linux-kernel - Re: [PATCH v4] panic: Avoid the extra noise dmesg

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Date:   Mon, 10 Dec 2018 10:49:22 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Feng Tang <feng.tang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...e.de>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sasha Levin <sashal@...nel.org>, stable@...nel.org
Subject: Re: [PATCH v4] panic: Avoid the extra noise dmesg

On Mon, Dec 10, 2018 at 10:17 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Fri,  7 Dec 2018 17:51:19 +0800
> Feng Tang <feng.tang@...el.com> wrote:
>
> > When kernel panic happens, it will first print the panic call stack,
> > then the ending msg like:
> >
> > [   35.743249] ---[ end Kernel panic - not syncing: Fatal exception
> > [   35.749975] ------------[ cut here ]------------
> >
> > The above message are very useful for debugging.
> >
> > But if system is configured to not reboot on panic, say the "panic_timeout"
> > parameter equals 0, it will likely print out many noisy message like
> > WARN() call stack for each and every CPU except the panic one, messages
> > like below:
>
>
> > Keeping the interrupt disabled will avoid the noisy message.
> >
> > When code runs to this point, it means user has chosed to not reboot, or
> > do any special handling by using the panic notifier method, the only reason
> > to enable the interrupt may be sysrq migic key and panic_blink function
> > (though may not work even with irq enabled).
> >
> > So make the irq disabled by default and add a cmdline parameter
> > "panic_keep_irq_on" to turn it on when needed.
> >
> > Signed-off-by: Feng Tang <feng.tang@...el.com>
> >
>
> Acked-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

I'm fine with the new boot param, but I think we need to leave it how
it was by default: systems that want to see the blinking aren't going
to have a screen to read about what boot param they need to add.
Currently, we'll blink and spew extra warnings. With this patch we'll
not blink and not spew: a headless machine will have no indication
that a panic happened.

-Kees

--
Kees Cook