Re: [PATCH] Btrfs: fix race between enabling quotas and subvolume creation

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2018/11/20 上午12:20, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> We have a race between enabling quotas end subvolume creation that cause
> subvolume creation to fail with -EINVAL, and the following diagram shows
> how it happens:
> 
>               CPU 0                                          CPU 1
> 
>  btrfs_ioctl()
>   btrfs_ioctl_quota_ctl()
>    btrfs_quota_enable()
>     mutex_lock(fs_info->qgroup_ioctl_lock)
> 
>                                                   btrfs_ioctl()
>                                                    create_subvol()
>                                                     btrfs_qgroup_inherit()
>                                                      -> save fs_info->quota_root
>                                                         into quota_root
>                                                      -> stores a NULL value
>                                                      -> tries to lock the mutex
>                                                         qgroup_ioctl_lock
>                                                         -> blocks waiting for
>                                                            the task at CPU0
> 
>    -> sets BTRFS_FS_QUOTA_ENABLED in fs_info
>    -> sets quota_root in fs_info->quota_root
>       (non-NULL value)
> 
>    mutex_unlock(fs_info->qgroup_ioctl_lock)
> 
>                                                      -> checks quota enabled
>                                                         flag is set
>                                                      -> returns -EINVAL because
>                                                         fs_info->quota_root was
>                                                         NULL before it acquired
>                                                         the mutex
>                                                         qgroup_ioctl_lock
>                                                    -> ioctl returns -EINVAL
> 
> Returning -EINVAL to user space will be confusing if all the arguments
> passed to the subvolume creation ioctl were valid.
> 
> Fix it by grabbing the value from fs_info->quota_root after acquiring
> the mutex.
> 
> Signed-off-by: Filipe Manana <fdmanana@suse.com>

Reviewed-by: Qu Wenruo <wqu@suse.com>

Thanks,
Qu

> ---
>  fs/btrfs/qgroup.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
> index ae1358253b7b..0bdf28499790 100644
> --- a/fs/btrfs/qgroup.c
> +++ b/fs/btrfs/qgroup.c
> @@ -2250,7 +2250,7 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans, u64 srcid,
>  	int i;
>  	u64 *i_qgroups;
>  	struct btrfs_fs_info *fs_info = trans->fs_info;
> -	struct btrfs_root *quota_root = fs_info->quota_root;
> +	struct btrfs_root *quota_root;
>  	struct btrfs_qgroup *srcgroup;
>  	struct btrfs_qgroup *dstgroup;
>  	u32 level_size = 0;
> @@ -2260,6 +2260,7 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans, u64 srcid,
>  	if (!test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags))
>  		goto out;
>  
> +	quota_root = fs_info->quota_root;
>  	if (!quota_root) {
>  		ret = -EINVAL;
>  		goto out;
> 


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