/*-
 * Copyright (c) 2014-2020 Mindaugas Rasiukevicius <rmind at noxt eu>
 * Copyright (c) 2010-2014 The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * This material is based upon work partially supported by The
 * NetBSD Foundation under a contract with Mindaugas Rasiukevicius.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

/*
 * NPF connection tracking for stateful filtering and translation.
 *
 * Overview
 *
 *      Packets can be incoming or outgoing with respect to an interface.
 *      Connection direction is identified by the direction of its first
 *      packet.  The meaning of incoming/outgoing packet in the context of
 *      connection direction can be confusing.  Therefore, we will use the
 *      terms "forwards stream" and "backwards stream", where packets in
 *      the forwards stream mean the packets travelling in the direction
 *      as the connection direction.
 *
 *      All connections have two keys and thus two entries:
 *
 *      - npf_conn_getforwkey(con)        -- for the forwards stream;
 *      - npf_conn_getbackkey(con, alen)  -- for the backwards stream.
 *
 *      Note: the keys are stored in npf_conn_t::c_keys[], which is used
 *      to allocate variable-length npf_conn_t structures based on whether
 *      the IPv4 or IPv6 addresses are used.
 *
 *      The key is an n-tuple used to identify the connection flow: see the
 *      npf_connkey.c source file for the description of the key layouts.
 *      The key may be formed using translated values in a case of NAT.
 *
 *      Connections can serve two purposes: for the implicit passing and/or
 *      to accommodate the dynamic NAT.  Connections for the former purpose
 *      are created by the rules with "stateful" attribute and are used for
 *      stateful filtering.  Such connections indicate that the packet of
 *      the backwards stream should be passed without inspection of the
 *      ruleset.  The other purpose is to associate a dynamic NAT mechanism
 *      with a connection.  Such connections are created by the NAT policies
 *      and they have a relationship with NAT translation structure via
 *      npf_conn_t::c_nat.  A single connection can serve both purposes,
 *      which is a common case.
 *
 * Connection life-cycle
 *
 *      Connections are established when a packet matches said rule or
 *      NAT policy.  Both keys of the established connection are inserted
 *      into the connection database.  A garbage collection thread
 *      periodically scans all connections and depending on connection
 *      properties (e.g. last activity time, protocol) removes connection
 *      entries and expires the actual connections.
 *
 *      Each connection has a reference count.  The reference is acquired
 *      on lookup and should be released by the caller.  It guarantees that
 *      the connection will not be destroyed, although it may be expired.
 *
 * Synchronization
 *
 *      Connection database is accessed in a lock-free manner by the main
 *      routines: npf_conn_inspect() and npf_conn_establish().  Since they
 *      are always called from a software interrupt, the database is
 *      protected using EBR.  The main place which can destroy a connection
 *      is npf_conn_worker().  The database itself can be replaced and
 *      destroyed in npf_conn_reload().
 *
 * ALG support
 *
 *      Application-level gateways (ALGs) can override generic connection
 *      inspection (npf_alg_conn() call in npf_conn_inspect() function) by
 *      performing their own lookup using different key.  Recursive call
 *      to npf_conn_inspect() is not allowed.  The ALGs ought to use the
 *      npf_conn_lookup() function for this purpose.
 *
 * Lock order
 *
 *      npf_t::config_lock ->
 *              conn_lock ->
 *                      npf_conn_t::c_lock
 */

#ifdef _KERNEL
#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: npf_conn.c,v 1.35 2023/01/22 18:39:35 riastradh Exp $");

#include <sys/param.h>
#include <sys/types.h>

#include <netinet/in.h>
#include <netinet/tcp.h>

#include <sys/atomic.h>
#include <sys/kmem.h>
#include <sys/mutex.h>
#include <net/pfil.h>
#include <sys/pool.h>
#include <sys/queue.h>
#include <sys/systm.h>
#endif

#define __NPF_CONN_PRIVATE
#include "npf_conn.h"
#include "npf_impl.h"

/* A helper to select the IPv4 or IPv6 connection cache. */
#define NPF_CONNCACHE(alen)     (((alen) >> 4) & 0x1)

