btrfs: fix null pointer deference at btrfs_sysfs_add_one+0x105

bdev is null when disk has disappeared and mounted with
the degrade option

stack trace
---------
btrfs_sysfs_add_one+0x105/0x1c0 [btrfs]
open_ctree+0x15f3/0x1fe0 [btrfs]
btrfs_mount+0x5db/0x790 [btrfs]
? alloc_pages_current+0xa4/0x160
mount_fs+0x34/0x1b0
vfs_kern_mount+0x62/0xf0
do_mount+0x22e/0xa80
? __get_free_pages+0x9/0x40
? copy_mount_options+0x31/0x170
SyS_mount+0x7e/0xc0
system_call_fastpath+0x16/0x1b
---------

reproducer:
-------
mkfs.btrfs -draid1 -mraid1 /dev/sdc /dev/sdd
(detach a disk)
devmgt detach /dev/sdc [1]
mount -o degrade /dev/sdd /btrfs
-------

[1] github.com/anajain/devmgt.git

Signed-off-by: Anand Jain <Anand.Jain@oracle.com>
Tested-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 8 insertions, 2 deletions

diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index 782374d8fd19..865f4cf9a769 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -578,8 +578,14 @@ static int add_device_membership(struct btrfs_fs_info *fs_info)
                 return -ENOMEM;
 
         list_for_each_entry(dev, &fs_devices->devices, dev_list) {
-                struct hd_struct *disk = dev->bdev->bd_part;
-                struct kobject *disk_kobj = &part_to_dev(disk)->kobj;
+                struct hd_struct *disk;
+                struct kobject *disk_kobj;
+
+                if (!dev->bdev)
+                        continue;
+
+                disk = dev->bdev->bd_part;
+                disk_kobj = &part_to_dev(disk)->kobj;
 
                 error = sysfs_create_link(fs_info->device_dir_kobj,
                                           disk_kobj, disk_kobj->name);