[PATCH v1 bpf-next 3/8] bpf: tcp: Support bpf_skb_load_bytes() for BPF_SOCK_OPS_RCVLOWAT_CB.

[Kuniyuki Iwashima <kuniyu@google.com>]

When a TCP skb is queued to sk->sk_receive_queue, BPF SOCK_OPS
prog can be called with BPF_SOCK_OPS_RCVLOWAT_CB.

In this hook, we want to parse the RPC descriptor in the skb
and adjust sk->sk_rcvlowat based on the RPC frame size.

However, we cannot access payload via bpf_sock_ops.data on
modern NICs with TCP header/data split on as the payload is
not placed in the linear area.

Let's support bpf_skb_load_bytes() for BPF_SOCK_OPS_RCVLOWAT_CB.

Two notes:

  1) bpf_sock_ops_kern.skb will be NULL when the BPF prog is
      invoked from recvmsg().

  2) Access to bpf_sock_ops.data will be disabled by passing
      0 end_offset to bpf_skops_init_skb().

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 net/core/filter.c | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index 5fa9189eb772..94d07a15b2ab 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -7718,6 +7718,38 @@ static const struct bpf_func_proto bpf_sk_assign_proto = {
 	.arg3_type	= ARG_ANYTHING,
 };
 
+BPF_CALL_4(bpf_sock_ops_skb_load_bytes, struct bpf_sock_ops_kern *, bpf_sock,
+	   u32, offset, void *, to, u32, len)
+{
+	int err;
+
+	if (bpf_sock->op != BPF_SOCK_OPS_RCVLOWAT_CB) {
+		err = -EOPNOTSUPP;
+		goto err_clear;
+	}
+
+	if (!bpf_sock->skb) {
+		err = -EPERM;
+		goto err_clear;
+	}
+
+	return ____bpf_skb_load_bytes(bpf_sock->skb, offset, to, len);
+
+err_clear:
+	memset(to, 0, len);
+	return err;
+}
+
+static const struct bpf_func_proto bpf_sock_ops_skb_load_bytes_proto = {
+	.func		= bpf_sock_ops_skb_load_bytes,
+	.gpl_only	= false,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_CTX,
+	.arg2_type	= ARG_ANYTHING,
+	.arg3_type	= ARG_PTR_TO_UNINIT_MEM,
+	.arg4_type	= ARG_CONST_SIZE,
+};
+
 static const u8 *bpf_search_tcp_opt(const u8 *op, const u8 *opend,
 				    u8 search_kind, const u8 *magic,
 				    u8 magic_len, bool *eol)
@@ -8574,6 +8606,8 @@ sock_ops_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 	case BPF_FUNC_get_netns_cookie:
 		return &bpf_get_netns_cookie_sock_ops_proto;
 #ifdef CONFIG_INET
+	case BPF_FUNC_skb_load_bytes:
+		return &bpf_sock_ops_skb_load_bytes_proto;
 	case BPF_FUNC_load_hdr_opt:
 		return &bpf_sock_ops_load_hdr_opt_proto;
 	case BPF_FUNC_store_hdr_opt:
-- 
2.54.0.563.g4f69b47b94-goog