Re: [PATCH 5.10-rt] printk: ignore consoles without write() callback

[Luis Claudio R. Goncalves]

On Fri, Feb 17, 2023 at 09:53:44AM +0106, John Ogness wrote:
> The ttynull driver does not provide an implementation for the write()
> callback. This leads to a NULL pointer dereference in the related
> printing kthread, which assumes it can call that callback.
> 
> Do not create kthreads for consoles that do not implement the write()
> callback. Also, for pr_flush(), ignore consoles that do not implement
> write() or write_atomic(), since there is no way those consoles can
> flush their output.
> 
> Link: https://lore.kernel.org/lkml/1831554214.546921.1676479103702.JavaMail.zimbra@xxxxxxx
> Reported-by: Michael Thalmeier <michael.thalmeier@xxxxxxx>
> Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>

Thank you! I will apply that to the next 5.10-rt build.

Luis

> ---
>  kernel/printk/printk.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index d2205872304d..64747c72fbea 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2267,6 +2267,10 @@ static int printk_kthread_func(void *data)
>  /* Must be called within console_lock(). */
>  static void start_printk_kthread(struct console *con)
>  {
> +	/* No need to start a printing thread if the console cannot print. */
> +	if (!con->write)
> +		return;
> +
>  	con->thread = kthread_run(printk_kthread_func, con,
>  				  "pr/%s%d", con->name, con->index);
>  	if (IS_ERR(con->thread)) {
> @@ -3566,6 +3570,8 @@ bool pr_flush(int timeout_ms, bool reset_on_progress)
>  		for_each_console(con) {
>  			if (!(con->flags & CON_ENABLED))
>  				continue;
> +			if (!con->write && !con->write_atomic)
> +				continue;
>  			printk_seq = atomic64_read(&con->printk_seq);
>  			if (printk_seq < seq)
>  				diff += seq - printk_seq;
> -- 
> 2.30.2
> 
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