/*
 * Connection flags: PFIL_IN and PFIL_OUT values are reserved for direction.
 */
CTASSERT(PFIL_ALL == (0x001 | 0x002));
#define CONN_ACTIVE     0x004   /* visible on inspection */
#define CONN_PASS       0x008   /* perform implicit passing */
#define CONN_EXPIRE     0x010   /* explicitly expire */
#define CONN_REMOVED    0x020   /* "forw/back" entries removed */

enum { CONN_TRACKING_OFF, CONN_TRACKING_ON };

static int      npf_conn_export(npf_t *, npf_conn_t *, nvlist_t *);

/*
 * npf_conn_sys{init,fini}: initialize/destroy connection tracking.
 */

void
npf_conn_init(npf_t *npf)
{
        npf_conn_params_t *params = npf_param_allocgroup(npf,
            NPF_PARAMS_CONN, sizeof(npf_conn_params_t));
        npf_param_t param_map[] = {
                {
                        "state.key.interface",
                        &params->connkey_interface,
                        .default_val = 1, // true
                        .min = 0, .max = 1
                },
                {
                        "state.key.direction",
                        &params->connkey_direction,
                        .default_val = 1, // true
                        .min = 0, .max = 1
                },
        };
        npf_param_register(npf, param_map, __arraycount(param_map));

        npf->conn_cache[0] = pool_cache_init(
            offsetof(npf_conn_t, c_keys[NPF_CONNKEY_V4WORDS * 2]),
            0, 0, 0, "npfcn4pl", NULL, IPL_NET, NULL, NULL, NULL);
        npf->conn_cache[1] = pool_cache_init(
            offsetof(npf_conn_t, c_keys[NPF_CONNKEY_V6WORDS * 2]),
            0, 0, 0, "npfcn6pl", NULL, IPL_NET, NULL, NULL, NULL);

        mutex_init(&npf->conn_lock, MUTEX_DEFAULT, IPL_NONE);
        atomic_store_relaxed(&npf->conn_tracking, CONN_TRACKING_OFF);
        npf->conn_db = npf_conndb_create();
        npf_conndb_sysinit(npf);

        npf_worker_addfunc(npf, npf_conn_worker);
}

void
npf_conn_fini(npf_t *npf)
{
        const size_t len = sizeof(npf_conn_params_t);

        /* Note: the caller should have flushed the connections. */
        KASSERT(atomic_load_relaxed(&npf->conn_tracking) == CONN_TRACKING_OFF);

        npf_conndb_destroy(npf->conn_db);
        pool_cache_destroy(npf->conn_cache[0]);
        pool_cache_destroy(npf->conn_cache[1]);
        mutex_destroy(&npf->conn_lock);

        npf_param_freegroup(npf, NPF_PARAMS_CONN, len);
        npf_conndb_sysfini(npf);
}

/*
 * npf_conn_load: perform the load by flushing the current connection
 * database and replacing it with the new one or just destroying.
 *
 * => The caller must disable the connection tracking and ensure that
 *    there are no connection database lookups or references in-flight.
 */
void
npf_conn_load(npf_t *npf, npf_conndb_t *ndb, bool track)
{
        npf_conndb_t *odb = NULL;

        KASSERT(npf_config_locked_p(npf));

        /*
         * The connection database is in the quiescent state.
         * Prevent G/C thread from running and install a new database.
         */
        mutex_enter(&npf->conn_lock);
        if (ndb) {
                KASSERT(atomic_load_relaxed(&npf->conn_tracking)
                    == CONN_TRACKING_OFF);
                odb = atomic_load_relaxed(&npf->conn_db);
                atomic_store_release(&npf->conn_db, ndb);
        }
        if (track) {
                /* After this point lookups start flying in. */
                membar_producer();
                atomic_store_relaxed(&npf->conn_tracking, CONN_TRACKING_ON);
        }
        mutex_exit(&npf->conn_lock);

        if (odb) {
                /*
                 * Flush all, no sync since the caller did it for us.
                 * Also, release the pool cache memory.
                 */
                npf_conndb_gc(npf, odb, true, false);
                npf_conndb_destroy(odb);
                pool_cache_invalidate(npf->conn_cache[0]);
                pool_cache_invalidate(npf->conn_cache[1]);
        }
}

