block: add IOC_PR_READ_RESERVATION ioctl

Add a Persistent Reservations ioctl to read the current reservation.
This calls the pr_ops->read_reservation() function that was previously
added in commit c787f1b ("block: Add PR callouts for read keys and
reservation") but was only used by the in-kernel SCSI target so far.

The IOC_PR_READ_RESERVATION ioctl is necessary so that userspace
applications that rely on Persistent Reservations ioctls have a way of
inspecting the current state. Cluster managers and validation tests need
this functionality.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: stefanhaRH

block/ioctl.c

@@ -477,6 +477,32 @@ static int blkdev_pr_read_keys(struct block_device *bdev, blk_mode_t mode,
 	return ret;
 }
 
+static int blkdev_pr_read_reservation(struct block_device *bdev,
+		blk_mode_t mode, struct pr_read_reservation __user *arg)
+{
+	const struct pr_ops *ops = bdev->bd_disk->fops->pr_ops;
+	struct pr_held_reservation rsv = {};
+	struct pr_read_reservation out = {};
+	int ret;
+
+	if (!blkdev_pr_allowed(bdev, mode))
+		return -EPERM;
+	if (!ops || !ops->pr_read_reservation)
+		return -EOPNOTSUPP;
+
+	ret = ops->pr_read_reservation(bdev, &rsv);
+	if (ret)
+		return ret;
+
+	out.key = rsv.key;
+	out.generation = rsv.generation;
+	out.type = rsv.type;
+
+	if (copy_to_user(arg, &out, sizeof(out)))
+		return -EFAULT;
+	return 0;
+}
+
 static int blkdev_flushbuf(struct block_device *bdev, unsigned cmd,
 		unsigned long arg)
 {
@@ -701,6 +727,8 @@ static int blkdev_common_ioctl(struct block_device *bdev, blk_mode_t mode,
 		return blkdev_pr_clear(bdev, mode, argp);
 	case IOC_PR_READ_KEYS:
 		return blkdev_pr_read_keys(bdev, mode, argp);
+	case IOC_PR_READ_RESERVATION:
+		return blkdev_pr_read_reservation(bdev, mode, argp);
 	default:
 		return blk_get_meta_cap(bdev, cmd, argp);
 	}

include/uapi/linux/pr.h

@@ -62,6 +62,12 @@ struct pr_read_keys {
 	__u64	keys_ptr;
 };
 
+struct pr_read_reservation {
+	__u64	key;
+	__u32	generation;
+	__u32	type;
+};
+
 #define PR_FL_IGNORE_KEY	(1 << 0)	/* ignore existing key */
 
 #define IOC_PR_REGISTER		_IOW('p', 200, struct pr_registration)
@@ -71,5 +77,6 @@ struct pr_read_keys {
 #define IOC_PR_PREEMPT_ABORT	_IOW('p', 204, struct pr_preempt)
 #define IOC_PR_CLEAR		_IOW('p', 205, struct pr_clear)
 #define IOC_PR_READ_KEYS	_IOWR('p', 206, struct pr_read_keys)
+#define IOC_PR_READ_RESERVATION	_IOR('p', 207, struct pr_read_reservation)
 
 #endif /* _UAPI_PR_H */