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    Subject[PATCH 5.15 760/917] nbd: fix max value for first_minor
    Date
    From: Yu Kuai <yukuai3@huawei.com>

    [ Upstream commit e4c4871a73944353ea23e319de27ef73ce546623 ]

    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>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    drivers/block/nbd.c | 6 +++---
    1 file changed, 3 insertions(+), 3 deletions(-)

    diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
    index 4f1b591c3a555..0d820c4dec176 100644
    --- a/drivers/block/nbd.c
    +++ b/drivers/block/nbd.c
    @@ -1749,11 +1749,11 @@ 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;
    }
    --
    2.33.0


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