/*
 * npf_conn_tracking: enable/disable connection tracking.
 */
void
npf_conn_tracking(npf_t *npf, bool track)
{
        KASSERT(npf_config_locked_p(npf));
        atomic_store_relaxed(&npf->conn_tracking,
            track ? CONN_TRACKING_ON : CONN_TRACKING_OFF);
}

static inline bool
npf_conn_trackable_p(const npf_cache_t *npc)
{
        const npf_t *npf = npc->npc_ctx;

        /*
         * Check if connection tracking is on.  Also, if layer 3 and 4 are
         * not cached - protocol is not supported or packet is invalid.
         */
        if (atomic_load_relaxed(&npf->conn_tracking) != CONN_TRACKING_ON) {
                return false;
        }
        if (!npf_iscached(npc, NPC_IP46) || !npf_iscached(npc, NPC_LAYER4)) {
                return false;
        }
        return true;
}

static inline void
conn_update_atime(npf_conn_t *con)
{
        struct timespec tsnow;

        getnanouptime(&tsnow);
        atomic_store_relaxed(&con->c_atime, tsnow.tv_sec);
}

/*
 * npf_conn_check: check that:
 *
 *      - the connection is active;
 *
 *      - the packet is travelling in the right direction with the respect
 *        to the connection direction (if interface-id is not zero);
 *
 *      - the packet is travelling on the same interface as the
 *        connection interface (if interface-id is not zero).
 */
static bool
npf_conn_check(const npf_conn_t *con, const nbuf_t *nbuf,
    const unsigned di, const npf_flow_t flow)
{
        const uint32_t flags = atomic_load_relaxed(&con->c_flags);
        const unsigned ifid = atomic_load_relaxed(&con->c_ifid);
        bool active;

        active = (flags & (CONN_ACTIVE | CONN_EXPIRE)) == CONN_ACTIVE;
        if (__predict_false(!active)) {
                return false;
        }
        if (ifid && nbuf) {
                const bool match = (flags & PFIL_ALL) == di;
                npf_flow_t pflow = match ? NPF_FLOW_FORW : NPF_FLOW_BACK;

                if (__predict_false(flow != pflow)) {
                        return false;
                }
                if (__predict_false(ifid != nbuf->nb_ifid)) {
                        return false;
                }
        }
        return true;
}

/*
 * npf_conn_lookup: lookup if there is an established connection.
 *
 * => If found, we will hold a reference for the caller.
 */
npf_conn_t *
npf_conn_lookup(const npf_cache_t *npc, const unsigned di, npf_flow_t *flow)
{
        npf_t *npf = npc->npc_ctx;
        const nbuf_t *nbuf = npc->npc_nbuf;
        npf_conn_t *con;
        npf_connkey_t key;

        /* Construct a key and lookup for a connection in the store. */
        if (!npf_conn_conkey(npc, &key, di, NPF_FLOW_FORW)) {
                return NULL;
        }
        con = npf_conndb_lookup(npf, &key, flow);
        if (con == NULL) {
                return NULL;
        }
        KASSERT(npc->npc_proto == atomic_load_relaxed(&con->c_proto));

        /* Extra checks for the connection and packet. */
        if (!npf_conn_check(con, nbuf, di, *flow)) {
                atomic_dec_uint(&con->c_refcnt);
                return NULL;
        }

        /* Update the last activity time. */
        conn_update_atime(con);
        return con;
}

/*
 * npf_conn_inspect: lookup a connection and inspecting the protocol data.
 *
 * => If found, we will hold a reference for the caller.
 */
npf_conn_t *
npf_conn_inspect(npf_cache_t *npc, const unsigned di, int *error)
{
        nbuf_t *nbuf = npc->npc_nbuf;
        npf_flow_t flow;
        npf_conn_t *con;
        bool ok;

        KASSERT(!nbuf_flag_p(nbuf, NBUF_DATAREF_RESET));
        if (!npf_conn_trackable_p(npc)) {
                return NULL;
        }

        /* Query ALG which may lookup connection for us. */
        if ((con = npf_alg_conn(npc, di)) != NULL) {
                /* Note: reference is held. */
                return con;
        }
        if (nbuf_head_mbuf(nbuf) == NULL) {
                *error = ENOMEM;
                return NULL;
        }
        KASSERT(!nbuf_flag_p(nbuf, NBUF_DATAREF_RESET));

