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

[Jonathan Lemon]

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.le...@gmail.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