[PATCH printk v4 5/6] printk: convert @syslog_lock to mutex

John Ogness
2021-07-15T19:52:36Z

@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outdated @seq value.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: John Ogness <john.ogness@linutronix.de>
---
kernel/printk/printk.c | 49 +++++++++++++++++-------------------------
1 file changed, 20 insertions(+), 29 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 9693791ab124..99160d0dafd6 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -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:
@@ -2932,7 +2924,6 @@ 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;

@@ -3037,9 +3028,9 @@ 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();
--
2.20.1