Re: [syzbot? printk?] no WARN_ON() messages printed before "Kernel panic - not syncing: panic_on_warn set ..."

[Dmitry Vyukov]

On Sat, Mar 16, 2019 at 3:14 PM Sergey Senozhatsky
<sergey.senozhatsky@xxxxxxxxx> wrote:
>
> On (03/16/19 19:18), Tetsuo Handa wrote:
> > On 2019/03/16 18:11, Dmitry Vyukov wrote:
> > > Such reports showed up always with low frequency. For all that I
> > > looked at we also always had a normal  non-truncated version, so I was
> > > never too worried.
> > >
> > > If something would write from userspace, that would show up in the
> > > output, or not?
> > >
> > > Perhaps syzkaller somehow manages to lower console output level?
> >
> > panic() calls console_verbose() before calling
> >
> >   pr_emerg("Kernel panic - not syncing: %s\n", buf);
> >
> > line. Then, someone might have changed console_loglevel enough to
> > suppress printk() output.
>
> Hmm... sysctl, may be?
>
> > > I figured out that we should restrict it from doing
> > > syslog(SYSLOG_ACTION_CONSOLE_OFF). And I also restricted its access o
> > > /dev/console. But maybe there is something else? It _should_ not be
> > > able to write to random sysctl's.
> >
> > Maybe try running with "ignore_loglevel" kernel command line option added?
>
> Right, that's something I would expect 0-day and syzkaller to do.

to double-check: enabling this won't lead to verbose/debug level of logging?