Re: [PATCH] btrfs: fix rw device counting in __btrfs_free_extra_devids

[Desmond Cheong Zhi Xi]

On 15/7/21 7:55 pm, Nikolay Borisov wrote:
> On 15.07.21 г. 13:34, Desmond Cheong Zhi Xi wrote:
> > 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Tested-by: syzbot+a70e2ad0879f160b9217@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
>
> 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?

> > ---
> >   fs/btrfs/volumes.c | 1 +
> >   1 file changed, 1 insertion(+)
> >
> > diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> > index 807502cd6510..916c25371658 100644
> > --- a/fs/btrfs/volumes.c
> > +++ b/fs/btrfs/volumes.c
> > @@ -1078,6 +1078,7 @@ static void __btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices,
> >   		if (test_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state)) {
> >   			list_del_init(&device->dev_alloc_list);
> >   			clear_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state);
> > +			fs_devices->rw_devices--;
> >   		}
> >   		list_del_init(&device->dev_list);
> >   		fs_devices->num_devices--;