linux-kernel - Re: [PATCH] btrfs: Fix btrfs_block

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Date:   Tue, 21 Apr 2020 17:53:37 +0800
From:   Xiyu Yang <xiyuyang19@...an.edu.cn>
To:     David Sterba <dsterba@...e.cz>
Cc:     Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, yuanxzhang@...an.edu.cn,
        kjlu@....edu, Xin Tan <tanxin.ctf@...il.com>
Subject: Re: [PATCH] btrfs: Fix btrfs_block_group refcnt leak

On Tue, Apr 21, 2020 at 12:43:15AM +0200, David Sterba wrote:
> On Mon, Apr 20, 2020 at 01:38:40PM +0800, Xiyu Yang wrote:
> > btrfs_remove_block_group() invokes btrfs_lookup_block_group(), which
> > returns a local reference of the blcok group that contains the given
> > bytenr to "block_group" with increased refcount.
> > 
> > When btrfs_remove_block_group() returns, "block_group" becomes invalid,
> > so the refcount should be decreased to keep refcount balanced.
> > 
> > The reference counting issue happens in several exception handling paths
> > of btrfs_remove_block_group(). When those error scenarios occur such as
> > btrfs_alloc_path() returns NULL, the function forgets to decrease its
> > refcnt increased by btrfs_lookup_block_group() and will cause a refcnt
> > leak.
> > 
> > Fix this issue by jumping to "out_put_group" label and calling
> > btrfs_put_block_group() when those error scenarios occur.
> > 
> > Signed-off-by: Xiyu Yang <xiyuyang19@...an.edu.cn>
> > Signed-off-by: Xin Tan <tanxin.ctf@...il.com>
> 
> Thanks for the fix. May I ask if this was found by code inspection or by
> some analysis tool?

Thanks for your advice about the patch! We are looking for some automated ways
to find this kind of bug.