        /* The main lookup of the connection (acquires a reference). */
        if ((con = npf_conn_lookup(npc, di, &flow)) == NULL) {
                return NULL;
        }

        /* Inspect the protocol data and handle state changes. */
        mutex_enter(&con->c_lock);
        ok = npf_state_inspect(npc, &con->c_state, flow);
        mutex_exit(&con->c_lock);

        /* If invalid state: let the rules deal with it. */
        if (__predict_false(!ok)) {
                npf_conn_release(con);
                npf_stats_inc(npc->npc_ctx, NPF_STAT_INVALID_STATE);
                return NULL;
        }
#if 0
        /*
         * TODO -- determine when this might be wanted/used.
         *
         * Note: skipping the connection lookup and ruleset inspection
         * on other interfaces will also bypass dynamic NAT.
         */
        if (atomic_load_relaxed(&con->c_flags) & CONN_GPASS) {
                /*
                 * Note: if tagging fails, then give this packet a chance
                 * to go through a regular ruleset.
                 */
                (void)nbuf_add_tag(nbuf, NPF_NTAG_PASS);
        }
#endif
        return con;
}

/*
 * npf_conn_establish: create a new connection, insert into the global list.
 *
 * => Connection is created with the reference held for the caller.
 * => Connection will be activated on the first reference release.
 */
npf_conn_t *
npf_conn_establish(npf_cache_t *npc, const unsigned di, bool global)
{
        npf_t *npf = npc->npc_ctx;
        const unsigned alen = npc->npc_alen;
        const unsigned idx = NPF_CONNCACHE(alen);
        const nbuf_t *nbuf = npc->npc_nbuf;
        npf_connkey_t *fw, *bk;
        npf_conndb_t *conn_db;
        npf_conn_t *con;
        int error = 0;

        KASSERT(!nbuf_flag_p(nbuf, NBUF_DATAREF_RESET));

        if (!npf_conn_trackable_p(npc)) {
                return NULL;
        }

        /* Allocate and initialize the new connection. */
        con = pool_cache_get(npf->conn_cache[idx], PR_NOWAIT);
        if (__predict_false(!con)) {
                npf_worker_signal(npf);
                return NULL;
        }
        NPF_PRINTF(("NPF: create conn %p\n", con));
        npf_stats_inc(npf, NPF_STAT_CONN_CREATE);

        mutex_init(&con->c_lock, MUTEX_DEFAULT, IPL_SOFTNET);
        atomic_store_relaxed(&con->c_flags, di & PFIL_ALL);
        atomic_store_relaxed(&con->c_refcnt, 0);
        con->c_rproc = NULL;
        con->c_nat = NULL;

        con->c_proto = npc->npc_proto;
        CTASSERT(sizeof(con->c_proto) >= sizeof(npc->npc_proto));
        con->c_alen = alen;

        /* Initialize the protocol state. */
        if (!npf_state_init(npc, &con->c_state)) {
                npf_conn_destroy(npf, con);
                return NULL;
        }
        KASSERT(npf_iscached(npc, NPC_IP46));

        fw = npf_conn_getforwkey(con);
        bk = npf_conn_getbackkey(con, alen);

        /*
         * Construct "forwards" and "backwards" keys.  Also, set the
         * interface ID for this connection (unless it is global).
         */
        if (!npf_conn_conkey(npc, fw, di, NPF_FLOW_FORW) ||
            !npf_conn_conkey(npc, bk, di ^ PFIL_ALL, NPF_FLOW_BACK)) {
                npf_conn_destroy(npf, con);
                return NULL;
        }
        con->c_ifid = global ? nbuf->nb_ifid : 0;

        /*
         * Set last activity time for a new connection and acquire
         * a reference for the caller before we make it visible.
         */
        conn_update_atime(con);
        atomic_store_relaxed(&con->c_refcnt, 1);

