[PATCH] printk: Monitor change of console loglevel.

[Tetsuo Handa]

We are seeing syzbot reports [1] where printk() messages prior to panic()
are missing for unknown reason. To test whether it is due to some testcase
changing console loglevel, let's panic() as soon as console loglevel has
changed. This patch is intended for testing on linux-next.git only, and
will be removed after we found what is wrong.

[1] https://lkml.kernel.org/r/127c9c3b-f878-174f-7065-66dc50fcabcf@xxxxxxxxxxxxxxxxxxx

Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@xxxxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxxx>
---
 kernel/printk/printk.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index e1e8250..f0b9463 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3338,3 +3338,23 @@ void kmsg_dump_rewind(struct kmsg_dumper *dumper)
 EXPORT_SYMBOL_GPL(kmsg_dump_rewind);
 
 #endif
+
+#ifdef CONFIG_DEBUG_AID_FOR_SYZBOT
+static int initial_loglevel;
+static void check_loglevel(struct timer_list *timer)
+{
+	if (console_loglevel < initial_loglevel)
+		panic("Console loglevel changed (%d->%d)!", initial_loglevel,
+		      console_loglevel);
+	mod_timer(timer, jiffies + HZ);
+}
+static int __init loglevelcheck_init(void)
+{
+	static DEFINE_TIMER(timer, check_loglevel);
+
+	initial_loglevel = console_loglevel;
+	mod_timer(&timer, jiffies + HZ);
+	return 0;
+}
+late_initcall(loglevelcheck_init);
+#endif
-- 
1.8.3.1