linux-kernel - [PATCH v1] serial: core: fix infinite loop in handle_tx() for PORT

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Message-ID: <20260204074327.226165-1-jiayuan.chen@linux.dev>
Date: Wed,  4 Feb 2026 15:43:20 +0800
From: Jiayuan Chen <jiayuan.chen@...ux.dev>
To: linux-serial@...r.kernel.org
Cc: Jiayuan Chen <jiayuan.chen@...pee.com>,
	Jiayuan Chen <jiayuan.chen@...ux.dev>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>,
	Petr Mladek <pmladek@...e.com>,
	Marcos Paulo de Souza <mpdesouza@...e.com>,
	Krzysztof Kozlowski <krzysztof.kozlowski@....qualcomm.com>,
	"Dr. David Alan Gilbert" <linux@...blig.org>,
	Joseph Tilahun <jtilahun@...ranis.com>,
	Sjur Braendeland <sjur.brandeland@...ricsson.com>,
	"David S. Miller" <davem@...emloft.net>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v1] serial: core: fix infinite loop in handle_tx() for PORT_UNKNOWN

From: Jiayuan Chen <jiayuan.chen@...pee.com>

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

Fixes: 9b27105b4a44 ("net-caif-driver: add CAIF serial driver (ldisc)")
Signed-off-by: Jiayuan Chen <jiayuan.chen@...pee.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@...ux.dev>
---
 drivers/tty/serial/serial_core.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 2805cad10511..0b2edf185cc7 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -643,7 +643,10 @@ static unsigned int uart_write_room(struct tty_struct *tty)
 	unsigned int ret;
 
 	port = uart_port_ref_lock(state, &flags);
-	ret = kfifo_avail(&state->port.xmit_fifo);
+	if (!state->port.xmit_buf)
+		ret = 0;
+	else
+		ret = kfifo_avail(&state->port.xmit_fifo);
 	uart_port_unlock_deref(port, flags);
 	return ret;
 }
-- 
2.43.0