On Thu, Jul 15, 2021 at 09:11:43PM +0800, Desmond Cheong Zhi Xi wrote:

On 15/7/21 7:55 pm, Nikolay Borisov wrote:

quoted

On 15.07.21 г. 13:34, Desmond Cheong Zhi Xi wrote:

quoted

Syzbot reports a warning in close_fs_devices that happens because
fs_devices->rw_devices is not 0 after calling btrfs_close_one_device
on each device.

This happens when a writeable device is removed in
__btrfs_free_extra_devids, but the rw device count is not decremented
accordingly. So when close_fs_devices is called, the removed device is
still counted and we get an off by 1 error.

Here is one call trace that was observed:
   btrfs_mount_root():
     btrfs_scan_one_device():
       device_list_add();   <---------------- device added
     btrfs_open_devices():
       open_fs_devices():
         btrfs_open_one_device();   <-------- rw device count ++
     btrfs_fill_super():
       open_ctree():
         btrfs_free_extra_devids():
	  __btrfs_free_extra_devids();  <--- device removed
	  fail_tree_roots:
	    btrfs_close_devices():
	      close_fs_devices();   <------- rw device count off by 1

Fixes: cf89af146b7e ("btrfs: dev-replace: fail mount if we don't have replace item with target device")
Reported-by: syzbot+a70e2ad0879f160b9217@syzkaller.appspotmail.com
Tested-by: syzbot+a70e2ad0879f160b9217@syzkaller.appspotmail.com
Signed-off-by: Desmond Cheong Zhi Xi <redacted>

Is there a reliable reproducer from syzbot? Can this be turned into an
xfstest?

Syzbot has some reliable reproducers here:
https://syzkaller.appspot.com/bug?id=113d9a01cbe0af3e291633ba7a7a3e983b86c3c0

Seems like it constructs two images in-memory then mounts them. I'm not 
sure if that's amenable to be converted into an xfstest?

It would need to be an image from the time the warning is reproduced,
I'm not sure how much timing is also important. But iirc adding raw test
images to fstests was not welcome, so it would have to be a reproducer
and given that the syzkaller source is not human readable I'm not sure
it would be welcome either.

Maybe there's some middle ground when the image is created by mkfs and
filled with the data and then the mount loop is started from shell. But
that means to untangle the C reproducer.