ptp: annotate data-race around q->head and q->tail

As I was working on a syzbot report, I found that KCSAN would
probably complain that reading q->head or q->tail without
barriers could lead to invalid results.

Add corresponding READ_ONCE() and WRITE_ONCE() to avoid
load-store tearing.

Fixes: d94ba80ebbea ("ptp: Added a brand new class driver for ptp clocks.")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Link: https://lore.kernel.org/r/20231109174859.3995880-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/drivers/ptp/ptp_chardev.c b/drivers/ptp/ptp_chardev.c
index 3f7a747..7513018 100644 (file)
--- a/drivers/ptp/ptp_chardev.c
+++ b/drivers/ptp/ptp_chardev.c
@@ -572,7 +572,8 @@ ssize_t ptp_read(struct posix_clock_context *pccontext, uint rdflags,
 
        for (i = 0; i < cnt; i++) {
                event[i] = queue->buf[queue->head];
-               queue->head = (queue->head + 1) % PTP_MAX_TIMESTAMPS;
+               /* Paired with READ_ONCE() in queue_cnt() */
+               WRITE_ONCE(queue->head, (queue->head + 1) % PTP_MAX_TIMESTAMPS);
        }
 
        spin_unlock_irqrestore(&queue->lock, flags);

diff --git a/drivers/ptp/ptp_clock.c b/drivers/ptp/ptp_clock.c
index 3134568..15b804b 100644 (file)
--- a/drivers/ptp/ptp_clock.c
+++ b/drivers/ptp/ptp_clock.c
@@ -57,10 +57,11 @@ static void enqueue_external_timestamp(struct timestamp_event_queue *queue,
        dst->t.sec = seconds;
        dst->t.nsec = remainder;
 
+       /* Both WRITE_ONCE() are paired with READ_ONCE() in queue_cnt() */
        if (!queue_free(queue))
-               queue->head = (queue->head + 1) % PTP_MAX_TIMESTAMPS;
+               WRITE_ONCE(queue->head, (queue->head + 1) % PTP_MAX_TIMESTAMPS);
 
-       queue->tail = (queue->tail + 1) % PTP_MAX_TIMESTAMPS;
+       WRITE_ONCE(queue->tail, (queue->tail + 1) % PTP_MAX_TIMESTAMPS);
 
        spin_unlock_irqrestore(&queue->lock, flags);
 }

diff --git a/drivers/ptp/ptp_private.h b/drivers/ptp/ptp_private.h
index 35fde0a..45f9002 100644 (file)
--- a/drivers/ptp/ptp_private.h
+++ b/drivers/ptp/ptp_private.h
@@ -85,9 +85,13 @@ struct ptp_vclock {
  * that a writer might concurrently increment the tail does not
  * matter, since the queue remains nonempty nonetheless.
  */
-static inline int queue_cnt(struct timestamp_event_queue *q)
+static inline int queue_cnt(const struct timestamp_event_queue *q)
 {
-       int cnt = q->tail - q->head;
+       /*
+        * Paired with WRITE_ONCE() in enqueue_external_timestamp(),
+        * ptp_read(), extts_fifo_show().
+        */
+       int cnt = READ_ONCE(q->tail) - READ_ONCE(q->head);
        return cnt < 0 ? PTP_MAX_TIMESTAMPS + cnt : cnt;
 }
 

diff --git a/drivers/ptp/ptp_sysfs.c b/drivers/ptp/ptp_sysfs.c
index 7d023d9..f7a499a 100644 (file)
--- a/drivers/ptp/ptp_sysfs.c
+++ b/drivers/ptp/ptp_sysfs.c
@@ -94,7 +94,8 @@ static ssize_t extts_fifo_show(struct device *dev,
        qcnt = queue_cnt(queue);
        if (qcnt) {
                event = queue->buf[queue->head];
-               queue->head = (queue->head + 1) % PTP_MAX_TIMESTAMPS;
+               /* Paired with READ_ONCE() in queue_cnt() */
+               WRITE_ONCE(queue->head, (queue->head + 1) % PTP_MAX_TIMESTAMPS);
        }
        spin_unlock_irqrestore(&queue->lock, flags);