[PATCH v3 bpf-next 1/9] tcp: Use a struct to represent a saved_syn

[Martin KaFai Lau <kafai@fb.com>]

The TCP_SAVE_SYN has both the network header and tcp header.
The total length of the saved syn packet is currently stored in
the first 4 bytes (u32) of an array and the actual packet data is
stored after that.

A latter patch will add a bpf helper that allows to get the tcp header
alone from the saved syn without the network header.  It will be more
convenient to have a direct offset to a specific header instead of
re-parsing it.  This requires to separately store the network hdrlen.
The total header length (i.e. network + tcp) is still needed for the
current usage in getsockopt.  Although this total length can be obtained
by looking into the tcphdr and then get the (th->doff << 2), this patch
chooses to directly store the tcp hdrlen in the second four bytes of
this newly created "struct saved_syn".  By using a new struct, it can
give a readable name to each individual header length.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
---
 include/linux/tcp.h        |  7 ++++++-
 include/net/request_sock.h |  8 +++++++-
 net/core/filter.c          |  4 ++--
 net/ipv4/tcp.c             |  9 +++++----
 net/ipv4/tcp_input.c       | 15 ++++++++-------
 5 files changed, 28 insertions(+), 15 deletions(-)

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 527d668a5275..5528912dc468 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -406,7 +406,7 @@ struct tcp_sock {
 	 * socket. Used to retransmit SYNACKs etc.
 	 */
 	struct request_sock __rcu *fastopen_rsk;
-	u32	*saved_syn;
+	struct saved_syn *saved_syn;
 };
 
 enum tsq_enum {
@@ -484,6 +484,11 @@ static inline void tcp_saved_syn_free(struct tcp_sock *tp)
 	tp->saved_syn = NULL;
 }
 
+static inline u32 tcp_saved_syn_len(const struct saved_syn *saved_syn)
+{
+	return saved_syn->network_hdrlen + saved_syn->tcp_hdrlen;
+}
+
 struct sk_buff *tcp_get_timestamping_opt_stats(const struct sock *sk);
 
 static inline u16 tcp_mss_clamp(const struct tcp_sock *tp, u16 mss)
diff --git a/include/net/request_sock.h b/include/net/request_sock.h
index cf8b33213bbc..b1b101814ecb 100644
--- a/include/net/request_sock.h
+++ b/include/net/request_sock.h
@@ -41,6 +41,12 @@ struct request_sock_ops {
 
 int inet_rtx_syn_ack(const struct sock *parent, struct request_sock *req);
 
+struct saved_syn {
+	u32 network_hdrlen;
+	u32 tcp_hdrlen;
+	u8 data[];
+};
+
 /* struct request_sock - mini sock to represent a connection request
  */
 struct request_sock {
@@ -60,7 +66,7 @@ struct request_sock {
 	struct timer_list		rsk_timer;
 	const struct request_sock_ops	*rsk_ops;
 	struct sock			*sk;
-	u32				*saved_syn;
+	struct saved_syn		*saved_syn;
 	u32				secid;
 	u32				peer_secid;
 };
diff --git a/net/core/filter.c b/net/core/filter.c
index 7124f0fe6974..250b5552a148 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -4550,9 +4550,9 @@ static int _bpf_getsockopt(struct sock *sk, int level, int optname,
 			tp = tcp_sk(sk);
 
 			if (optlen <= 0 || !tp->saved_syn ||
-			    optlen > tp->saved_syn[0])
+			    optlen > tcp_saved_syn_len(tp->saved_syn))
 				goto err_clear;
-			memcpy(optval, tp->saved_syn + 1, optlen);
+			memcpy(optval, tp->saved_syn->data, optlen);
 			break;
 		default:
 			goto err_clear;
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 27de9380ed14..8a774b5094e9 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -3791,20 +3791,21 @@ static int do_tcp_getsockopt(struct sock *sk, int level,
 
 		lock_sock(sk);
 		if (tp->saved_syn) {
-			if (len < tp->saved_syn[0]) {
-				if (put_user(tp->saved_syn[0], optlen)) {
+			if (len < tcp_saved_syn_len(tp->saved_syn)) {
+				if (put_user(tcp_saved_syn_len(tp->saved_syn),
+					     optlen)) {
 					release_sock(sk);
 					return -EFAULT;
 				}
 				release_sock(sk);
 				return -EINVAL;
 			}
-			len = tp->saved_syn[0];
+			len = tcp_saved_syn_len(tp->saved_syn);
 			if (put_user(len, optlen)) {
 				release_sock(sk);
 				return -EFAULT;
 			}
-			if (copy_to_user(optval, tp->saved_syn + 1, len)) {
+			if (copy_to_user(optval, tp->saved_syn->data, len)) {
 				release_sock(sk);
 				return -EFAULT;
 			}
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index a018bafd7bdf..6c38ca9de17e 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -6598,13 +6598,14 @@ static void tcp_reqsk_record_syn(const struct sock *sk,
 {
 	if (tcp_sk(sk)->save_syn) {
 		u32 len = skb_network_header_len(skb) + tcp_hdrlen(skb);
-		u32 *copy;
-
-		copy = kmalloc(len + sizeof(u32), GFP_ATOMIC);
-		if (copy) {
-			copy[0] = len;
-			memcpy(&copy[1], skb_network_header(skb), len);
-			req->saved_syn = copy;
+		struct saved_syn *saved_syn;
+
+		saved_syn = kmalloc(len + sizeof(*saved_syn), GFP_ATOMIC);
+		if (saved_syn) {
+			saved_syn->network_hdrlen = skb_network_header_len(skb);
+			saved_syn->tcp_hdrlen = tcp_hdrlen(skb);
+			memcpy(saved_syn->data, skb_network_header(skb), len);
+			req->saved_syn = saved_syn;
 		}
 	}
 }
-- 
2.24.1