Date:   Thu, 10 May 2018 22:00:24 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Joe Stringer <joe@...d.net.nz>
CC:     <daniel@...earbox.net>, <netdev@...r.kernel.org>, <ast@...nel.org>,
        <john.fastabend@...il.com>
Subject: Re: [RFC bpf-next 07/11] bpf: Add helper to retrieve socket in BPF

On Wed, May 09, 2018 at 02:07:05PM -0700, Joe Stringer wrote:
> This patch adds a new BPF helper function, sk_lookup() which allows BPF
> programs to find out if there is a socket listening on this host, and
> returns a socket pointer which the BPF program can then access to
> determine, for instance, whether to forward or drop traffic. sk_lookup()
> takes a reference on the socket, so when a BPF program makes use of this
> function, it must subsequently pass the returned pointer into the newly
> added sk_release() to return the reference.
> 
> By way of example, the following pseudocode would filter inbound
> connections at XDP if there is no corresponding service listening for
> the traffic:
> 
>   struct bpf_sock_tuple tuple;
>   struct bpf_sock_ops *sk;
> 
>   populate_tuple(ctx, &tuple); // Extract the 5tuple from the packet
>   sk = bpf_sk_lookup(ctx, &tuple, sizeof tuple, netns, 0);
>   if (!sk) {
>     // Couldn't find a socket listening for this traffic. Drop.
>     return TC_ACT_SHOT;
>   }
>   bpf_sk_release(sk, 0);
>   return TC_ACT_OK;
> 
> Signed-off-by: Joe Stringer <joe@...d.net.nz>
> ---
>  include/uapi/linux/bpf.h                  |  39 +++++++++++-
>  kernel/bpf/verifier.c                     |   8 ++-
>  net/core/filter.c                         | 102 ++++++++++++++++++++++++++++++
>  tools/include/uapi/linux/bpf.h            |  40 +++++++++++-
>  tools/testing/selftests/bpf/bpf_helpers.h |   7 ++
>  5 files changed, 193 insertions(+), 3 deletions(-)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index d615c777b573..29f38838dbca 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -1828,6 +1828,25 @@ union bpf_attr {
>   * 	Return
>   * 		0 on success, or a negative error in case of failure.
>   *
> + * struct bpf_sock_ops *bpf_sk_lookup(ctx, tuple, tuple_size, netns, flags)
> + * 	Decription
> + * 		Look for socket matching 'tuple'. The return value must be checked,
> + * 		and if non-NULL, released via bpf_sk_release().
> + * 		@ctx: pointer to ctx
> + * 		@tuple: pointer to struct bpf_sock_tuple
> + * 		@tuple_size: size of the tuple
> + * 		@flags: flags value
> + * 	Return
> + * 		pointer to socket ops on success, or
> + * 		NULL in case of failure
> + *
> + *  int bpf_sk_release(sock, flags)
> + * 	Description
> + * 		Release the reference held by 'sock'.
> + * 		@sock: Pointer reference to release. Must be found via bpf_sk_lookup().
> + * 		@flags: flags value
> + * 	Return
> + * 		0 on success, or a negative error in case of failure.
>   */
>  #define __BPF_FUNC_MAPPER(FN)		\
>  	FN(unspec),			\
> @@ -1898,7 +1917,9 @@ union bpf_attr {
>  	FN(xdp_adjust_tail),		\
>  	FN(skb_get_xfrm_state),		\
>  	FN(get_stack),			\
> -	FN(skb_load_bytes_relative),
> +	FN(skb_load_bytes_relative),	\
> +	FN(sk_lookup),			\
> +	FN(sk_release),
>  
>  /* integer value in 'imm' field of BPF_CALL instruction selects which helper
>   * function eBPF program intends to call
> @@ -2060,6 +2081,22 @@ struct bpf_sock {
>  				 */
>  };
>  
> +struct bpf_sock_tuple {
> +	union {
> +		__be32 ipv6[4];
> +		__be32 ipv4;
> +	} saddr;
> +	union {
> +		__be32 ipv6[4];
> +		__be32 ipv4;
> +	} daddr;
> +	__be16 sport;
> +	__be16 dport;
> +	__u32 dst_if;
> +	__u8 family;
> +	__u8 proto;
> +};
> +
>  #define XDP_PACKET_HEADROOM 256
>  
>  /* User return codes for XDP prog type.
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 92b9a5dc465a..579012c483e4 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -153,6 +153,12 @@ static const struct bpf_verifier_ops * const bpf_verifier_ops[] = {
>   * PTR_TO_MAP_VALUE, PTR_TO_SOCKET_OR_NULL becomes PTR_TO_SOCKET when the type
>   * passes through a NULL-check conditional. For the branch wherein the state is
>   * changed to CONST_IMM, the verifier releases the reference.
> + *
> + * For each helper function that allocates a reference, such as bpf_sk_lookup(),
> + * there is a corresponding release function, such as bpf_sk_release(). When
> + * a reference type passes into the release function, the verifier also releases
> + * the reference. If any unchecked or unreleased reference remains at the end of
> + * the program, the verifier rejects it.
