From 3e1530bc230f2cdc0a00795e4b061e465a2451bd Mon Sep 17 00:00:00 2001
From: John Ogness <john.ogness@linutronix.de>
Date: Fri, 17 Feb 2023 09:53:44 +0106
Subject: [PATCH 317/327] printk: ignore consoles without write() callback

The ttynull driver does not provide an implementation for the write()
callback. This leads to a NULL pointer dereference in the related
printing kthread, which assumes it can call that callback.

Do not create kthreads for consoles that do not implement the write()
callback. Also, for pr_flush(), ignore consoles that do not implement
write() or write_atomic(), since there is no way those consoles can
flush their output.

Link: https://lore.kernel.org/lkml/1831554214.546921.1676479103702.JavaMail.zimbra@hale.at
Reported-by: Michael Thalmeier <michael.thalmeier@hale.at>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
---
 kernel/printk/printk.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 32617271ade3..db268a1dd434 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2267,6 +2267,10 @@ static int printk_kthread_func(void *data)
 /* Must be called within console_lock(). */
 static void start_printk_kthread(struct console *con)
 {
+	/* No need to start a printing thread if the console cannot print. */
+	if (!con->write)
+		return;
+
 	con->thread = kthread_run(printk_kthread_func, con,
 				  "pr/%s%d", con->name, con->index);
 	if (IS_ERR(con->thread)) {
@@ -3589,6 +3593,8 @@ bool pr_flush(int timeout_ms, bool reset_on_progress)
 		for_each_console(con) {
 			if (!(con->flags & CON_ENABLED))
 				continue;
+			if (!con->write && !con->write_atomic)
+				continue;
 			printk_seq = atomic64_read(&con->printk_seq);
 			if (printk_seq < seq)
 				diff += seq - printk_seq;
-- 
2.49.0

