Re: [PATCH printk 3/6] printk: Evaluate flush policy in nbcon_allow_unsafe_takeover()

[Breno Leitao <leitao@debian.org>]

On Mon, Nov 09, 2026 at 03:34:32PM +0106, John Ogness wrote:
> On 2026-11-09, Breno Leitao <leitao@debian.org> wrote:
> > That is coming from:
> >
> >    550 static int nbcon_context_try_acquire_hostile(struct nbcon_context *ctxt,
> >    551                                              struct nbcon_state *cur)
> >    552 {
> >    553         unsigned int cpu = smp_processor_id();
> >    554         struct console *con = ctxt->console;
> >    555         struct nbcon_state new;
> >    556
> >    557         if (!ctxt->allow_unsafe_takeover)
> >    558                 return -EPERM;
> >    559
> >    560         /* Ensure caller is allowed to perform unsafe hostile takeovers. */
> >    561         if (WARN_ON_ONCE(ctxt->prio != NBCON_PRIO_PANIC))
> >    562                 return -EPERM;
> 
> Wow, you managed to get a hostile takeover. Out of curiousity, do you
> know how dangerous this really is? Can this basically just result in
> garbage on the network? Or is there dangers of running into the forest
> with NULL or garbage pointers? And is the danger acceptable?

Sure, I was playing with different scenarios, and this was done with a
test module that forces the takeover:

1) Acquire netcon_ext with nbcon_device_try_acquire(), so the console
   is owned at NORMAL priority and in an unsafe state

2) mdelay() for a few seconds without releasing it (simulating
   a possible netpoll busy TX queue -- or some other networkdelay)

3) echo b > /proc/sysrq-trigger from another CPU

--breno