linux-kernel - Re: [syzbot? printk?] no WARN_ON() messages printed before "Kernel panic

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Date:   Mon, 18 Mar 2019 14:27:26 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: [syzbot? printk?] no WARN_ON() messages printed before "Kernel panic - not syncing: panic_on_warn set ..."

Dmitry Vyukov wrote:
> > Then, we need to find what test is changing console_loglevel.
> > Maybe add debug BUG_ON() in linux-next.git using CONFIG_DEBUG_AID_FOR_SYZBOT ?
> 
> Is there a single place to catch this? I could run syzkaller locally
> first with the check.
> 

There is no such place. But not so many places change permanently.
For x86, you can test with below patch applied.

---
 drivers/tty/sysrq.c    |  1 +
 kernel/printk/printk.c |  3 +++
 kernel/sysctl.c        | 23 ++++++++++++++++++++++-
 3 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index fa0ce7d..ad73520 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -91,6 +91,7 @@ static void sysrq_handle_loglevel(int key)
 	i = key - '0';
 	console_loglevel = CONSOLE_LOGLEVEL_DEFAULT;
 	pr_info("Loglevel set to %d\n", i);
+	WARN_ONCE(i < CONSOLE_LOGLEVEL_DEFAULT, "Reducing console_loglevel to %d", i);
 	console_loglevel = i;
 }
 static struct sysrq_key_op sysrq_loglevel_op = {
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 02ca827..70d1f8c 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1533,11 +1533,13 @@ int do_syslog(int type, char __user *buf, int len, int source)
 	case SYSLOG_ACTION_CONSOLE_OFF:
 		if (saved_console_loglevel == LOGLEVEL_DEFAULT)
 			saved_console_loglevel = console_loglevel;
+		WARN_ONCE(minimum_console_loglevel < CONSOLE_LOGLEVEL_DEFAULT, "Reducing console_loglevel to %d", minimum_console_loglevel);
 		console_loglevel = minimum_console_loglevel;
 		break;
 	/* Enable logging to console */
 	case SYSLOG_ACTION_CONSOLE_ON:
 		if (saved_console_loglevel != LOGLEVEL_DEFAULT) {
+			WARN_ONCE(saved_console_loglevel < CONSOLE_LOGLEVEL_DEFAULT, "Reducing console_loglevel to %d", saved_console_loglevel);
 			console_loglevel = saved_console_loglevel;
 			saved_console_loglevel = LOGLEVEL_DEFAULT;
 		}
@@ -1548,6 +1550,7 @@ int do_syslog(int type, char __user *buf, int len, int source)
 			return -EINVAL;
 		if (len < minimum_console_loglevel)
 			len = minimum_console_loglevel;
+		WARN_ONCE(len < CONSOLE_LOGLEVEL_DEFAULT, "Reducing console_loglevel to %d", len);
 		console_loglevel = len;
 		/* Implicitly re-enable logging to console */
 		saved_console_loglevel = LOGLEVEL_DEFAULT;
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index b3df3ab..2170421 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -273,6 +273,27 @@ static int sysrq_sysctl_handler(struct ctl_table *table, int write,
 int sysctl_legacy_va_layout;
 #endif
 
+static int proc_dointvec_loglevel(struct ctl_table *table, int write,
+				  void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+	if (write && buffer && *lenp) {
+		size_t len = *lenp;
+		char *kbuf, *p;
+
+		if (len > PAGE_SIZE - 1)
+			len = PAGE_SIZE - 1;
+		p = kbuf = memdup_user_nul(buffer, len);
+		if (IS_ERR(kbuf))
+			return PTR_ERR(kbuf);
+		while (*p && (*p < '0' || *p > '9'))
+			p++;
+		len = *p ? strtoul(p, &p, 10) : CONSOLE_LOGLEVEL_DEFAULT;
+		WARN_ONCE(len < CONSOLE_LOGLEVEL_DEFAULT, "Reducing console_loglevel to %d", (int) len);
+		kfree(kbuf);
+	}
+	return proc_dointvec(table, write, buffer, lenp, ppos);
+}
+
 /* The default sysctl tables: */
 
 static struct ctl_table sysctl_base_table[] = {
@@ -839,7 +860,7 @@ static int sysrq_sysctl_handler(struct ctl_table *table, int write,
 		.data		= &console_loglevel,
 		.maxlen		= 4*sizeof(int),
 		.mode		= 0644,
-		.proc_handler	= proc_dointvec,
+		.proc_handler	= proc_dointvec_loglevel,
 	},
 	{
 		.procname	= "printk_ratelimit",
-- 
1.8.3.1