Re: [PATCH] btrfs: tests: Fix memory leak in btrfs_test_qgroups()

[Qu Wenruo]

在 2025/12/25 20:57, Zilin Guan 写道:
> btrfs_alloc_dummy_root() allocates a root with a reference count of 1.
> Then btrfs_insert_fs_root() is used to insert the root into the fs_info.
> On success, it increments the reference count. On failure, it does not.
>
> Currently, if btrfs_insert_fs_root() fails, the error handling path
> jumps to the out label immediately without decrementing the reference
> count of tmp_root, leading to a memory leak.
>
> Fix this by calling btrfs_put_root() unconditionally after
> btrfs_insert_fs_root(). This correctly handles both cases: on success,
> it drops the local reference, leaving the root with the reference held
> by fs_info; on failure, it drops the sole reference, freeing the root.
>
> Fixes: 4785e24fa5d23 ("btrfs: don't take an extra root ref at allocation time")
> Signed-off-by: Zilin Guan <zilin@xxxxxxxxxx>
> ---
>   fs/btrfs/tests/qgroup-tests.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/fs/btrfs/tests/qgroup-tests.c b/fs/btrfs/tests/qgroup-tests.c
> index e9124605974b..0d51e0abaeac 100644
> --- a/fs/btrfs/tests/qgroup-tests.c
> +++ b/fs/btrfs/tests/qgroup-tests.c
> @@ -517,11 +517,11 @@ int btrfs_test_qgroups(u32 sectorsize, u32 nodesize)
>   	tmp_root->root_key.objectid = BTRFS_FS_TREE_OBJECTID;
>   	root->fs_info->fs_root = tmp_root;
>   	ret = btrfs_insert_fs_root(root->fs_info, tmp_root);
> +	btrfs_put_root(tmp_root);
>   	if (ret) {
>   		test_err("couldn't insert fs root %d", ret);
>   		goto out;

This will lead to double free.

If btrfs_insert_fs_root() failed, btrfs_put_root() will do the cleaning 
and free the root.

Then btrfs_free_dummy_root() will call btrfs_put_root() again on the 
root, cause use-after-free.

So your analyze is completely wrong.

Thanks,
Qu

>   	}
> -	btrfs_put_root(tmp_root);
>   
>   	tmp_root = btrfs_alloc_dummy_root(fs_info);
>   	if (IS_ERR(tmp_root)) {
> @@ -532,11 +532,11 @@ int btrfs_test_qgroups(u32 sectorsize, u32 nodesize)
>   
>   	tmp_root->root_key.objectid = BTRFS_FIRST_FREE_OBJECTID;
>   	ret = btrfs_insert_fs_root(root->fs_info, tmp_root);
> +	btrfs_put_root(tmp_root);
>   	if (ret) {
>   		test_err("couldn't insert fs root %d", ret);
>   		goto out;
>   	}
> -	btrfs_put_root(tmp_root);
>   
>   	test_msg("running qgroup tests");
>   	ret = test_no_shared_qgroup(root, sectorsize, nodesize);