[PATCH v2 09/17] net: psp: update the TCP MSS to reflect PSP packet overhead

Daniel Zahka
2025-06-25T13:52:22Z

From: Jakub Kicinski <kuba@kernel.org>

PSP eats 32B of header space. Adjust MSS appropriately.

We can either modify tcp_mtu_to_mss() / tcp_mss_to_mtu()
or reuse icsk_ext_hdr_len. The former option is more TCP
specific and has runtime overhead. The latter is a bit
of a hack as PSP is not an ext_hdr. If one squints hard
enough, UDP encap is just a more practical version of
IPv6 exthdr, so go with the latter. Happy to change.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
---

Notes:
v1:
- https://lore.kernel.org/netdev/20240510030435.120935-8-kuba@kernel.org/

include/net/psp/functions.h | 12 ++++++++++++
include/net/psp/types.h | 3 +++
net/ipv4/tcp_ipv4.c | 4 ++--
net/ipv6/ipv6_sockglue.c | 6 +++++-
net/ipv6/tcp_ipv6.c | 6 +++---
net/psp/psp_sock.c | 5 +++++
6 files changed, 30 insertions(+), 6 deletions(-)

diff --git a/include/net/psp/functions.h b/include/net/psp/functions.h
index f0488f29fb19..90a60817012c 100644
--- a/include/net/psp/functions.h
+++ b/include/net/psp/functions.h
@@ -128,6 +128,13 @@ static inline struct psp_assoc *psp_skb_get_assoc_rcu(struct sk_buff *skb)
rcu_dereference(skb->sk->psp_assoc);
return pas;
}
+
+static inline unsigned int psp_sk_overhead(const struct sock *sk)
+{
+ bool has_psp = rcu_access_pointer(sk->psp_assoc);
+
+ return has_psp ? PSP_HDR_SIZE + PSP_TRL_SIZE : 0;
+}
#else
static inline void psp_sk_assoc_free(struct sock *sk) { }
static inline void
@@ -167,6 +174,11 @@ static inline struct psp_assoc *psp_skb_get_assoc_rcu(struct sk_buff *skb)
{
return NULL;
}
+
+static inline unsigned int psp_sk_overhead(const struct sock *sk)
+{
+ return 0;
+}
#endif

static inline unsigned long
diff --git a/include/net/psp/types.h b/include/net/psp/types.h
index 3b7b977e62a3..5b0c2474a042 100644
--- a/include/net/psp/types.h
+++ b/include/net/psp/types.h
@@ -95,6 +95,9 @@ struct psp_dev_caps {
#define PSP_V1_KEY 32
#define PSP_MAX_KEY 32

+#define PSP_HDR_SIZE 16 /* We don't support optional fields, yet */
+#define PSP_TRL_SIZE 16 /* AES-GCM/GMAC trailer size */
+
struct psp_skb_ext {
__be32 spi;
u16 dev_id;
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 6273e3b0885c..0183f8d6c2dc 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -291,9 +291,9 @@ int tcp_v4_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
inet->inet_dport = usin->sin_port;
sk_daddr_set(sk, daddr);

- inet_csk(sk)->icsk_ext_hdr_len = 0;
+ inet_csk(sk)->icsk_ext_hdr_len = psp_sk_overhead(sk);
if (inet_opt)
- inet_csk(sk)->icsk_ext_hdr_len = inet_opt->opt.optlen;
+ inet_csk(sk)->icsk_ext_hdr_len += inet_opt->opt.optlen;

tp->rx_opt.mss_clamp = TCP_MSS_DEFAULT;

diff --git a/net/ipv6/ipv6_sockglue.c b/net/ipv6/ipv6_sockglue.c
index 1e225e6489ea..94b9ffd35e80 100644
--- a/net/ipv6/ipv6_sockglue.c
+++ b/net/ipv6/ipv6_sockglue.c
@@ -49,6 +49,7 @@
#include <net/xfrm.h>
#include <net/compat.h>
#include <net/seg6.h>
+#include <net/psp.h>

#include <linux/uaccess.h>

@@ -107,7 +108,10 @@ struct ipv6_txoptions *ipv6_update_options(struct sock *sk,
!((1 << sk->sk_state) & (TCPF_LISTEN | TCPF_CLOSE)) &&
inet_sk(sk)->inet_daddr != LOOPBACK4_IPV6) {
struct inet_connection_sock *icsk = inet_csk(sk);
- icsk->icsk_ext_hdr_len = opt->opt_flen + opt->opt_nflen;
+
+ icsk->icsk_ext_hdr_len =
+ psp_sk_overhead(sk) +
+ opt->opt_flen + opt->opt_nflen;
icsk->icsk_sync_mss(sk, icsk->icsk_pmtu_cookie);
}
}
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 033d953c0835..7fccf5c93084 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -301,10 +301,10 @@ static int tcp_v6_connect(struct sock *sk, struct sockaddr *uaddr,
sk->sk_gso_type = SKB_GSO_TCPV6;
ip6_dst_store(sk, dst, NULL, NULL);

- icsk->icsk_ext_hdr_len = 0;
+ icsk->icsk_ext_hdr_len = psp_sk_overhead(sk);
if (opt)
- icsk->icsk_ext_hdr_len = opt->opt_flen +
- opt->opt_nflen;
+ icsk->icsk_ext_hdr_len += opt->opt_flen +
+ opt->opt_nflen;

tp->rx_opt.mss_clamp = IPV6_MIN_MTU - sizeof(struct tcphdr) - sizeof(struct ipv6hdr);

diff --git a/net/psp/psp_sock.c b/net/psp/psp_sock.c
index 9028f45a2dda..4bfd8643de4e 100644
--- a/net/psp/psp_sock.c
+++ b/net/psp/psp_sock.c
@@ -188,6 +188,7 @@ int psp_sock_assoc_set_tx(struct sock *sk, struct psp_dev *psd,
u32 version, struct psp_key_parsed *key,
struct netlink_ext_ack *extack)
{
+ struct inet_connection_sock *icsk;
struct psp_assoc *pas, *dummy;
int err;

@@ -239,6 +240,10 @@ int psp_sock_assoc_set_tx(struct sock *sk, struct psp_dev *psd,
tcp_write_collapse_fence(sk);
pas->upgrade_seq = tcp_sk(sk)->rcv_nxt;

+ icsk = inet_csk(sk);
+ icsk->icsk_ext_hdr_len += psp_sk_overhead(sk);
+ icsk->icsk_sync_mss(sk, icsk->icsk_pmtu_cookie);
+
exit_free_dummy:
kfree(dummy);
exit_unlock:
--
2.47.1