Re: [PATCH bpf-next v5 1/2] bpf: support BPF_PROG_QUERY for progs attached to sockmap

[Jakub Sitnicki]

On Thu, Jan 13, 2022 at 10:00 AM CET, Di Zhu wrote:
> Right now there is no way to query whether BPF programs are
> attached to a sockmap or not.
>
> we can use the standard interface in libbpf to query, such as:
> bpf_prog_query(mapFd, BPF_SK_SKB_STREAM_PARSER, 0, NULL, ...);
> the mapFd is the fd of sockmap.
>
> Signed-off-by: Di Zhu <zhudi2@xxxxxxxxxx>
> Acked-by: Yonghong Song <yhs@xxxxxx>
> ---
>  include/linux/bpf.h  |  9 +++++
>  kernel/bpf/syscall.c |  5 +++
>  net/core/sock_map.c  | 78 ++++++++++++++++++++++++++++++++++++++++----
>  3 files changed, 85 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 6e947cd91152..c4ca14c9f838 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -2071,6 +2071,9 @@ int bpf_prog_test_run_syscall(struct bpf_prog *prog,
>  int sock_map_get_from_fd(const union bpf_attr *attr, struct bpf_prog *prog);
>  int sock_map_prog_detach(const union bpf_attr *attr, enum bpf_prog_type ptype);
>  int sock_map_update_elem_sys(struct bpf_map *map, void *key, void *value, u64 flags);
> +int sock_map_bpf_prog_query(const union bpf_attr *attr,
> +			    union bpf_attr __user *uattr);
> +
>  void sock_map_unhash(struct sock *sk);
>  void sock_map_close(struct sock *sk, long timeout);
>  #else
> @@ -2124,6 +2127,12 @@ static inline int sock_map_update_elem_sys(struct bpf_map *map, void *key, void
>  {
>  	return -EOPNOTSUPP;
>  }
> +
> +static inline int sock_map_bpf_prog_query(const union bpf_attr *attr,
> +					  union bpf_attr __user *uattr)
> +{
> +	return -EINVAL;
> +}
>  #endif /* CONFIG_BPF_SYSCALL */
>  #endif /* CONFIG_NET && CONFIG_BPF_SYSCALL */
>
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index fa4505f9b611..9e0631f091a6 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -3318,6 +3318,11 @@ static int bpf_prog_query(const union bpf_attr *attr,
>  	case BPF_FLOW_DISSECTOR:
>  	case BPF_SK_LOOKUP:
>  		return netns_bpf_prog_query(attr, uattr);
> +	case BPF_SK_SKB_STREAM_PARSER:
> +	case BPF_SK_SKB_STREAM_VERDICT:
> +	case BPF_SK_MSG_VERDICT:
> +	case BPF_SK_SKB_VERDICT:
> +		return sock_map_bpf_prog_query(attr, uattr);
>  	default:
>  		return -EINVAL;
>  	}
> diff --git a/net/core/sock_map.c b/net/core/sock_map.c
> index 1827669eedd6..a424f51041ca 100644
> --- a/net/core/sock_map.c
> +++ b/net/core/sock_map.c
> @@ -1416,38 +1416,50 @@ static struct sk_psock_progs *sock_map_progs(struct bpf_map *map)
>  	return NULL;
>  }
>
> -static int sock_map_prog_update(struct bpf_map *map, struct bpf_prog *prog,
> -				struct bpf_prog *old, u32 which)
> +static int sock_map_prog_lookup(struct bpf_map *map, struct bpf_prog ***pprog,
> +				u32 which)
>  {
>  	struct sk_psock_progs *progs = sock_map_progs(map);
> -	struct bpf_prog **pprog;
>
>  	if (!progs)
>  		return -EOPNOTSUPP;
>
>  	switch (which) {
>  	case BPF_SK_MSG_VERDICT:
> -		pprog = &progs->msg_parser;
> +		*pprog = &progs->msg_parser;
>  		break;
>  #if IS_ENABLED(CONFIG_BPF_STREAM_PARSER)
>  	case BPF_SK_SKB_STREAM_PARSER:
> -		pprog = &progs->stream_parser;
> +		*pprog = &progs->stream_parser;
>  		break;
>  #endif
>  	case BPF_SK_SKB_STREAM_VERDICT:
>  		if (progs->skb_verdict)
>  			return -EBUSY;
> -		pprog = &progs->stream_verdict;
> +		*pprog = &progs->stream_verdict;
>  		break;
>  	case BPF_SK_SKB_VERDICT:
>  		if (progs->stream_verdict)
>  			return -EBUSY;
> -		pprog = &progs->skb_verdict;
> +		*pprog = &progs->skb_verdict;
>  		break;
>  	default:
>  		return -EOPNOTSUPP;
>  	}
>
> +	return 0;
> +}
> +
> +static int sock_map_prog_update(struct bpf_map *map, struct bpf_prog *prog,
> +				struct bpf_prog *old, u32 which)
> +{
> +	struct bpf_prog **pprog;
> +	int ret;
> +
> +	ret = sock_map_prog_lookup(map, &pprog, which);
> +	if (ret)
> +		return ret;
> +
>  	if (old)
>  		return psock_replace_prog(pprog, prog, old);
>
> @@ -1455,6 +1467,58 @@ static int sock_map_prog_update(struct bpf_map *map, struct bpf_prog *prog,
>  	return 0;
>  }
>
> +int sock_map_bpf_prog_query(const union bpf_attr *attr,
> +			    union bpf_attr __user *uattr)
> +{
> +	__u32 __user *prog_ids = u64_to_user_ptr(attr->query.prog_ids);
> +	u32 prog_cnt = 0, flags = 0, ufd = attr->target_fd;
> +	struct bpf_prog **pprog;
> +	struct bpf_prog *prog;
> +	struct bpf_map *map;
> +	struct fd f;
> +	u32 id = 0;
> +	int ret;
> +
> +	if (attr->query.query_flags)
> +		return -EINVAL;
> +
> +	f = fdget(ufd);
> +	map = __bpf_map_get(f);
> +	if (IS_ERR(map))
> +		return PTR_ERR(map);
> +
> +	rcu_read_lock();
> +
> +	ret = sock_map_prog_lookup(map, &pprog, attr->query.attach_type);
> +	if (ret)
> +		goto end;
> +
> +	prog = *pprog;
> +	prog_cnt = !prog ? 0 : 1;
> +
> +	if (!attr->query.prog_cnt || !prog_ids || !prog_cnt)
> +		goto end;
> +
> +	id = prog->aux->id;

^ This looks like a concurrent read/write.

Would wrap with READ_ONCE() and corresponding WRITE_ONCE() in
bpf_prog_free_id(). See [1] for rationale.

[1] https://github.com/google/kernel-sanitizers/blob/master/other/READ_WRITE_ONCE.md

> +
> +	/* we do not hold the refcnt, the bpf prog may be released
> +	 * asynchronously and the id would be set to 0.
> +	 */
> +	if (id == 0)
> +		prog_cnt = 0;
> +
> +end:
> +	rcu_read_unlock();
> +
> +	if (copy_to_user(&uattr->query.attach_flags, &flags, sizeof(flags)) ||
> +	    (id != 0 && copy_to_user(prog_ids, &id, sizeof(u32))) ||
> +	    copy_to_user(&uattr->query.prog_cnt, &prog_cnt, sizeof(prog_cnt)))
> +		ret = -EFAULT;
> +
> +	fdput(f);
> +	return ret;
> +}
> +
>  static void sock_map_unlink(struct sock *sk, struct sk_psock_link *link)
>  {
>  	switch (link->map->map_type) {