        /*
         * Insert both keys (entries representing directions) of the
         * connection.  At this point it becomes visible, but we activate
         * the connection later.
         */
        mutex_enter(&con->c_lock);
        conn_db = atomic_load_consume(&npf->conn_db);
        if (!npf_conndb_insert(conn_db, fw, con, NPF_FLOW_FORW)) {
                error = EISCONN;
                goto err;
        }
        if (!npf_conndb_insert(conn_db, bk, con, NPF_FLOW_BACK)) {
                npf_conn_t *ret __diagused;
                ret = npf_conndb_remove(conn_db, fw);
                KASSERT(ret == con);
                error = EISCONN;
                goto err;
        }
err:
        /*
         * If we have hit the duplicate: mark the connection as expired
         * and let the G/C thread to take care of it.  We cannot do it
         * here since there might be references acquired already.
         */
        if (error) {
                atomic_or_uint(&con->c_flags, CONN_REMOVED | CONN_EXPIRE);
                atomic_dec_uint(&con->c_refcnt);
                npf_stats_inc(npf, NPF_STAT_RACE_CONN);
        } else {
                NPF_PRINTF(("NPF: establish conn %p\n", con));
        }

        /* Finally, insert into the connection list. */
        npf_conndb_enqueue(conn_db, con);
        mutex_exit(&con->c_lock);

        return error ? NULL : con;
}

void
npf_conn_destroy(npf_t *npf, npf_conn_t *con)
{
        const unsigned idx __unused = NPF_CONNCACHE(con->c_alen);

        KASSERT(atomic_load_relaxed(&con->c_refcnt) == 0);

        if (con->c_nat) {
                /* Release any NAT structures. */
                npf_nat_destroy(con, con->c_nat);
        }
        if (con->c_rproc) {
                /* Release the rule procedure. */
                npf_rproc_release(con->c_rproc);
        }

        /* Destroy the state. */
        npf_state_destroy(&con->c_state);
        mutex_destroy(&con->c_lock);

        /* Free the structure, increase the counter. */
        pool_cache_put(npf->conn_cache[idx], con);
        npf_stats_inc(npf, NPF_STAT_CONN_DESTROY);
        NPF_PRINTF(("NPF: conn %p destroyed\n", con));
}

/*
 * npf_conn_setnat: associate NAT entry with the connection, update and
 * re-insert connection entry using the translation values.
 *
 * => The caller must be holding a reference.
 */
int
npf_conn_setnat(const npf_cache_t *npc, npf_conn_t *con,
    npf_nat_t *nt, unsigned ntype)
{
        static const unsigned nat_type_which[] = {
                /* See the description in npf_nat_which(). */
                [NPF_NATOUT] = NPF_DST,
                [NPF_NATIN] = NPF_SRC,
        };
        npf_t *npf = npc->npc_ctx;
        npf_conn_t *ret __diagused;
        npf_conndb_t *conn_db;
        npf_connkey_t *bk;
        npf_addr_t *taddr;
        in_port_t tport;
        uint32_t flags;

        KASSERT(atomic_load_relaxed(&con->c_refcnt) > 0);

        npf_nat_gettrans(nt, &taddr, &tport);
        KASSERT(ntype == NPF_NATOUT || ntype == NPF_NATIN);

        /* Acquire the lock and check for the races. */
        mutex_enter(&con->c_lock);
        flags = atomic_load_relaxed(&con->c_flags);
        if (__predict_false(flags & CONN_EXPIRE)) {
                /* The connection got expired. */
                mutex_exit(&con->c_lock);
                return EINVAL;
        }
        KASSERT((flags & CONN_REMOVED) == 0);

        if (__predict_false(con->c_nat != NULL)) {
                /* Race with a duplicate packet. */
                mutex_exit(&con->c_lock);
                npf_stats_inc(npc->npc_ctx, NPF_STAT_RACE_NAT);
                return EISCONN;
        }

        /* Remove the "backwards" key. */
        conn_db = atomic_load_consume(&npf->conn_db);
        bk = npf_conn_getbackkey(con, con->c_alen);
        ret = npf_conndb_remove(conn_db, bk);
        KASSERT(ret == con);

        /* Set the source/destination IDs to the translation values. */
        npf_conn_adjkey(bk, taddr, tport, nat_type_which[ntype]);

        /* Finally, re-insert the "backwards" key. */
        if (!npf_conndb_insert(conn_db, bk, con, NPF_FLOW_BACK)) {
                /*
                 * Race: we have hit the duplicate, remove the "forwards"
                 * key and expire our connection; it is no longer valid.
                 */
                npf_connkey_t *fw = npf_conn_getforwkey(con);
                ret = npf_conndb_remove(conn_db, fw);
                KASSERT(ret == con);

                atomic_or_uint(&con->c_flags, CONN_REMOVED | CONN_EXPIRE);
                mutex_exit(&con->c_lock);

                npf_stats_inc(npc->npc_ctx, NPF_STAT_RACE_NAT);
                return EISCONN;
        }

