block: fix BLKSECTGET ioctl when max_sectors is greater than USHRT_MAX

BLKSECTGET ioctl loads the request queue's max_sectors as unsigned
short value to the argument pointer.  So if the max_sector is greater
than USHRT_MAX, the upper 16 bits of that is just discarded.

In such case, USHRT_MAX is more preferable than the lower 16 bits of
max_sectors.
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: "James E.J. Bottomley" <JBottomley@parallels.com>
Cc: Douglas Gilbert <dgilbert@interlog.com>
Cc: linux-scsi@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/compat_ioctl.c

@@ -663,6 +663,7 @@ long compat_blkdev_ioctl(struct file *file, unsigned cmd, unsigned long arg)
 	fmode_t mode = file->f_mode;
 	struct backing_dev_info *bdi;
 	loff_t size;
+	unsigned int max_sectors;
 
 	/*
 	 * O_NDELAY can be altered using fcntl(.., F_SETFL, ..), so we have
@@ -718,8 +719,9 @@ long compat_blkdev_ioctl(struct file *file, unsigned cmd, unsigned long arg)
 	case BLKSSZGET: /* get block device hardware sector size */
 		return compat_put_int(arg, bdev_logical_block_size(bdev));
 	case BLKSECTGET:
-		return compat_put_ushort(arg,
+		max_sectors = min_t(unsigned int, USHRT_MAX,
 				    queue_max_sectors(bdev_get_queue(bdev)));
+		return compat_put_ushort(arg, max_sectors);
 	case BLKROTATIONAL:
 		return compat_put_ushort(arg,
 					 !blk_queue_nonrot(bdev_get_queue(bdev)));

block/ioctl.c

@@ -278,6 +278,7 @@ int blkdev_ioctl(struct block_device *bdev, fmode_t mode, unsigned cmd,
 	struct backing_dev_info *bdi;
 	loff_t size;
 	int ret, n;
+	unsigned int max_sectors;
 
 	switch(cmd) {
 	case BLKFLSBUF:
@@ -375,7 +376,9 @@ int blkdev_ioctl(struct block_device *bdev, fmode_t mode, unsigned cmd,
 	case BLKDISCARDZEROES:
 		return put_uint(arg, bdev_discard_zeroes_data(bdev));
 	case BLKSECTGET:
-		return put_ushort(arg, queue_max_sectors(bdev_get_queue(bdev)));
+		max_sectors = min_t(unsigned int, USHRT_MAX,
+				    queue_max_sectors(bdev_get_queue(bdev)));
+		return put_ushort(arg, max_sectors);
 	case BLKROTATIONAL:
 		return put_ushort(arg, !blk_queue_nonrot(bdev_get_queue(bdev)));
 	case BLKRASET: