linux-kernel - Re: [PATCH] serial: drop debugging WARN_ON_ONCE() from uart

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Date: Mon, 3 Jun 2024 09:10:26 +0200
From: Jiri Slaby <jirislaby@...nel.org>
To: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
 gregkh@...uxfoundation.org
Cc: linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	Al Cooper <alcooperx@...il.com>,
	Matthias Brugger <matthias.bgg@...il.com>,
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
	Kumaravel Thiagarajan <kumaravel.thiagarajan@...rochip.com>,
	Tharun Kumar P <tharunkumar.pasumarthi@...rochip.com>,
	Russell King <linux@...linux.org.uk>,
	Vineet Gupta <vgupta@...nel.org>,
	Richard Genoud <richard.genoud@...il.com>,
	Nicolas Ferre <nicolas.ferre@...rochip.com>,
	Alexandre Belloni <alexandre.belloni@...tlin.com>,
	Claudiu Beznea <claudiu.beznea@...on.dev>,
	Alexander Shiyan <shc_work@...l.ru>,
	Baruch Siach <baruch@...s.co.il>,
	"Maciej W. Rozycki" <macro@...am.me.uk>,
	Shawn Guo <shawnguo@...nel.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Fabio Estevam <festevam@...il.com>,
	Neil Armstrong <neil.armstrong@...ar.smtp.subspace.kernel.org>
Subject: Re: [PATCH] serial: drop debugging WARN_ON_ONCE() from uart_write()

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@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=f78380e4eae53c64125c
> Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>

Ugh, definitely:

Acked-by: Jiri Slaby <jirislaby@...nel.org>

> ---
> 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;
>   	}

-- 
js
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