Re: [REGRESSION] disabling and re-enabling magic sysrq fails after kernel 6.11

[Peter Seiderer <ps.report@gmx.net>]

Hello Ned, Christian, *,

On Sun, 27 Oct 2024 15:06:09 +0100, Christian Heusel <christian@heusel.eu> wrote:

> On 24/10/26 07:15PM, Ned T. Crigler wrote:
> > Hi,
>
> Hey Ned,
>
> > It looks like starting with kernel 6.11, disabling and re-enabling
> > magic
> > sysrq fails with these errors in dmesg:
> >
> > kernel: input: input_handler_check_methods: only one event processing
> > method can be defined (sysrq)
> > kernel: sysrq: Failed to register input handler, error -22
> >
> > after doing:
> >
> > # echo 0 > /proc/sys/kernel/sysrq
> > # echo 438 > /proc/sys/kernel/sysrq
> > # echo 0 > /proc/sys/kernel/sysrq
> > # echo 438 > /proc/sys/kernel/sysrq
> > # echo 0 > /proc/sys/kernel/sysrq
> > # echo 438 > /proc/sys/kernel/sysrq
>
> I have found that this issue is also present in the latest mainline
> release and bisected it to the following commit:
>
>     d469647bafd9 ("Input: simplify event handling logic")
>

After the mentioned commit a call sysrq_register_handler() -->
input_register_handler(&sysrq_handler) with sysrq_handler.filter set
will result in sysrq_handler.events set to input_handler_events_filter,
see drivers/input/input.c (line 2607 to 2608):

2596 int input_register_handler(struct input_handler *handler)
2597 {
2598         struct input_dev *dev;
2599         int error;
2600
2601         error = input_handler_check_methods(handler);
2602         if (error)
2603                 return error;
2604
2605         INIT_LIST_HEAD(&handler->h_list);
2606
2607         if (handler->filter)
2608                 handler->events = input_handler_events_filter;
2609         else if (handler->event)
2610                 handler->events = input_handler_events_default;
2611         else if (!handler->events)
2612                 handler->events = input_handler_events_null;

So the second call will fail at the check 'input_handler_check_methods(handler)'
which only allows one method to be set, see drivers/input/input.c:

2517 static int input_handler_check_methods(const struct input_handler *handler)
2518 {
2519         int count = 0;
2520
2521         if (handler->filter)
2522                 count++;
2523         if (handler->events)
2524                 count++;
2525         if (handler->event)
2526                 count++;
2527
2528         if (count > 1) {
2529                 pr_err("%s: only one event processing method can be defined      (%s)\n",
2530                        __func__, handler->name);
2531                 return -EINVAL;
2532         }
2533
2534         return 0;
2535 }


A quick fix/hack for the sysrq case:

--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -1045,7 +1045,7 @@ static inline void sysrq_register_handler(void)
        int error;

        sysrq_of_get_keyreset_config();
-
+       sysrq_handler.events = NULL;
        error = input_register_handler(&sysrq_handler);
        if (error)
                pr_err("Failed to register input handler, error %d", error);
lines 1-13/13 (END)

Regards,
Peter


> The additional authors / maintainers have been added to the recipients
> lists.
>
> I hope I didn't overlook any pending fixes.
>
> > --
> > Ned T. Crigler
>
> Cheers,
> Chris