nbd: fix max value for 'first_minor'

commit b1a811633f73 ("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: b1a811633f73 ("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>

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 0ee104fbb6286318d94c09517f288361b46546d8..29f9ac8817d7b0eff91d7d9fd2292af6804b445a 100644 (file)
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -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;
        }