Commit b371cbb5 authored by John Ogness's avatar John Ogness Committed by Petr Mladek
Browse files

printk: convert @syslog_lock to mutex 



@syslog_lock was a raw_spin_lock to simplify the transition of
removing @logbuf_lock and the safe buffers. With that transition
complete, and since all uses of @syslog_lock are within sleepable
contexts, @syslog_lock can become a mutex.

Note that until now register_console() would disable interrupts
using irqsave, which implies that it may be called with interrupts
disabled. And indeed, there is one possible call chain on parisc
where this happens:

handle_interruption(code=1) /* High-priority machine check (HPMC) */
  pdc_console_restart()
    pdc_console_init_force()
      register_console()

However, register_console() calls console_lock(), which might sleep.
So it has never been allowed to call register_console() from an
atomic context and the above call chain is a bug.

Note that the removal of read_syslog_seq_irq() is slightly changing
the behavior of SYSLOG_ACTION_READ by testing against a possibly
outdated @seq value. However, the value of @seq could have changed
after the test, so it is not a new window. A follow-up commit closes
this window.

Signed-off-by: default avatarJohn Ogness <john.ogness@linutronix.de>
Reviewed-by: default avatarPetr Mladek <pmladek@suse.com>
Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210715193359.25946-6-john.ogness@linutronix.de
parent 85e3e7fb

kernel/printk/printk.c

+20 −29
Original line number Diff line number Diff line
@@ -356,7 +356,7 @@ enum log_flags { 
};



/* syslog_lock protects syslog_* variables and write access to clear_seq. */

static DEFINE_RAW_SPINLOCK(syslog_lock);

static DEFINE_MUTEX(syslog_lock);



#ifdef CONFIG_PRINTK

DECLARE_WAIT_QUEUE_HEAD(log_wait);
@@ -1497,9 +1497,9 @@ static int syslog_print(char __user *buf, int size) 
		size_t n;

		size_t skip;



		raw_spin_lock_irq(&syslog_lock);

		mutex_lock(&syslog_lock);

		if (!prb_read_valid(prb, syslog_seq, &r)) {

			raw_spin_unlock_irq(&syslog_lock);

			mutex_unlock(&syslog_lock);

			break;

		}

		if (r.info->seq != syslog_seq) {
@@ -1528,7 +1528,7 @@ static int syslog_print(char __user *buf, int size) 
			syslog_partial += n;

		} else

			n = 0;

		raw_spin_unlock_irq(&syslog_lock);

		mutex_unlock(&syslog_lock);



		if (!n)

			break;
@@ -1592,9 +1592,9 @@ static int syslog_print_all(char __user *buf, int size, bool clear) 
	}



	if (clear) {

		raw_spin_lock_irq(&syslog_lock);

		mutex_lock(&syslog_lock);

		latched_seq_write(&clear_seq, seq);

		raw_spin_unlock_irq(&syslog_lock);

		mutex_unlock(&syslog_lock);

	}



	kfree(text);
@@ -1603,21 +1603,9 @@ static int syslog_print_all(char __user *buf, int size, bool clear) 


static void syslog_clear(void)

{

	raw_spin_lock_irq(&syslog_lock);

	mutex_lock(&syslog_lock);

	latched_seq_write(&clear_seq, prb_next_seq(prb));

	raw_spin_unlock_irq(&syslog_lock);

}



/* Return a consistent copy of @syslog_seq. */

static u64 read_syslog_seq_irq(void)

{

	u64 seq;



	raw_spin_lock_irq(&syslog_lock);

	seq = syslog_seq;

	raw_spin_unlock_irq(&syslog_lock);



	return seq;

	mutex_unlock(&syslog_lock);

}



int do_syslog(int type, char __user *buf, int len, int source)
@@ -1626,6 +1614,7 @@ int do_syslog(int type, char __user *buf, int len, int source) 
	bool clear = false;

	static int saved_console_loglevel = LOGLEVEL_DEFAULT;

	int error;

	u64 seq;



	error = check_syslog_permissions(type, source);

	if (error)
@@ -1644,8 +1633,12 @@ int do_syslog(int type, char __user *buf, int len, int source) 
		if (!access_ok(buf, len))

			return -EFAULT;



		error = wait_event_interruptible(log_wait,

				prb_read_valid(prb, read_syslog_seq_irq(), NULL));

		/* Get a consistent copy of @syslog_seq. */

		mutex_lock(&syslog_lock);

		seq = syslog_seq;

		mutex_unlock(&syslog_lock);



		error = wait_event_interruptible(log_wait, prb_read_valid(prb, seq, NULL));

		if (error)

			return error;

		error = syslog_print(buf, len);
@@ -1693,10 +1686,10 @@ int do_syslog(int type, char __user *buf, int len, int source) 
		break;

	/* Number of chars in the log buffer */

	case SYSLOG_ACTION_SIZE_UNREAD:

		raw_spin_lock_irq(&syslog_lock);

		mutex_lock(&syslog_lock);

		if (!prb_read_valid_info(prb, syslog_seq, &info, NULL)) {

			/* No unread messages. */

			raw_spin_unlock_irq(&syslog_lock);

			mutex_unlock(&syslog_lock);

			return 0;

		}

		if (info.seq != syslog_seq) {
@@ -1714,7 +1707,6 @@ int do_syslog(int type, char __user *buf, int len, int source) 
		} else {

			bool time = syslog_partial ? syslog_time : printk_time;

			unsigned int line_count;

			u64 seq;



			prb_for_each_info(syslog_seq, prb, seq, &info,

					  &line_count) {
@@ -1724,7 +1716,7 @@ int do_syslog(int type, char __user *buf, int len, int source) 
			}

			error -= syslog_partial;

		}

		raw_spin_unlock_irq(&syslog_lock);

		mutex_unlock(&syslog_lock);

		break;

	/* Size of the log buffer */

	case SYSLOG_ACTION_SIZE_BUFFER:
@@ -2929,7 +2921,6 @@ static int try_enable_new_console(struct console *newcon, bool user_specified) 
 */

void register_console(struct console *newcon)

{

	unsigned long flags;

	struct console *bcon = NULL;

	int err;
@@ -3034,9 +3025,9 @@ void register_console(struct console *newcon) 
		exclusive_console_stop_seq = console_seq;



		/* Get a consistent copy of @syslog_seq. */

		raw_spin_lock_irqsave(&syslog_lock, flags);

		mutex_lock(&syslog_lock);

		console_seq = syslog_seq;

		raw_spin_unlock_irqrestore(&syslog_lock, flags);

		mutex_unlock(&syslog_lock);

	}

	console_unlock();

	console_sysfs_notify();