        /* Associate the NAT entry and release the lock. */
        con->c_nat = nt;
        mutex_exit(&con->c_lock);
        return 0;
}

/*
 * npf_conn_expire: explicitly mark connection as expired.
 *
 * => Must be called with: a) reference held  b) the relevant lock held.
 *    The relevant lock should prevent from connection destruction, e.g.
 *    npf_t::conn_lock or npf_natpolicy_t::n_lock.
 */
void
npf_conn_expire(npf_conn_t *con)
{
        atomic_or_uint(&con->c_flags, CONN_EXPIRE);
}

/*
 * npf_conn_pass: return true if connection is "pass" one, otherwise false.
 */
bool
npf_conn_pass(const npf_conn_t *con, npf_match_info_t *mi, npf_rproc_t **rp)
{
        KASSERT(atomic_load_relaxed(&con->c_refcnt) > 0);
        if (__predict_true(atomic_load_relaxed(&con->c_flags) & CONN_PASS)) {
                mi->mi_retfl = atomic_load_relaxed(&con->c_retfl);
                mi->mi_rid = con->c_rid;
                *rp = con->c_rproc;
                return true;
        }
        return false;
}

/*
 * npf_conn_setpass: mark connection as a "pass" one and associate the
 * rule procedure with it.
 */
void
npf_conn_setpass(npf_conn_t *con, const npf_match_info_t *mi, npf_rproc_t *rp)
{
        KASSERT((atomic_load_relaxed(&con->c_flags) & CONN_ACTIVE) == 0);
        KASSERT(atomic_load_relaxed(&con->c_refcnt) > 0);
        KASSERT(con->c_rproc == NULL);

        /*
         * No need for atomic since the connection is not yet active.
         * If rproc is set, the caller transfers its reference to us,
         * which will be released on npf_conn_destroy().
         */
        atomic_or_uint(&con->c_flags, CONN_PASS);
        con->c_rproc = rp;
        if (rp) {
                con->c_rid = mi->mi_rid;
                con->c_retfl = mi->mi_retfl;
        }
}

/*
 * npf_conn_release: release a reference, which might allow G/C thread
 * to destroy this connection.
 */
void
npf_conn_release(npf_conn_t *con)
{
        const unsigned flags = atomic_load_relaxed(&con->c_flags);

        if ((flags & (CONN_ACTIVE | CONN_EXPIRE)) == 0) {
                /* Activate: after this, connection is globally visible. */
                atomic_or_uint(&con->c_flags, CONN_ACTIVE);
        }
        KASSERT(atomic_load_relaxed(&con->c_refcnt) > 0);
        atomic_dec_uint(&con->c_refcnt);
}

/*
 * npf_conn_getnat: return the associated NAT entry, if any.
 */
npf_nat_t *
npf_conn_getnat(const npf_conn_t *con)
{
        return con->c_nat;
}

/*
 * npf_conn_expired: criterion to check if connection is expired.
 */
bool
npf_conn_expired(npf_t *npf, const npf_conn_t *con, uint64_t tsnow)
{
        const unsigned flags = atomic_load_relaxed(&con->c_flags);
        const int etime = npf_state_etime(npf, &con->c_state, con->c_proto);
        int elapsed;

        if (__predict_false(flags & CONN_EXPIRE)) {
                /* Explicitly marked to be expired. */
                return true;
        }

        /*
         * Note: another thread may update 'atime' and it might
         * become greater than 'now'.
         */
        elapsed = (int64_t)tsnow - atomic_load_relaxed(&con->c_atime);
        return elapsed > etime;
}

/*
 * npf_conn_remove: unlink the connection and mark as expired.
 */
void
npf_conn_remove(npf_conndb_t *cd, npf_conn_t *con)
{
        /* Remove both entries of the connection. */
        mutex_enter(&con->c_lock);
        if ((atomic_load_relaxed(&con->c_flags) & CONN_REMOVED) == 0) {
                npf_connkey_t *fw, *bk;
                npf_conn_t *ret __diagused;

                fw = npf_conn_getforwkey(con);
                ret = npf_conndb_remove(cd, fw);
                KASSERT(ret == con);

                bk = npf_conn_getbackkey(con, NPF_CONNKEY_ALEN(fw));
                ret = npf_conndb_remove(cd, bk);
                KASSERT(ret == con);
        }

