linux-kernel - [PATCH] tty: ttynull: implement console write

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Date:   Tue, 14 Feb 2023 11:23:17 +0100
From:   Michael Thalmeier <michael.thalmeier@...e.at>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Vincent Whitchurch <vincent.whitchurch@...s.com>
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH] tty: ttynull: implement console write

Since commit 3579b59c7edc475013ae769a2d26d99733c95f13 ("printk: refactor
and rework printing logic"), con->write is called without checking if it
is implemented in the console driver. This does make some sense, because
for all "normal" consoles it is vital to have a write function.
As the ttynull console driver does not need any console output the write
function was not implemented. This caused a "unable to handle kernel NULL
pointer dereference" when the write function is called now.

To fix this issue, implement an empty write function.

Signed-off-by: Michael Thalmeier <michael.thalmeier@...e.at>
Cc: stable@...r.kernel.org
---
 drivers/tty/ttynull.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/tty/ttynull.c b/drivers/tty/ttynull.c
index 1d4438472442..6e9323544a7d 100644
--- a/drivers/tty/ttynull.c
+++ b/drivers/tty/ttynull.c
@@ -40,6 +40,12 @@ static unsigned int ttynull_write_room(struct tty_struct *tty)
 	return 65536;
 }

+
+static void ttynull_console_write(struct console *co, const char *buf,
+				  unsigned count)
+{
+}
+
 static const struct tty_operations ttynull_ops = {
 	.open = ttynull_open,
 	.close = ttynull_close,
@@ -56,6 +62,7 @@ static struct tty_driver *ttynull_device(struct console *c, int *index)

 static struct console ttynull_console = {
 	.name = "ttynull",
+	.write = ttynull_console_write,
 	.device = ttynull_device,
 };

-- 
2.39.1