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authorJohn Ogness <john.ogness@linutronix.de>2022-11-16 19:21:44 +0300
committerPetr Mladek <pmladek@suse.com>2022-12-02 13:25:02 +0300
commit6f8836756f3cbb02bfd1f0e033516585250318a9 (patch)
treeda70918f2ac3f0ad26c930085eabeab7b285a915 /kernel/printk
parent2c6b4b7065a7006271a9c12b1a1e6c56dc6d3796 (diff)
downloadlinux-6f8836756f3cbb02bfd1f0e033516585250318a9.tar.xz
printk, xen: fbfront: create/use safe function for forcing preferred
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. Signed-off-by: John Ogness <john.ogness@linutronix.de> Reviewed-by: Petr Mladek <pmladek@suse.com> Signed-off-by: Petr Mladek <pmladek@suse.com> Link: https://lore.kernel.org/r/20221116162152.193147-33-john.ogness@linutronix.de
Diffstat (limited to 'kernel/printk')
-rw-r--r--kernel/printk/printk.c49
1 files changed, 46 insertions, 3 deletions
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index e58a6ccb945c..bda9e8ec6817 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -248,9 +248,10 @@ int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
void console_list_lock(void)
{
/*
- * In unregister_console(), synchronize_srcu() is called with the
- * console_list_lock held. Therefore it is not allowed that the
- * console_list_lock is taken with the srcu_lock held.
+ * In unregister_console() and console_force_preferred_locked(),
+ * synchronize_srcu() is called with the console_list_lock held.
+ * Therefore it is not allowed that the console_list_lock is taken
+ * with the srcu_lock held.
*
* Detecting if this context is really in the read-side critical
* section is only possible if the appropriate debug options are
@@ -3490,6 +3491,48 @@ 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;
+
+ /*
+ * Delete, but do not re-initialize the entry. This allows the console
+ * to continue to appear registered (via any hlist_unhashed_lockless()
+ * checks), even though it was briefly removed from the console list.
+ */
+ 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.
+ */
+ synchronize_srcu(&console_srcu);
+
+ con->flags |= CON_CONSDEV;
+ WARN_ON(!con->device);
+
+ /* Only the new head can have CON_CONSDEV set. */
+ console_srcu_write_flags(cur_pref_con, cur_pref_con->flags & ~CON_CONSDEV);
+ hlist_add_head_rcu(&con->node, &console_list);
+}
+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.