[PATCH v2] tty: ttynull: implement console write

[Michael Thalmeier]

Since commit a699449bb13b ("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.

Fixes: a699449bb13b ("printk: refactor and rework printing logic")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Michael Thalmeier <michael.thalmeier@xxxxxxx>
---
Changes in v2:
- Reference correct commit SHA-1 ID
- Add Fixes tag
- Link to v1: https://lore.kernel.org/all/20230214102317.382769-1-michael.thalmeier@xxxxxxx

---
 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