Re: [PATCH 1/3] Btrfs: fix a block group ref counter leak after failure to remove block group

[Filipe Manana <fdmanana@kernel.org>]

On Wed, Jun 3, 2020 at 8:32 AM Nikolay Borisov <nborisov@suse.com> wrote:
>
>
>
> On 1.06.20 г. 21:12 ч., fdmanana@kernel.org wrote:
> > From: Filipe Manana <fdmanana@suse.com>
> >
> > When removing a block group, if we fail to delete the block group's item
> > from the extent tree, we jump to the 'out' label and end up decrementing
> > the block group's reference count once only (by 1), resulting in a counter
> > leak because the block group at that point was already removed from the
> > block group cache rbtree - so we have to decrement the reference count
> > twice, once for the rbtree and once for our lookup at the start of the
> > function.
>
> However I'm having hard time reconciling this. The block group is
> removed from the block_group_cache_tree after we've called
> btrfs_del_item. So if btrfs_del_item or btrfs_search_slot fail the code
> jumps at out_put_group and puts the reference acquired at the beginning
> of the function via btrfs_lookup_block_group.
>
> I think what you meant is if we fail to delete the block group's item
> from the freespace tree, that is if we fail
> remove_block_group_free_space, then we'd have a ref leak.

What I meant is exactly what I wrote:
if we fail to delete the block group's item from the extent tree (the
call to remove_block_group_item()),
we end up decrementing the reference count only once because we jump
to the out label - but we
should have decremented it twice, once for the rbtree removal, which
happened before, and once for
the lookup at the start of the function.

Thanks.

> With this
> modification to the changelog:
>
> Reviewed-by: Nikolay Borisov <nborisov@suse.com>
>
> >
> > To make things less error prone, decrement the reference count for the
> > rbtree immediately after removing the block group from it. This also
> > eleminates the need for two different exit labels on error, renaming
> > 'out_put_label' to just 'out' and removing the old 'out'.
>
> I agree with this.
>
> >
> > Fixes: f6033c5e333238 ("btrfs: fix block group leak when removing fails")
> > Signed-off-by: Filipe Manana <fdmanana@suse.com>
>
> <snip>