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

From: Nikolay Borisov <nborisov@suse.com>
To: fdmanana@kernel.org, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH 1/3] Btrfs: fix a block group ref counter leak after failure to remove block group
Date: Wed, 3 Jun 2020 10:44:55 +0300	[thread overview]
Message-ID: <207d40cc-051c-4c4a-62c8-3343e53294ad@suse.com> (raw)
In-Reply-To: <0b010680-f286-f858-8cfd-94dc888ca7a4@suse.com>

On 3.06.20 г. 10:32 ч., Nikolay Borisov 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. With this
> modification to the changelog:
> 

Looking again in this function without this patch the sequence of
remove_block_group_free_space/btrfs_put_block-group/remove_block_group_item
is really bogus.

1. If remove_block_group_free_space fails the code would jump to
out_put_group which would leak the ref count for the rb tree

2. If remove_block_group_item (removal from extent tree) fails then the
code would jump to out: which won't drop the reference taken in
btrfs_remove_block_group...

> 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>
> 