On Mon 2022-11-07 15:22:31, John Ogness wrote:
> With commit 9e124fe16ff2("xen: Enable console tty by default in domU
> if it's not a dummy") a hack was implemented to make sure that the
> tty console remains the console behind the /dev/console device. The
> main problem with the hack is that, after getting the console pointer
> to the tty console, it is assumed the pointer is still valid after
> releasing the console_sem. This assumption is incorrect and unsafe.
>
> Make the hack safe by introducing a new function
> console_force_preferred_locked() and perform the full operation
> under the console_list_lock.
>
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3457,6 +3458,43 @@ int unregister_console(struct console *console)
> }
> EXPORT_SYMBOL(unregister_console);
>
> +/**
> + * console_force_preferred_locked - force a registered console preferred
> + * @con: The registered console to force preferred.
> + *
> + * Must be called under console_list_lock().
> + */
> +void console_force_preferred_locked(struct console *con)
> +{
> + struct console *cur_pref_con;
> +
> + if (!console_is_registered_locked(con))
> + return;
> +
> + cur_pref_con = console_first();
> +
> + /* Already preferred? */
> + if (cur_pref_con == con)
> + return;
> +
> + hlist_del_init_rcu(&con->node);
We actually should re-initialize the node only after all existing
console list walks are finished. Se we should use here:
hlist_del_rcu(&con->node);
> +
> + /*
> + * Ensure that all SRCU list walks have completed so that the console
> + * can be added to the beginning of the console list and its forward
> + * list pointer can be re-initialized.
The comment is right ;-)
> + */
> + synchronize_srcu(&console_srcu);
> +
> + con->flags |= CON_CONSDEV;
> + WARN_ON(!con->device);
> +
> + /* Only the new head can have CON_CONSDEV set. */
> + WRITE_ONCE(cur_pref_con->flags, cur_pref_con->flags & ~CON_CONSDEV);
As mentioned in the reply for 7th patch, I would prefer to hide this
WRITE_ONCE into a wrapper, e.g. console_set_flag(). It might also
check that the console_list_lock is taken...
> + hlist_add_behind_rcu(&con->node, console_list.first);
> +}
> +EXPORT_SYMBOL(console_force_preferred_locked);
> +
> /*
> * Initialize the console device. This is called *early*, so
> * we can't necessarily depend on lots of kernel help here.
Best Regards,
Petr
