[PATCH v2] btrfs: fix null pointer deref when target device is missing

[Anand Jain <anand.jain@oracle.com>]

The replace target device can be missing when mounted with -o degraded,
but we wont allocate a missing btrfs_device to it. So check the device
before access.

BUG: unable to handle kernel NULL pointer dereference at 00000000000000b0
IP: btrfs_destroy_dev_replace_tgtdev+0x43/0xf0 [btrfs]
Call Trace:
btrfs_dev_replace_cancel+0x15f/0x180 [btrfs]
btrfs_ioctl+0x2216/0x2590 [btrfs]
do_vfs_ioctl+0x625/0x650
SyS_ioctl+0x4e/0x80
do_syscall_64+0x5d/0x160
entry_SYSCALL64_slow_path+0x25/0x25

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
v1->v2: Fix change log. Fix $subject.
  Old $subject
    btrfs: fix NPD when target device is missing

 fs/btrfs/dev-replace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c
index c97969b2abbd..e279f04b3388 100644
--- a/fs/btrfs/dev-replace.c
+++ b/fs/btrfs/dev-replace.c
@@ -312,7 +312,7 @@ void btrfs_after_dev_replace_commit(struct btrfs_fs_info *fs_info)
 
 static char* btrfs_dev_name(struct btrfs_device *device)
 {
-	if (test_bit(BTRFS_DEV_STATE_MISSING, &device->dev_state))
+	if (!device || test_bit(BTRFS_DEV_STATE_MISSING, &device->dev_state))
 		return "<missing disk>";
 	else
 		return rcu_str_deref(device->name);
-- 
2.15.0