        /* Flag the removal and expiration. */
        atomic_or_uint(&con->c_flags, CONN_REMOVED | CONN_EXPIRE);
        mutex_exit(&con->c_lock);
}

/*
 * npf_conn_worker: G/C to run from a worker thread or via npfk_gc().
 */
void
npf_conn_worker(npf_t *npf)
{
        npf_conndb_t *conn_db = atomic_load_consume(&npf->conn_db);
        npf_conndb_gc(npf, conn_db, false, true);
}

/*
 * npf_conndb_export: construct a list of connections prepared for saving.
 * Note: this is expected to be an expensive operation.
 */
int
npf_conndb_export(npf_t *npf, nvlist_t *nvl)
{
        npf_conn_t *head, *con;
        npf_conndb_t *conn_db;

        /*
         * Note: acquire conn_lock to prevent from the database
         * destruction and G/C thread.
         */
        mutex_enter(&npf->conn_lock);
        if (atomic_load_relaxed(&npf->conn_tracking) != CONN_TRACKING_ON) {
                mutex_exit(&npf->conn_lock);
                return 0;
        }
        conn_db = atomic_load_relaxed(&npf->conn_db);
        head = npf_conndb_getlist(conn_db);
        con = head;
        while (con) {
                nvlist_t *con_nvl;

                con_nvl = nvlist_create(0);
                if (npf_conn_export(npf, con, con_nvl) == 0) {
                        nvlist_append_nvlist_array(nvl, "conn-list", con_nvl);
                }
                nvlist_destroy(con_nvl);

                if ((con = npf_conndb_getnext(conn_db, con)) == head) {
                        break;
                }
        }
        mutex_exit(&npf->conn_lock);
        return 0;
}

/*
 * npf_conn_export: serialize a single connection.
 */
static int
npf_conn_export(npf_t *npf, npf_conn_t *con, nvlist_t *nvl)
{
        nvlist_t *knvl;
        npf_connkey_t *fw, *bk;
        unsigned flags, alen;

        flags = atomic_load_relaxed(&con->c_flags);
        if ((flags & (CONN_ACTIVE|CONN_EXPIRE)) != CONN_ACTIVE) {
                return ESRCH;
        }
        nvlist_add_number(nvl, "flags", flags);
        nvlist_add_number(nvl, "proto", con->c_proto);
        if (con->c_ifid) {
                char ifname[IFNAMSIZ];
                npf_ifmap_copyname(npf, con->c_ifid, ifname, sizeof(ifname));
                nvlist_add_string(nvl, "ifname", ifname);
        }
        nvlist_add_binary(nvl, "state", &con->c_state, sizeof(npf_state_t));

        fw = npf_conn_getforwkey(con);
        alen = NPF_CONNKEY_ALEN(fw);
        KASSERT(alen == con->c_alen);
        bk = npf_conn_getbackkey(con, alen);

        knvl = npf_connkey_export(npf, fw);
        nvlist_move_nvlist(nvl, "forw-key", knvl);

        knvl = npf_connkey_export(npf, bk);
        nvlist_move_nvlist(nvl, "back-key", knvl);

        /* Let the address length be based on on first key. */
        nvlist_add_number(nvl, "alen", alen);

        if (con->c_nat) {
                npf_nat_export(npf, con->c_nat, nvl);
        }
        return 0;
}

/*
 * npf_conn_import: fully reconstruct a single connection from a
 * nvlist and insert into the given database.
 */
int
npf_conn_import(npf_t *npf, npf_conndb_t *cd, const nvlist_t *cdict,
    npf_ruleset_t *natlist)
{
        npf_conn_t *con;
        npf_connkey_t *fw, *bk;
        const nvlist_t *nat, *conkey;
        unsigned flags, alen, idx;
        const char *ifname;
        const void *state;
        size_t len;

        /*
         * To determine the length of the connection, which depends
         * on the address length in the connection keys.
         */
        alen = dnvlist_get_number(cdict, "alen", 0);
        idx = NPF_CONNCACHE(alen);

