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Message-Id: <20220924000454.3319186-7-john.ogness@linutronix.de>
Date: Sat, 24 Sep 2022 02:10:42 +0206
From: John Ogness <john.ogness@...utronix.de>
To: Petr Mladek <pmladek@...e.com>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>,
Steven Rostedt <rostedt@...dmis.org>,
Thomas Gleixner <tglx@...utronix.de>,
linux-kernel@...r.kernel.org,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: [PATCH printk 06/18] printk: Protect [un]register_console() with a mutex
From: Thomas Gleixner <tglx@...utronix.de>
Unprotected list walks are a brilliant idea. Especially in the context of
hotpluggable consoles.
The new list lock provides not only synchronization for console list
manipulation, but also for manipulation of console->flags:
console_list_lock();
console_lock();
/* may now manipulate the console list and/or console->flags */
console_unlock();
console_list_unlock();
Therefore it is safe to iterate the console list and read console->flags
if holding either the console lock or the console list lock.
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: John Ogness <john.ogness@...utronix.de>
---
include/linux/console.h | 30 +++++++++++++--
kernel/printk/printk.c | 84 ++++++++++++++++++++++++++++++++++-------
2 files changed, 98 insertions(+), 16 deletions(-)
diff --git a/include/linux/console.h b/include/linux/console.h
index 8c1686e2c233..24344f9b0bc1 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -157,10 +157,34 @@ struct console {
struct console *next;
};
-/*
- * for_each_console() allows you to iterate on each console
+#ifdef CONFIG_LOCKDEP
+extern void lockdep_assert_console_list_lock_held(void);
+#else
+static inline void lockdep_assert_console_list_lock_held(void) { }
+#endif
+
+extern void console_list_lock(void) __acquires(console_mutex);
+extern void console_list_unlock(void) __releases(console_mutex);
+
+/**
+ * for_each_registered_console() - Iterator over registered consoles
+ * @con: struct console pointer used as loop cursor
+ *
+ * Requires console_list_lock to be held. Can only be invoked from
+ * preemptible context.
+ */
+#define for_each_registered_console(con) \
+ lockdep_assert_console_list_lock_held(); \
+ for (con = console_drivers; con != NULL; con = con->next)
+
+/**
+ * for_each_console() - Iterator over registered consoles
+ * @con: struct console pointer used as loop cursor
+ *
+ * Requires console_lock to be held which guarantees that the
+ * list is immutable.
*/
-#define for_each_console(con) \
+#define for_each_console(con) \
for (con = console_drivers; con != NULL; con = con->next)
extern int console_set_on_cmdline;
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index e4f1e7478b52..8c56f6071873 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -79,10 +79,17 @@ int oops_in_progress;
EXPORT_SYMBOL(oops_in_progress);
/*
- * console_sem protects the console_drivers list, and also
- * provides serialisation for access to the entire console
- * driver system.
+ * console_sem protects the console_drivers list, and also provides
+ * serialization for access to the entire console driver system.
+ *
+ * console_mutex serializes register/unregister.
+ *
+ * console_sem must be taken inside a console_mutex locked section
+ * for any list manipulation in order to keep the console BKL
+ * machinery happy. This requirement also applies to manipulation
+ * of console->flags.
*/
+static DEFINE_MUTEX(console_mutex);
static DEFINE_SEMAPHORE(console_sem);
struct console *console_drivers;
EXPORT_SYMBOL_GPL(console_drivers);
@@ -103,6 +110,12 @@ static int __read_mostly suppress_panic_printk;
static struct lockdep_map console_lock_dep_map = {
.name = "console_lock"
};
+
+void lockdep_assert_console_list_lock_held(void)
+{
+ lockdep_assert_held(&console_mutex);
+}
+
#endif
enum devkmsg_log_bits {
@@ -220,6 +233,28 @@ int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
}
#endif /* CONFIG_PRINTK && CONFIG_SYSCTL */
+/**
+ * console_list_lock - Lock the console list
+ *
+ * For non-console related list walks, e.g. procfs, sysfs...