>   */
>  
>  /* verifier_state + insn_idx are pushed to stack when branch is encountered */
> @@ -277,7 +283,7 @@ static bool arg_type_is_refcounted(enum bpf_arg_type type)
>   */
>  static bool is_release_function(enum bpf_func_id func_id)
>  {
> -	return false;
> +	return func_id == BPF_FUNC_sk_release;
>  }
>  
>  /* string representation of 'enum bpf_reg_type' */
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 4c35152fb3a8..751c255d17d3 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -58,8 +58,12 @@
>  #include <net/busy_poll.h>
>  #include <net/tcp.h>
>  #include <net/xfrm.h>
> +#include <net/udp.h>
>  #include <linux/bpf_trace.h>
>  #include <net/xdp_sock.h>
> +#include <net/inet_hashtables.h>
> +#include <net/inet6_hashtables.h>
> +#include <net/net_namespace.h>
>  
>  /**
>   *	sk_filter_trim_cap - run a packet through a socket filter
> @@ -4032,6 +4036,96 @@ static const struct bpf_func_proto bpf_skb_get_xfrm_state_proto = {
>  };
>  #endif
>  
> +struct sock *
> +sk_lookup(struct net *net, struct bpf_sock_tuple *tuple) {
Would it be possible to have another version that
returns a sk without taking its refcnt?
It may have performance benefit.

> +	int dst_if = (int)tuple->dst_if;
> +	struct in6_addr *src6;
> +	struct in6_addr *dst6;
> +
> +	if (tuple->family == AF_INET6) {
> +		src6 = (struct in6_addr *)&tuple->saddr.ipv6;
> +		dst6 = (struct in6_addr *)&tuple->daddr.ipv6;
> +	} else if (tuple->family != AF_INET) {
> +		return ERR_PTR(-EOPNOTSUPP);
> +	}
> +
> +	if (tuple->proto == IPPROTO_TCP) {
> +		if (tuple->family == AF_INET)
> +			return inet_lookup(net, &tcp_hashinfo, NULL, 0,
> +					   tuple->saddr.ipv4, tuple->sport,
> +					   tuple->daddr.ipv4, tuple->dport,
> +					   dst_if);
> +		else
> +			return inet6_lookup(net, &tcp_hashinfo, NULL, 0,
> +					    src6, tuple->sport,
> +					    dst6, tuple->dport, dst_if);
> +	} else if (tuple->proto == IPPROTO_UDP) {
> +		if (tuple->family == AF_INET)
> +			return udp4_lib_lookup(net, tuple->saddr.ipv4,
> +					       tuple->sport, tuple->daddr.ipv4,
> +					       tuple->dport, dst_if);
> +		else
> +			return udp6_lib_lookup(net, src6, tuple->sport,
> +					       dst6, tuple->dport, dst_if);
> +	} else {
> +		return ERR_PTR(-EOPNOTSUPP);
> +	}
> +
> +	return NULL;
> +}
> +
> +BPF_CALL_5(bpf_sk_lookup, struct sk_buff *, skb,
> +	   struct bpf_sock_tuple *, tuple, u32, len, u32, netns_id, u64, flags)
> +{
> +	struct net *caller_net = dev_net(skb->dev);
> +	struct sock *sk = NULL;
> +	struct net *net;
> +
> +	/* XXX: Perform verification-time checking of tuple size? */
> +	if (unlikely(len != sizeof(struct bpf_sock_tuple) || flags))
> +		goto out;
> +
> +	net = get_net_ns_by_id(caller_net, netns_id);
> +	if (unlikely(!net))
> +		goto out;
> +
> +	sk = sk_lookup(net, tuple);
> +	put_net(net);
> +	if (IS_ERR_OR_NULL(sk))
> +		sk = NULL;
> +	else
> +		sk = sk_to_full_sk(sk);
> +out:
> +	return (unsigned long) sk;
> +}
> +
> +static const struct bpf_func_proto bpf_sk_lookup_proto = {
> +	.func		= bpf_sk_lookup,
> +	.gpl_only	= false,
> +	.ret_type	= RET_PTR_TO_SOCKET_OR_NULL,
> +	.arg1_type	= ARG_PTR_TO_CTX,
> +	.arg2_type	= ARG_PTR_TO_MEM,
> +	.arg3_type	= ARG_CONST_SIZE,
> +	.arg4_type	= ARG_ANYTHING,
> +	.arg5_type	= ARG_ANYTHING,
> +};
> +
> +BPF_CALL_2(bpf_sk_release, struct sock *, sk, u64, flags)
> +{
> +	sock_gen_put(sk);
> +	if (unlikely(flags))
> +		return -EINVAL;
> +	return 0;
> +}
> +
> +static const struct bpf_func_proto bpf_sk_release_proto = {
> +	.func		= bpf_sk_release,
> +	.gpl_only	= false,
> +	.ret_type	= RET_INTEGER,
> +	.arg1_type	= ARG_PTR_TO_SOCKET,
> +	.arg2_type	= ARG_ANYTHING,
> +};
> +
>  static const struct bpf_func_proto *
>  bpf_base_func_proto(enum bpf_func_id func_id)
>  {
