On Wed 2019-05-08 19:31:06, Tetsuo Handa wrote: > Hello. > > We are again getting corrupted reports where message from WARN() is missing. > For example, https://syzkaller.appspot.com/text?tag=CrashLog&x=1720cac8a00000 > was > titled as "WARNING in cgroup_exit" because the > "WARNING: CPU: 0 PID: 7870 at kernel/cgroup/cgroup.c:6008 > cgroup_exit+0x51a/0x5d0" > line is there but > https://syzkaller.appspot.com/text?tag=CrashLog&x=1670a602a00000 > was titled as "corrupted report (2)" because the > "WARNING: CPU: 0 PID: 10223 at kernel/cgroup/cgroup.c:6008 > cgroup_exit+0x51a/0x5d0" > line is missing. Also, it is unlikely that there was no printk() for a few > minutes. > Thus, I suspect something is again suppressing console output.
It is just a wild speculation. It is highly unlikely that the console_loglevel was manipulated to hide messages. There are very few locations where console_loglevel is manipulated. Anyway, the new "panic_print" feature from Feng Tang might help here. It is in mm tree, see https://marc.info/?l=linux-mm-commits&m=155614613719648&w=2 > Since this problem is happening in 5.1.0-next-20190507, do we want to try > below one? > > 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); I am not sure why panic() is needed. I would personally start with pr_emerg(). Anyway, I somehow doubt that this is the reason. > + 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 > > > > > > By the way, recently we are hitting false positives caused by "WARNING:" > string from not WARN() messages but plain printk() messages (e.g. > > > https://syzkaller.appspot.com/bug?id=31bdef63e48688854fde93e6edf390922b70f8a4 > > https://syzkaller.appspot.com/bug?id=faae4720a75cadb8cd0dbda5c4d3542228d37340 > > ) and we need to avoid emitting "WARNING:" string from plain printk() messages > during fuzzing testing. I guess we want to add something like > CONFIG_DEBUG_AID_FOR_SYZBOT to all kernels in order to mask such string... I wonder who catches the string "WARNING" and how the system is killed. panic_on_warn should cause that WARN() macro calls panic(). Simple printk() should not cause this. Best Regards, Petr
