Subject: [RFC PATCH v2 13/21] net/tcp: Pad TCP options out to a fixed size for netgpu

From: Jonathan Lemon <[email-masked]>
Date: Mon, 27 Jul 2020 15:44:36 -0700

From: Jonathan Lemon <[email-masked]>

The "header splitting" feature used by netgpu doesn't actually parse
the incoming packet header.  Instead, it splits the packet at a fixed
offset.  In order for this to work, the sender needs to send packets
with a fixed header size.

Signed-off-by: Jonathan Lemon <[email-masked]>
---
 net/ipv4/tcp_output.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index d8f16f6a9b02..e8a74d0f7ad2 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -438,6 +438,7 @@ struct tcp_out_options {
 	u8 ws;			/* window scale, 0 to disable */
 	u8 num_sack_blocks;	/* number of SACK blocks to include */
 	u8 hash_size;		/* bytes in hash_location */
+	u8 pad_size;		/* additional nops for padding */
 	__u8 *hash_location;	/* temporary pointer, overloaded */
 	__u32 tsval, tsecr;	/* need to include OPTION_TS */
 	struct tcp_fastopen_cookie *fastopen_cookie;	/* Fast open cookie */
@@ -562,6 +563,17 @@ static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
 	smc_options_write(ptr, &options);
 
 	mptcp_options_write(ptr, opts);
+
+#if IS_ENABLED(CONFIG_NETGPU)
+	/* pad out options */
+	if (opts->pad_size) {
+		int len = opts->pad_size;
+		u8 *p = (u8 *)ptr;
+
+		while (len--)
+			*p++ = TCPOPT_NOP;
+	}
+#endif
 }
 
 static void smc_set_option(const struct tcp_sock *tp,
@@ -826,6 +838,14 @@ static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb
 			opts->num_sack_blocks * TCPOLEN_SACK_PERBLOCK;
 	}
 
+#if IS_ENABLED(CONFIG_NETGPU)
+	/* force padding */
+	if (size < 20) {
+		opts->pad_size = 20 - size;
+		size += opts->pad_size;
+	}
+#endif
+
 	return size;
 }
 
-- 
2.24.1