Re: [PATCH] serial: drop debugging WARN_ON_ONCE() from uart_write()

[Jiri Slaby]

On 28. 05. 24, 17:05, Tetsuo Handa wrote:
> syzbot is reporting lockdep warning upon
>
>    int disc = 7;
>    ioctl(open("/dev/ttyS3", O_RDONLY), TIOCSETD, &disc);
>
> sequence. Do like what commit 5f1149d2f4bf ("serial: drop debugging
> WARN_ON_ONCE() from uart_put_char()") does.
>
> Reported-by: syzbot+f78380e4eae53c64125c@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=f78380e4eae53c64125c
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>

Ugh, definitely:

Acked-by: Jiri Slaby <jirislaby@xxxxxxxxxx>

> ---
> Example is https://syzkaller.appspot.com/text?tag=CrashReport&x=100271ec980000 .
> But not using this example, for this link will disappear eventually.
>
> By the way, do we want to also guard uart_port_lock'ed section using
> printk_deferred_enter()/printk_deferred_exit(), for trying to use e.g.
> WARN_ON() inside such section will result in the same lockdep warning?
At this point, I don't know the answer.

> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -622,7 +622,7 @@ static ssize_t uart_write(struct tty_struct *tty, const u8 *buf, size_t count)
>   		return -EL3HLT;
>   
>   	port = uart_port_lock(state, flags);
> -	if (WARN_ON_ONCE(!state->port.xmit_buf)) {
> +	if (!state->port.xmit_buf) {
>   		uart_port_unlock(port, flags);
>   		return 0;
>   	}

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