[PATCH 2/4] btrfs: stop using uninitiazlised fs_info in device_list_add()

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

fs_info is born during mount, and operations before the mount such as
scanning and assembling of the device volume should happen without any
reference to fs_info.

However the patch commit a9261d4125c9 (btrfs: harden agaist duplicate
fsid on scanned devices) used fs_info to call btrfs_warn_in_rcu() and
btrfs_info_in_rcu(), so if fs_info is NULL, the stacked functions which
leads to btrfs_printk() which shall print "unknown" instead of sb->s_id.
Or even might UAF as reported in [1].

So do the right thing, don't use fs_info instead use NO_FS_INFO in
btrfs_warn_in_rcu() and btrfs_info_in_rcu() for the btrfs_printk()
to take care of it properly.

Link:
 [1] https://www.spinics.net/lists/linux-btrfs/msg96524.html
Fixes: a9261d4125c9 (btrfs: harden agaist duplicate fsid on scanned devices)
Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
 fs/btrfs/volumes.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 6fd90270e2c7..0301c3d693d8 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -889,14 +889,14 @@ static noinline struct btrfs_device *device_list_add(const char *path,
 			if (device->bdev != path_bdev) {
 				bdput(path_bdev);
 				mutex_unlock(&fs_devices->device_list_mutex);
-				btrfs_warn_in_rcu(device->fs_info,
+				btrfs_warn_in_rcu(NO_FS_INFO,
 			"duplicate device fsid:devid for %pU:%llu old:%s new:%s",
 					disk_super->fsid, devid,
 					rcu_str_deref(device->name), path);
 				return ERR_PTR(-EEXIST);
 			}
 			bdput(path_bdev);
-			btrfs_info_in_rcu(device->fs_info,
+			btrfs_info_in_rcu(NO_FS_INFO,
 				"device fsid %pU devid %llu moved old:%s new:%s",
 				disk_super->fsid, devid,
 				rcu_str_deref(device->name), path);
-- 
2.23.0