Re: [PATCH] tty: discard flip-buffer input for TCSAFLUSH

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] tty: missing tty_unthrottle() in set_termios() on TERMIOS_FLUSH
--

commit 757d2dda2b0d44401d4d977f6413afe8f7669c45
Author: Hungyu Lin <dennylin0707@gmail.com>

tty: discard flip-buffer input for TCSAFLUSH

This commit updates set_termios() to call tty_buffer_flush() instead of only
invoking the line discipline flush_buffer callback when TERMIOS_FLUSH is set,
so pending input in the tty flip buffers is also discarded on TCSAFLUSH.

> diff --git a/drivers/tty/tty_ioctl.c b/drivers/tty/tty_ioctl.c
> index 90c70d8d14e3a..45cb77185dbb2 100644
> --- a/drivers/tty/tty_ioctl.c
> +++ b/drivers/tty/tty_ioctl.c
> @@ -496,8 +496,8 @@ static int set_termios(struct tty_struct *tty, void __user *arg, int opt)
>  
>  		ld = tty_ldisc_ref(tty);
>  		if (ld != NULL) {
> -			if ((opt & TERMIOS_FLUSH) && ld->ops->flush_buffer)
> -				ld->ops->flush_buffer(tty);
> +			if (opt & TERMIOS_FLUSH)
> +				tty_buffer_flush(tty, ld);

[Severity: High]
Should this path also call tty_unthrottle(tty) after flushing the buffers,
similar to how __tty_perform_flush() handles TCIFLUSH and TCIOFLUSH?

drivers/tty/tty_ioctl.c:__tty_perform_flush() {
	switch (arg) {
	case TCIFLUSH:
		if (ld && ld->ops->flush_buffer) {
			ld->ops->flush_buffer(tty);
			tty_unthrottle(tty);
		}
		break;
}

When a terminal using N_TTY fills ldata->read_buf and queues additional bytes
in the flip buffers, n_tty_receive_buf_common() sets ldata->no_room and calls
n_tty_check_throttle() -> tty_throttle_safe(), which sets TTY_THROTTLED and
throttles the underlying driver (or on a PTY pair, fills port->buf up to
mem_limit so the master blocks on write_wait).

Previously, set_termios() only called ld->ops->flush_buffer(tty), leaving the
flip buffer data intact to refill ldata->read_buf via flush_to_ldisc(), so a
subsequent n_tty_read() would still see input_available_p(tty, 0) and reach
n_tty_check_unthrottle(tty).

With tty_buffer_flush(tty, ld) now emptying both the flip buffers and
ldata->read_buf without calling tty_unthrottle(tty), a subsequent n_tty_read()
sees no available input and blocks in n_tty_wait_for_input() before reaching
n_tty_check_unthrottle(tty):

drivers/tty/n_tty.c:n_tty_read() {
		if (!input_available_p(tty, 0)) {
			up_read(&tty->termios_rwsem);
			tty_buffer_flush_work(tty->port);
			down_read(&tty->termios_rwsem);
			if (!input_available_p(tty, 0)) {
				int ret = n_tty_wait_for_input(tty, file, &wait,
							       &timeout);
				if (ret <= 0) {
					retval = ret;
					break;
				}
				continue;
			}
		}
}

Can this leave a throttled serial/USB-serial device unable to receive new
input to wake the reader, or leave a PTY master permanently blocked on
write_wait after tcsetattr() with TCSAFLUSH?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261027054304.35543-1-dennylin0707@gmail.com?part=1