tcp: TCP experimental option for SMC

The SMC protocol [1] relies on the use of a new TCP experimental
option [2, 3]. With this option, SMC capabilities are exchanged
between peers during the TCP three way handshake. This patch adds
support for this experimental option to TCP.

References:
[1] SMC-R Informational RFC: http://www.rfc-editor.org/info/rfc7609
[2] Shared Use of TCP Experimental Options RFC 6994:
    https://tools.ietf.org/rfc/rfc6994.txt
[3] IANA ExID SMCR:
http://www.iana.org/assignments/tcp-parameters/tcp-parameters.xhtml#tcp-exids

Signed-off-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

4 files changed, 120 insertions, 3 deletions

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 8f36277e82e9..f6e1c00e300e 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -270,6 +270,7 @@
 #include <linux/time.h>
 #include <linux/slab.h>
 #include <linux/errqueue.h>
+#include <linux/static_key.h>
 
 #include <net/icmp.h>
 #include <net/inet_common.h>
@@ -302,6 +303,11 @@ EXPORT_SYMBOL(sysctl_tcp_wmem);
 atomic_long_t tcp_memory_allocated;     /* Current allocated memory. */
 EXPORT_SYMBOL(tcp_memory_allocated);
 
+#if IS_ENABLED(CONFIG_SMC)
+DEFINE_STATIC_KEY_FALSE(tcp_have_smc);
+EXPORT_SYMBOL(tcp_have_smc);
+#endif
+
 /*
  * Current number of TCP sockets.
  */
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 893286db4623..337f6011528a 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -76,6 +76,8 @@
 #include <asm/unaligned.h>
 #include <linux/errqueue.h>
 #include <trace/events/tcp.h>
+#include <linux/unaligned/access_ok.h>
+#include <linux/static_key.h>
 
 int sysctl_tcp_fack __read_mostly;
 int sysctl_tcp_max_reordering __read_mostly = 300;
@@ -3737,6 +3739,21 @@ static void tcp_parse_fastopen_option(int len, const unsigned char *cookie,
         foc->exp = exp_opt;
 }
 
+static void smc_parse_options(const struct tcphdr *th,
+                              struct tcp_options_received *opt_rx,
+                              const unsigned char *ptr,
+                              int opsize)
+{
+#if IS_ENABLED(CONFIG_SMC)
+        if (static_branch_unlikely(&tcp_have_smc)) {
+                if (th->syn && !(opsize & 1) &&
+                    opsize >= TCPOLEN_EXP_SMC_BASE &&
+                    get_unaligned_be32(ptr) == TCPOPT_SMC_MAGIC)
+                        opt_rx->smc_ok = 1;
+        }
+#endif
+}
+
 /* Look for tcp options. Normally only called on SYN and SYNACK packets.
  * But, this can also be called on packets in the established flow when
  * the fast version below fails.
@@ -3844,6 +3861,9 @@ void tcp_parse_options(const struct net *net,
                                         tcp_parse_fastopen_option(opsize -
                                                 TCPOLEN_EXP_FASTOPEN_BASE,
                                                 ptr + 2, th->syn, foc, true);
+                                else
+                                        smc_parse_options(th, opt_rx, ptr,
+                                                          opsize);
                                 break;
 
                         }
@@ -5598,6 +5618,16 @@ static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
         return false;
 }
 
+static void smc_check_reset_syn(struct tcp_sock *tp)
+{
+#if IS_ENABLED(CONFIG_SMC)
+        if (static_branch_unlikely(&tcp_have_smc)) {
+                if (tp->syn_smc && !tp->rx_opt.smc_ok)
+                        tp->syn_smc = 0;
+        }
+#endif
+}
+
 static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
                                          const struct tcphdr *th)
 {
@@ -5704,6 +5734,8 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
                  * is initialized. */
                 tp->copied_seq = tp->rcv_nxt;
 
+                smc_check_reset_syn(tp);
+
                 smp_mb();
 
                 tcp_finish_connect(sk, skb);
@@ -6157,6 +6189,9 @@ static void tcp_openreq_init(struct request_sock *req,
         ireq->ir_rmt_port = tcp_hdr(skb)->source;
         ireq->ir_num = ntohs(tcp_hdr(skb)->dest);
         ireq->ir_mark = inet_request_mark(sk, skb);
+#if IS_ENABLED(CONFIG_SMC)
+        ireq->smc_ok = rx_opt->smc_ok;
+#endif
 }
 
 struct request_sock *inet_reqsk_alloc(const struct request_sock_ops *ops,
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index a952357054f4..056009f1c14f 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -23,6 +23,7 @@
 #include <linux/slab.h>
 #include <linux/sysctl.h>
 #include <linux/workqueue.h>
+#include <linux/static_key.h>
 #include <net/tcp.h>
 #include <net/inet_common.h>
 #include <net/xfrm.h>
@@ -416,6 +417,21 @@ void tcp_ca_openreq_child(struct sock *sk, const struct dst_entry *dst)
 }
 EXPORT_SYMBOL_GPL(tcp_ca_openreq_child);
 
