block: disallow Persistent Reservation on partitions

Refuse Persistent Reservation operations on partitions as reservation
on partitions doesn't make sense.

Besides, introduce blkdev_pr_allowed() helper, where more policies could
be placed here later.
Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230613084008.93795-2-jefflexu@linux.alibaba.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/ioctl.c

@@ -254,13 +254,25 @@ int blkdev_compat_ptr_ioctl(struct block_device *bdev, blk_mode_t mode,
 EXPORT_SYMBOL(blkdev_compat_ptr_ioctl);
 #endif
 
+static bool blkdev_pr_allowed(struct block_device *bdev)
+{
+	/* no sense to make reservations for partitions */
+	if (bdev_is_partition(bdev))
+		return false;
+
+	if (capable(CAP_SYS_ADMIN))
+		return true;
+
+	return false;
+}
+
 static int blkdev_pr_register(struct block_device *bdev,
 		struct pr_registration __user *arg)
 {
 	const struct pr_ops *ops = bdev->bd_disk->fops->pr_ops;
 	struct pr_registration reg;
 
-	if (!capable(CAP_SYS_ADMIN))
+	if (!blkdev_pr_allowed(bdev))
 		return -EPERM;
 	if (!ops || !ops->pr_register)
 		return -EOPNOTSUPP;
@@ -278,7 +290,7 @@ static int blkdev_pr_reserve(struct block_device *bdev,
 	const struct pr_ops *ops = bdev->bd_disk->fops->pr_ops;
 	struct pr_reservation rsv;
 
-	if (!capable(CAP_SYS_ADMIN))
+	if (!blkdev_pr_allowed(bdev))
 		return -EPERM;
 	if (!ops || !ops->pr_reserve)
 		return -EOPNOTSUPP;
@@ -296,7 +308,7 @@ static int blkdev_pr_release(struct block_device *bdev,
 	const struct pr_ops *ops = bdev->bd_disk->fops->pr_ops;
 	struct pr_reservation rsv;
 
-	if (!capable(CAP_SYS_ADMIN))
+	if (!blkdev_pr_allowed(bdev))
 		return -EPERM;
 	if (!ops || !ops->pr_release)
 		return -EOPNOTSUPP;
@@ -314,7 +326,7 @@ static int blkdev_pr_preempt(struct block_device *bdev,
 	const struct pr_ops *ops = bdev->bd_disk->fops->pr_ops;
 	struct pr_preempt p;
 
-	if (!capable(CAP_SYS_ADMIN))
+	if (!blkdev_pr_allowed(bdev))
 		return -EPERM;
 	if (!ops || !ops->pr_preempt)
 		return -EOPNOTSUPP;
@@ -332,7 +344,7 @@ static int blkdev_pr_clear(struct block_device *bdev,
 	const struct pr_ops *ops = bdev->bd_disk->fops->pr_ops;
 	struct pr_clear c;
 
-	if (!capable(CAP_SYS_ADMIN))
+	if (!blkdev_pr_allowed(bdev))
 		return -EPERM;
 	if (!ops || !ops->pr_clear)
 		return -EOPNOTSUPP;