Re: [PATCH] btrfs: Fix btrfs_block_group refcnt leak

From: David Sterba
Date: Mon Apr 20 2020 - 18:44:01 EST


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@xxxxxxxxxxxx>
> Signed-off-by: Xin Tan <tanxin.ctf@xxxxxxxxx>

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.