netdev - Re: [PATCH bpf-next v2 2/4] bpf: support cloning sk storage on accept()

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Date:   Mon, 12 Aug 2019 12:17:16 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Stanislav Fomichev <sdf@...gle.com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     davem@...emloft.net, ast@...nel.org,
        Martin KaFai Lau <kafai@...com>, Yonghong Song <yhs@...com>
Subject: Re: [PATCH bpf-next v2 2/4] bpf: support cloning sk storage on
 accept()

On 8/9/19 6:10 PM, Stanislav Fomichev wrote:
> Add new helper bpf_sk_storage_clone which optionally clones sk storage
> and call it from sk_clone_lock.
> 
> Cc: Martin KaFai Lau <kafai@...com>
> Cc: Yonghong Song <yhs@...com>
> Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
[...]
> +int bpf_sk_storage_clone(const struct sock *sk, struct sock *newsk)
> +{
> +	struct bpf_sk_storage *new_sk_storage = NULL;
> +	struct bpf_sk_storage *sk_storage;
> +	struct bpf_sk_storage_elem *selem;
> +	int ret;
> +
> +	RCU_INIT_POINTER(newsk->sk_bpf_storage, NULL);
> +
> +	rcu_read_lock();
> +	sk_storage = rcu_dereference(sk->sk_bpf_storage);
> +
> +	if (!sk_storage || hlist_empty(&sk_storage->list))
> +		goto out;
> +
> +	hlist_for_each_entry_rcu(selem, &sk_storage->list, snode) {
> +		struct bpf_sk_storage_elem *copy_selem;
> +		struct bpf_sk_storage_map *smap;
> +		struct bpf_map *map;
> +		int refold;
> +
> +		smap = rcu_dereference(SDATA(selem)->smap);
> +		if (!(smap->map.map_flags & BPF_F_CLONE))
> +			continue;
> +
> +		map = bpf_map_inc_not_zero(&smap->map, false);
> +		if (IS_ERR(map))
> +			continue;
> +
> +		copy_selem = bpf_sk_storage_clone_elem(newsk, smap, selem);
> +		if (!copy_selem) {
> +			ret = -ENOMEM;
> +			bpf_map_put(map);
> +			goto err;
> +		}
> +
> +		if (new_sk_storage) {
> +			selem_link_map(smap, copy_selem);
> +			__selem_link_sk(new_sk_storage, copy_selem);
> +		} else {
> +			ret = sk_storage_alloc(newsk, smap, copy_selem);
> +			if (ret) {
> +				kfree(copy_selem);
> +				atomic_sub(smap->elem_size,
> +					   &newsk->sk_omem_alloc);
> +				bpf_map_put(map);
> +				goto err;
> +			}
> +
> +			new_sk_storage = rcu_dereference(copy_selem->sk_storage);
> +		}
> +		bpf_map_put(map);

The map get/put combination /under/ RCU read lock seems a bit odd to me, could
you exactly describe the race that this would be preventing?

> +	}
> +
> +out:
> +	rcu_read_unlock();
> +	return 0;
> +
> +err:
> +	rcu_read_unlock();
> +
> +	bpf_sk_storage_free(newsk);
> +	return ret;
> +}
Thanks,
Daniel