linux-kernel - Re: [crash] NULL pointer dereference at IP: [<ffffffff812e9ccb>] uart

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Message-ID: <20091012124501.3c1ac94f@lxorguk.ukuu.org.uk>
Date:	Mon, 12 Oct 2009 12:45:01 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg KH <gregkh@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [crash] NULL pointer dereference at IP: [<ffffffff812e9ccb>]
 uart_close+0x2a/0x1e4

> Unfortunately 46d57a449aa13d9c6adcc9d1dbc7b9a0ecfb69d8 is not easy to 
> revert, at all. Would be glad to try any patch - fixes or some manual 
> revert as well (if that's feasible).

The patch doesn't do anything but create local variables of state->port.
I'm now glad I did it that way as it makes it easy to spot the mistake.

the tty_port lock is ->lock, the uart lock is ->lock both are different..

Spot the mistake:

 	if (state->flags & UIF_INITIALIZED) {
 		unsigned long flags;
 		spin_lock_irqsave(&port->lock, flags);
-		port->ops->stop_rx(port);
+		uport->ops->stop_rx(uport);
 		spin_unlock_irqrestore(&port->lock, flags);
 		/*
 		 * Before we drop DTR, make sure the UART transmitter
 		 * has completely drained; this is especially
 		 * important if there is a transmit FIFO!
 		 */
-		uart_wait_until_sent(tty, port->timeout);
+		uart_wait_until_sent(tty, uport->timeout);
 	}


so uart_close takes the wrong lock. I've checked the rest of the patch for
the same error and I don't see any other screwups.

uart: uart_close crash fix

From: Alan Cox <alan@...ux.intel.com>

uart_close takes the wrong lock at one point due to a bug introduced in the
port/uport renaming and tty_port local introduction in
46d57a449aa13d9c6adcc9d1dbc7b9a0ecfb69d8. The race in question is very
hard to hit but fortunately Ingo hit it twice..

Signed-off-by: Alan Cox <alan@...ux.intel.com>
---

 drivers/serial/serial_core.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)


diff --git a/drivers/serial/serial_core.c b/drivers/serial/serial_core.c
index 1689bda..3005231 100644
--- a/drivers/serial/serial_core.c
+++ b/drivers/serial/serial_core.c
@@ -1316,9 +1316,9 @@ static void uart_close(struct tty_struct *tty, struct file *filp)
 	 */
 	if (port->flags & ASYNC_INITIALIZED) {
 		unsigned long flags;
-		spin_lock_irqsave(&port->lock, flags);
+		spin_lock_irqsave(&uport->lock, flags);
 		uport->ops->stop_rx(uport);
-		spin_unlock_irqrestore(&port->lock, flags);
+		spin_unlock_irqrestore(&uport->lock, flags);
 		/*
 		 * Before we drop DTR, make sure the UART transmitter
 		 * has completely drained; this is especially


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