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SubjectRe: [syzbot? printk?] no WARN ON() messages printed before "Kernel panic - not syncin g: panic on warn set ..."
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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
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