netdev - [RFC PATCH 03/21] tcp: Pad TCP options out to a fixed size

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Date:   Thu, 18 Jun 2020 09:09:23 -0700
From:   Jonathan Lemon <jonathan.lemon@...il.com>
To:     <netdev@...r.kernel.org>
CC:     <kernel-team@...com>, <axboe@...nel.dk>
Subject: [RFC PATCH 03/21] tcp: Pad TCP options out to a fixed size

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.

(Obviously not for upstream committing, just for prototyping)

Signed-off-by: Jonathan Lemon <jonathan.lemon@...il.com>
---
 net/ipv4/tcp_output.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index a50e1990a845..afc996ef2d4e 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 +563,15 @@ static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
 	smc_options_write(ptr, &options);
 
 	mptcp_options_write(ptr, opts);
+
+	/* pad out options for netgpu */
+	if (opts->pad_size) {
+		int len = opts->pad_size;
+		u8 *p = (u8 *)ptr;
+
+		while (len--)
+			*p++ = TCPOPT_NOP;
+	}
 }
 
 static void smc_set_option(const struct tcp_sock *tp,
@@ -824,6 +834,12 @@ static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb
 			opts->num_sack_blocks * TCPOLEN_SACK_PERBLOCK;
 	}
 
+	/* force padding for netgpu */
+	if (size < 20) {
+		opts->pad_size = 20 - size;
+		size += opts->pad_size;
+	}
+
 	return size;
 }
 
-- 
2.24.1