bpf: export bpf_sock for BPF_PROG_TYPE_CGROUP_SOCK_ADDR prog type

And let it use bpf_sk_storage_{get,delete} helpers to access socket
storage. Kernel context (struct bpf_sock_addr_kern) already has sk
member, so I just expose it to the BPF hooks. Using PTR_TO_SOCKET
instead of PTR_TO_SOCK_COMMON should be safe because the hook is
called on bind/connect.

Cc: Martin Lau <kafai@fb.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/uapi/linux/bpf.h

@@ -3247,6 +3247,7 @@ struct bpf_sock_addr {
 	__u32 msg_src_ip6[4];	/* Allows 1,2,4-byte read an 4-byte write.
 				 * Stored in network byte order.
 				 */
+	__bpf_md_ptr(struct bpf_sock *, sk);
 };
 
 /* User bpf_sock_ops struct to access socket values and specify request ops

net/core/filter.c

@@ -5922,6 +5922,10 @@ sock_addr_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 	case BPF_FUNC_skc_lookup_tcp:
 		return &bpf_sock_addr_skc_lookup_tcp_proto;
 #endif /* CONFIG_INET */
+	case BPF_FUNC_sk_storage_get:
+		return &bpf_sk_storage_get_proto;
+	case BPF_FUNC_sk_storage_delete:
+		return &bpf_sk_storage_delete_proto;
 	default:
 		return bpf_base_func_proto(func_id);
 	}
@@ -6828,6 +6832,13 @@ static bool sock_addr_is_valid_access(int off, int size,
 		if (size != size_default)
 			return false;
 		break;
+	case offsetof(struct bpf_sock_addr, sk):
+		if (type != BPF_READ)
+			return false;
+		if (size != sizeof(__u64))
+			return false;
+		info->reg_type = PTR_TO_SOCKET;
+		break;
 	default:
 		if (type == BPF_READ) {
 			if (size != size_default)
@@ -7778,6 +7789,11 @@ static u32 sock_addr_convert_ctx_access(enum bpf_access_type type,
 			struct bpf_sock_addr_kern, struct in6_addr, t_ctx,
 			s6_addr32[0], BPF_SIZE(si->code), off, tmp_reg);
 		break;
+	case offsetof(struct bpf_sock_addr, sk):
+		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct bpf_sock_addr_kern, sk),
+				      si->dst_reg, si->src_reg,
+				      offsetof(struct bpf_sock_addr_kern, sk));
+		break;
 	}
 
 	return insn - insn_buf;