+static void smc_check_reset_syn_req(struct tcp_sock *oldtp,
+                                    struct request_sock *req,
+                                    struct tcp_sock *newtp)
+{
+#if IS_ENABLED(CONFIG_SMC)
+        struct inet_request_sock *ireq;
+
+        if (static_branch_unlikely(&tcp_have_smc)) {
+                ireq = inet_rsk(req);
+                if (oldtp->syn_smc && !ireq->smc_ok)
+                        newtp->syn_smc = 0;
+        }
+#endif
+}
+
 /* This is not only more efficient than what we used to do, it eliminates
  * a lot of code duplication between IPv4/IPv6 SYN recv processing. -DaveM
  *
@@ -433,6 +449,9 @@ struct sock *tcp_create_openreq_child(const struct sock *sk,
                 struct tcp_request_sock *treq = tcp_rsk(req);
                 struct inet_connection_sock *newicsk = inet_csk(newsk);
                 struct tcp_sock *newtp = tcp_sk(newsk);
+                struct tcp_sock *oldtp = tcp_sk(sk);
+
+                smc_check_reset_syn_req(oldtp, req, newtp);
 
                 /* Now setup tcp_sock */
                 newtp->pred_flags = 0;
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 1f01f4c9c738..c8fc512e0bbb 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -41,6 +41,7 @@
 #include <linux/compiler.h>
 #include <linux/gfp.h>
 #include <linux/module.h>
+#include <linux/static_key.h>
 
 #include <trace/events/tcp.h>
 
@@ -422,6 +423,22 @@ static inline bool tcp_urg_mode(const struct tcp_sock *tp)
 #define OPTION_MD5              (1 << 2)
 #define OPTION_WSCALE           (1 << 3)
 #define OPTION_FAST_OPEN_COOKIE (1 << 8)
+#define OPTION_SMC              (1 << 9)
+
+static void smc_options_write(__be32 *ptr, u16 *options)
+{
+#if IS_ENABLED(CONFIG_SMC)
+        if (static_branch_unlikely(&tcp_have_smc)) {
+                if (unlikely(OPTION_SMC & *options)) {
+                        *ptr++ = htonl((TCPOPT_NOP  << 24) |
+                                       (TCPOPT_NOP  << 16) |
+                                       (TCPOPT_EXP <<  8) |
+                                       (TCPOLEN_EXP_SMC_BASE));
+                        *ptr++ = htonl(TCPOPT_SMC_MAGIC);
+                }
+        }
+#endif
+}
 
 struct tcp_out_options {
         u16 options;            /* bit field of OPTION_* */
@@ -540,6 +557,41 @@ static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
                 }
                 ptr += (len + 3) >> 2;
         }
+
+        smc_options_write(ptr, &options);
+}
+
+static void smc_set_option(const struct tcp_sock *tp,
+                           struct tcp_out_options *opts,
+                           unsigned int *remaining)
+{
+#if IS_ENABLED(CONFIG_SMC)
+        if (static_branch_unlikely(&tcp_have_smc)) {
+                if (tp->syn_smc) {
+                        if (*remaining >= TCPOLEN_EXP_SMC_BASE_ALIGNED) {
+                                opts->options |= OPTION_SMC;
+                                *remaining -= TCPOLEN_EXP_SMC_BASE_ALIGNED;
+                        }
+                }
+        }
+#endif
+}
+
+static void smc_set_option_cond(const struct tcp_sock *tp,
+                                const struct inet_request_sock *ireq,
+                                struct tcp_out_options *opts,
+                                unsigned int *remaining)
+{
+#if IS_ENABLED(CONFIG_SMC)
+        if (static_branch_unlikely(&tcp_have_smc)) {
+                if (tp->syn_smc && ireq->smc_ok) {
+                        if (*remaining >= TCPOLEN_EXP_SMC_BASE_ALIGNED) {
+                                opts->options |= OPTION_SMC;
+                                *remaining -= TCPOLEN_EXP_SMC_BASE_ALIGNED;
+                        }
+                }
+        }
+#endif
 }
 
 /* Compute TCP options for SYN packets. This is not the final
@@ -607,11 +659,14 @@ static unsigned int tcp_syn_options(struct sock *sk, struct sk_buff *skb,
                 }
         }
 
+        smc_set_option(tp, opts, &remaining);
+
         return MAX_TCP_OPTION_SPACE - remaining;
 }
 
 /* Set up TCP options for SYN-ACKs. */
-static unsigned int tcp_synack_options(struct request_sock *req,
+static unsigned int tcp_synack_options(const struct sock *sk,
+                                       struct request_sock *req,
                                        unsigned int mss, struct sk_buff *skb,
                                        struct tcp_out_options *opts,
                                        const struct tcp_md5sig_key *md5,
@@ -667,6 +722,8 @@ static unsigned int tcp_synack_options(struct request_sock *req,
                 }
         }
 
+        smc_set_option_cond(tcp_sk(sk), ireq, opts, &remaining);
+
         return MAX_TCP_OPTION_SPACE - remaining;
 }
 
@@ -3195,8 +3252,8 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
         md5 = tcp_rsk(req)->af_specific->req_md5_lookup(sk, req_to_sk(req));
 #endif
         skb_set_hash(skb, tcp_rsk(req)->txhash, PKT_HASH_TYPE_L4);
-        tcp_header_size = tcp_synack_options(req, mss, skb, &opts, md5, foc) +
-                          sizeof(*th);
+        tcp_header_size = tcp_synack_options(sk, req, mss, skb, &opts, md5,
+                                             foc) + sizeof(*th);
 
         skb_push(skb, tcp_header_size);
         skb_reset_transport_header(skb);