Commit 455ce986 authored by Jiayuan Chen's avatar Jiayuan Chen Committed by Greg Kroah-Hartman
Browse files

serial: core: fix infinite loop in handle_tx() for PORT_UNKNOWN 

uart_write_room() and uart_write() behave inconsistently when
xmit_buf is NULL (which happens for PORT_UNKNOWN ports that were
never properly initialized):

- uart_write_room() returns kfifo_avail() which can be > 0
- uart_write() checks xmit_buf and returns 0 if NULL

This inconsistency causes an infinite loop in drivers that rely on
tty_write_room() to determine if they can write:

  while (tty_write_room(tty) > 0) {
      written = tty->ops->write(...);
      // written is always 0, loop never exits
  }

For example, caif_serial's handle_tx() enters an infinite loop when
used with PORT_UNKNOWN serial ports, causing system hangs.

Fix by making uart_write_room() also check xmit_buf and return 0 if
it's NULL, consistent with uart_write().

Reproducer: https://gist.github.com/mrpre/d9a694cc0e19828ee3bc3b37983fde13



Signed-off-by: default avatarJiayuan Chen <jiayuan.chen@shopee.com>
Cc: stable <stable@kernel.org>
Link: https://patch.msgid.link/20260204074327.226165-1-jiayuan.chen@linux.dev


Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent d54801cd

drivers/tty/serial/serial_core.c

+4 −1
Original line number Diff line number Diff line
@@ -643,6 +643,9 @@ static unsigned int uart_write_room(struct tty_struct *tty) 
	unsigned int ret;



	port = uart_port_ref_lock(state, &flags);

	if (!state->port.xmit_buf)

		ret = 0;

	else

		ret = kfifo_avail(&state->port.xmit_fifo);

	uart_port_unlock_deref(port, flags);

	return ret;