);
+			tcflush(STDIN_FILENO, TCIFLUSH);
 			if (op->flags & F_VCONSOLE)
 				termio_clear(STDOUT_FILENO);
+			bp = logname;
+			*bp = '\0';
 			continue;
 		}
 #endif
@@ -1874,15 +1839,10 @@ static char *get_logname(struct options *op, struct termios *tp, struct chardata
 		while (cp->eol == '\0') {
 
 			char key;
-			int force_echo = 0;
-
-			/* 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) {
+			debug("read from FD\n");
+			if (read(STDIN_FILENO, &c, 1) < 1) {
+				debug("read failed\n");
 
 				/* The terminal could be open with O_NONBLOCK when
 				 * -L (force CLOCAL) is specified...  */
@@ -1958,7 +1918,7 @@ 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 || force_echo)
+				if ((tp->c_lflag & ECHO) == 0)
 					write_all(1, &c, 1);	/* echo the character */
 				*bp++ = ascval;			/* and store it */
 				break;