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

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Date:   Tue, 21 Apr 2020 00:43:15 +0200
From:   David Sterba <dsterba@...e.cz>
To:     Xiyu Yang <xiyuyang19@...an.edu.cn>
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 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?

> @@ -1132,6 +1132,9 @@ int btrfs_remove_block_group(struct btrfs_trans_handle *trans,
>  		btrfs_delayed_refs_rsv_release(fs_info, 1);
>  	btrfs_free_path(path);
>  	return ret;
> +out_put_group:
> +	btrfs_put_block_group(block_group);
> +	goto out;

As Filipe noted, the trailing gotos are not a great pattern, what we'd
like to do is a more linear sequence where the resource
allocation/freeing nesting is obvious, like:


	x = allocate(X);
	if (!x)
		goto out;
	...
	y = allocate(Y);
	if (!y)
		goto out_free_x;
	z = allocate(Z);
	if (!z)
		goto out_free_y;
	...
	free(z);
out_free_y:
	free(y);
out_free_x:
	free(x);
out:
	return;


(where allocate/free can be refcount inc/dec and similar). Sometimes
it's not that straightforward and the freeing block needs conditionals,
but from code reading perspective this is still better than potentially
wild gotos.