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 01:37:20PM +0106, John Ogness wrote:
> @@ -1572,7 +1608,12 @@ static int __nbcon_atomic_flush_pending_con(struct console *con, enum pr_flush_r
>  	ctxt->console			= con;
>  	ctxt->spinwait_max_us		= 2000;
>  	ctxt->prio			= nbcon_get_default_prio();
> -	ctxt->allow_unsafe_takeover	= nbcon_allow_unsafe_takeover();
> +	ctxt->allow_unsafe_takeover	= nbcon_allow_unsafe_takeover(con, reason);


Thanks for the series, I've tried it here, and I got:

  sysrq: Resetting
    ------------[ cut here ]------------
    WARNING: kernel/printk/nbcon.c:561 at nbcon_context_try_acquire+0x72c/0x790, CPU#0: sh/212
    Modules linked in: nbcon_hold(OE+)
    CPU: 0 UID: 0 PID: 212 Comm: sh Tainted: G           OE       7.4.0-rc1-next-20261105upstream-00006-g4170adac69fb #1 PREEMPT(full)
    Hardware name: linux,dummy-virt (DT)
    pc : nbcon_context_try_acquire+0x72c/0x790
    lr : nbcon_context_try_acquire+0x508/0x790
    ...
    Call trace:
     nbcon_context_try_acquire+0x72c/0x790 (P)
     __nbcon_atomic_flush_pending_con+0x1c0/0x358
     nbcon_atomic_flush_pending+0x224/0x390
     printk_trigger_flush+0x50/0x1c8
     emergency_restart+0x54/0x98
     sysrq_handle_reboot+0x5c/0x88
     __handle_sysrq+0x298/0x3c0
     write_sysrq_trigger+0x178/0x1d0
     proc_reg_write+0x148/0x270
     vfs_write+0x1d4/0x7c8
     ksys_write+0xbc/0x168
     __arm64_sys_write+0x88/0xa8
     invoke_syscall+0x74/0x188
     do_el0_svc+0x108/0x190
     el0_svc+0x60/0x1d8
     el0t_64_sync_handler+0x68/0xe0
     el0t_64_sync+0x198/0x1a0

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;

Thanks,
--breno