+ */
+void console_list_lock(void)
+{
+ mutex_lock(&console_mutex);
+}
+EXPORT_SYMBOL(console_list_lock);
+
+/**
+ * console_list_unlock - Unlock the console list
+ *
+ * Counterpart to console_list_lock()
+ */
+void console_list_unlock(void)
+{
+ mutex_unlock(&console_mutex);
+}
+EXPORT_SYMBOL(console_list_unlock);
+
/*
* Helper macros to handle lockdep when locking/unlocking console_sem. We use
* macros instead of functions so that _RET_IP_ contains useful information.
@@ -2978,17 +3013,21 @@ struct tty_driver *console_device(int *index)
void console_stop(struct console *console)
{
__pr_flush(console, 1000, true);
+ console_list_lock();
console_lock();
console->flags &= ~CON_ENABLED;
console_unlock();
+ console_list_unlock();
}
EXPORT_SYMBOL(console_stop);
void console_start(struct console *console)
{
+ console_list_lock();
console_lock();
console->flags |= CON_ENABLED;
console_unlock();
+ console_list_unlock();
__pr_flush(console, 1000, true);
}
EXPORT_SYMBOL(console_start);
@@ -3081,6 +3120,8 @@ static void try_enable_default_console(struct console *newcon)
(con->flags & CON_BOOT) ? "boot" : "", \
con->name, con->index, ##__VA_ARGS__)
+static int console_unregister_locked(struct console *console);
+
/*
* The console driver calls this routine during kernel initialization
* to register the console printing procedure with printk() and to
@@ -3107,13 +3148,14 @@ void register_console(struct console *newcon)
bool realcon_enabled = false;
int err;
- for_each_console(con) {
+ console_list_lock();
+ for_each_registered_console(con) {
if (WARN(con == newcon, "console '%s%d' already registered\n",
con->name, con->index))
- return;
+ goto unlock;
}
- for_each_console(con) {
+ for_each_registered_console(con) {
if (con->flags & CON_BOOT)
bootcon_enabled = true;
else
@@ -3124,7 +3166,7 @@ void register_console(struct console *newcon)
if (newcon->flags & CON_BOOT && realcon_enabled) {
pr_info("Too late to register bootconsole %s%d\n",
newcon->name, newcon->index);
- return;
+ goto unlock;
}
/*
@@ -3155,7 +3197,7 @@ void register_console(struct console *newcon)
/* printk() messages are not printed to the Braille console. */
if (err || newcon->flags & CON_BRL)
- return;
+ goto unlock;
/*
* If we have a bootconsole, and are switching to a real console,
@@ -3209,14 +3251,17 @@ void register_console(struct console *newcon)
if (bootcon_enabled &&
((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV) &&
!keep_bootcon) {
- for_each_console(con)
+ for_each_console(con) {
if (con->flags & CON_BOOT)
- unregister_console(con);
+ console_unregister_locked(con);
+ }
}
+unlock:
+ console_list_unlock();
}
EXPORT_SYMBOL(register_console);
-int unregister_console(struct console *console)
+static int console_unregister_locked(struct console *console)
{
struct console *con;
int res;
@@ -3269,6 +3314,16 @@ int unregister_console(struct console *console)
return res;
}
+
+int unregister_console(struct console *console)
+{
+ int res;
+
+ console_list_lock();
+ res = console_unregister_locked(console);
+ console_list_unlock();
+ return res;
+}
EXPORT_SYMBOL(unregister_console);
/*
@@ -3320,7 +3375,8 @@ static int __init printk_late_init(void)
struct console *con;
int ret;
- for_each_console(con) {
+ console_list_lock();
+ for_each_registered_console(con) {
if (!(con->flags & CON_BOOT))
continue;
@@ -3337,9 +3393,11 @@ static int __init printk_late_init(void)
*/
pr_warn("bootconsole [%s%d] uses init memory and must be disabled even before the real one is ready\n",
con->name, con->index);
- unregister_console(con);
+ console_unregister_locked(con);
}
}
+ console_list_unlock();
+
ret = cpuhp_setup_state_nocalls(CPUHP_PRINTK_DEAD, "printk:dead", NULL,
console_cpu_notify);
WARN_ON(ret < 0);
--
2.30.2
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