Commit e4c4871a authored Nov 02, 2021 by Yu Kuai Committed by Jens Axboe Nov 02, 2021

nbd: fix max value for 'first_minor'



commit b1a81163 ("block: nbd: add sanity check for first_minor")
checks that 'first_minor' should not be greater than 0xff, which is
wrong. Whitout the commit, the details that when user pass 0x100000,
it ends up create sysfs dir "/sys/block/43:0" are as follows:

nbd_dev_add
 disk->first_minor = index << part_shift
  -> default part_shift is 5, first_minor is 0x2000000
  device_add_disk
   ddev->devt = MKDEV(disk->major, disk->first_minor)
    -> (0x2b << 20) | (0x2000000) = 0x2b00000
   device_add
    device_create_sys_dev_entry
	 format_dev_t
	  sprintf(buffer, "%u:%u", MAJOR(dev), MINOR(dev));
	   -> got 43:0
	  sysfs_create_link -> /sys/block/43:0

By the way, with the wrong fix, when part_shift is the default value,
only 8 ndb devices can be created since 8 << 5 is greater than 0xff.

Since the max bits for 'first_minor' should be the same as what
MKDEV() does, which is 20. Change the upper bound of 'first_minor'
from 0xff to 0xfffff.

Fixes: b1a81163 ("block: nbd: add sanity check for first_minor")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20211102015237.2309763-2-yebin10@huawei.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>

parent e1528830

drivers/block/nbd.c

+3 −3

Original line number	Diff line number	Diff line
		@@ -1808,11 +1808,11 @@ static struct nbd_device *nbd_dev_add(int index, unsigned int refs)
		disk->major = NBD_MAJOR;

		/* Too big first_minor can cause duplicate creation of
		* sysfs files/links, since first_minor will be truncated to
		* byte in __device_add_disk().
		* sysfs files/links, since MKDEV() expect that the max bits of
		* first_minor is 20.
		*/
		disk->first_minor = index << part_shift;
		if (disk->first_minor > 0xff) {
		if (disk->first_minor > MINORMASK) {
		err = -EINVAL;
		goto out_free_idr;
		}