> @@ -4181,6 +4275,10 @@ tc_cls_act_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>  	case BPF_FUNC_skb_get_xfrm_state:
>  		return &bpf_skb_get_xfrm_state_proto;
>  #endif
> +	case BPF_FUNC_sk_lookup:
> +		return &bpf_sk_lookup_proto;
> +	case BPF_FUNC_sk_release:
> +		return &bpf_sk_release_proto;
>  	default:
>  		return bpf_base_func_proto(func_id);
>  	}
> @@ -4292,6 +4390,10 @@ sk_skb_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>  		return &bpf_get_socket_uid_proto;
>  	case BPF_FUNC_sk_redirect_map:
>  		return &bpf_sk_redirect_map_proto;
> +	case BPF_FUNC_sk_lookup:
> +		return &bpf_sk_lookup_proto;
> +	case BPF_FUNC_sk_release:
> +		return &bpf_sk_release_proto;
>  	default:
>  		return bpf_base_func_proto(func_id);
>  	}
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index fff51c187d1e..29f38838dbca 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -117,6 +117,7 @@ enum bpf_map_type {
>  	BPF_MAP_TYPE_DEVMAP,
>  	BPF_MAP_TYPE_SOCKMAP,
>  	BPF_MAP_TYPE_CPUMAP,
> +	BPF_MAP_TYPE_XSKMAP,
>  };
>  
>  enum bpf_prog_type {
> @@ -1827,6 +1828,25 @@ union bpf_attr {
>   * 	Return
>   * 		0 on success, or a negative error in case of failure.
>   *
> + * struct bpf_sock_ops *bpf_sk_lookup(ctx, tuple, tuple_size, netns, flags)
> + * 	Decription
> + * 		Look for socket matching 'tuple'. The return value must be checked,
> + * 		and if non-NULL, released via bpf_sk_release().
> + * 		@ctx: pointer to ctx
> + * 		@tuple: pointer to struct bpf_sock_tuple
> + * 		@tuple_size: size of the tuple
> + * 		@flags: flags value
> + * 	Return
> + * 		pointer to socket ops on success, or
> + * 		NULL in case of failure
> + *
> + *  int bpf_sk_release(sock, flags)
> + * 	Description
> + * 		Release the reference held by 'sock'.
> + * 		@sock: Pointer reference to release. Must be found via bpf_sk_lookup().
> + * 		@flags: flags value
> + * 	Return
> + * 		0 on success, or a negative error in case of failure.
>   */
>  #define __BPF_FUNC_MAPPER(FN)		\
>  	FN(unspec),			\
> @@ -1897,7 +1917,9 @@ union bpf_attr {
>  	FN(xdp_adjust_tail),		\
>  	FN(skb_get_xfrm_state),		\
>  	FN(get_stack),			\
> -	FN(skb_load_bytes_relative),
> +	FN(skb_load_bytes_relative),	\
> +	FN(sk_lookup),			\
> +	FN(sk_release),
>  
>  /* integer value in 'imm' field of BPF_CALL instruction selects which helper
>   * function eBPF program intends to call
> @@ -2059,6 +2081,22 @@ struct bpf_sock {
>  				 */
>  };
>  
> +struct bpf_sock_tuple {
> +	union {
> +		__be32 ipv6[4];
> +		__be32 ipv4;
> +	} saddr;
> +	union {
> +		__be32 ipv6[4];
> +		__be32 ipv4;
> +	} daddr;
> +	__be16 sport;
> +	__be16 dport;
> +	__u32 dst_if;
> +	__u8 family;
> +	__u8 proto;
> +};
> +
>  #define XDP_PACKET_HEADROOM 256
>  
>  /* User return codes for XDP prog type.
> diff --git a/tools/testing/selftests/bpf/bpf_helpers.h b/tools/testing/selftests/bpf/bpf_helpers.h
> index 265f8e0e8ada..4dc311ea0c16 100644
> --- a/tools/testing/selftests/bpf/bpf_helpers.h
> +++ b/tools/testing/selftests/bpf/bpf_helpers.h
> @@ -103,6 +103,13 @@ static int (*bpf_skb_get_xfrm_state)(void *ctx, int index, void *state,
>  	(void *) BPF_FUNC_skb_get_xfrm_state;
>  static int (*bpf_get_stack)(void *ctx, void *buf, int size, int flags) =
>  	(void *) BPF_FUNC_get_stack;
> +static struct bpf_sock *(*bpf_sk_lookup)(void *ctx,
> +					 struct bpf_sock_tuple *tuple,
> +					 int size, unsigned int netns_id,
> +					 unsigned long long flags) =
> +	(void *) BPF_FUNC_sk_lookup;
> +static int (*bpf_sk_release)(struct bpf_sock *sk, unsigned long long flags) =
> +	(void *) BPF_FUNC_sk_release;
>  
>  /* llvm builtin functions that eBPF C program may use to
>   * emit BPF_LD_ABS and BPF_LD_IND instructions
> -- 
> 2.14.1
> 

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