        /* Allocate a connection and initialize it (clear first). */
        con = pool_cache_get(npf->conn_cache[idx], PR_WAITOK);
        memset(con, 0, sizeof(npf_conn_t));
        mutex_init(&con->c_lock, MUTEX_DEFAULT, IPL_SOFTNET);
        npf_stats_inc(npf, NPF_STAT_CONN_CREATE);

        con->c_proto = dnvlist_get_number(cdict, "proto", 0);
        flags = dnvlist_get_number(cdict, "flags", 0);
        flags &= PFIL_ALL | CONN_ACTIVE | CONN_PASS;
        atomic_store_relaxed(&con->c_flags, flags);
        conn_update_atime(con);

        ifname = dnvlist_get_string(cdict, "ifname", NULL);
        if (ifname && (con->c_ifid = npf_ifmap_register(npf, ifname)) == 0) {
                goto err;
        }

        state = dnvlist_get_binary(cdict, "state", &len, NULL, 0);
        if (!state || len != sizeof(npf_state_t)) {
                goto err;
        }
        memcpy(&con->c_state, state, sizeof(npf_state_t));

        /* Reconstruct NAT association, if any. */
        if ((nat = dnvlist_get_nvlist(cdict, "nat", NULL)) != NULL &&
            (con->c_nat = npf_nat_import(npf, nat, natlist, con)) == NULL) {
                goto err;
        }

        /*
         * Fetch and copy the keys for each direction.
         */
        fw = npf_conn_getforwkey(con);
        conkey = dnvlist_get_nvlist(cdict, "forw-key", NULL);
        if (conkey == NULL || !npf_connkey_import(npf, conkey, fw)) {
                goto err;
        }
        bk = npf_conn_getbackkey(con, NPF_CONNKEY_ALEN(fw));
        conkey = dnvlist_get_nvlist(cdict, "back-key", NULL);
        if (conkey == NULL || !npf_connkey_import(npf, conkey, bk)) {
                goto err;
        }

        /* Guard against the contradicting address lengths. */
        if (NPF_CONNKEY_ALEN(fw) != alen || NPF_CONNKEY_ALEN(bk) != alen) {
                goto err;
        }

        /* Insert the entries and the connection itself. */
        if (!npf_conndb_insert(cd, fw, con, NPF_FLOW_FORW)) {
                goto err;
        }
        if (!npf_conndb_insert(cd, bk, con, NPF_FLOW_BACK)) {
                npf_conndb_remove(cd, fw);
                goto err;
        }

        NPF_PRINTF(("NPF: imported conn %p\n", con));
        npf_conndb_enqueue(cd, con);
        return 0;
err:
        npf_conn_destroy(npf, con);
        return EINVAL;
}

/*
 * npf_conn_find: lookup a connection in the list of connections
 */
int
npf_conn_find(npf_t *npf, const nvlist_t *req, nvlist_t *resp)
{
        const nvlist_t *key_nv;
        npf_conn_t *con;
        npf_connkey_t key;
        npf_flow_t flow;
        int error;

        key_nv = dnvlist_get_nvlist(req, "key", NULL);
        if (!key_nv || !npf_connkey_import(npf, key_nv, &key)) {
                return EINVAL;
        }
        con = npf_conndb_lookup(npf, &key, &flow);
        if (con == NULL) {
                return ESRCH;
        }
        if (!npf_conn_check(con, NULL, 0, NPF_FLOW_FORW)) {
                atomic_dec_uint(&con->c_refcnt);
                return ESRCH;
        }
        error = npf_conn_export(npf, con, resp);
        nvlist_add_number(resp, "flow", flow);
        atomic_dec_uint(&con->c_refcnt);
        return error;
}

#if defined(DDB) || defined(_NPF_TESTING)

void
npf_conn_print(npf_conn_t *con)
{
        const npf_connkey_t *fw = npf_conn_getforwkey(con);
        const npf_connkey_t *bk = npf_conn_getbackkey(con, NPF_CONNKEY_ALEN(fw));
        const unsigned flags = atomic_load_relaxed(&con->c_flags);
        const unsigned proto = con->c_proto;
        struct timespec tspnow;

        getnanouptime(&tspnow);
        printf("%p:\n\tproto %d flags 0x%x tsdiff %ld etime %d\n", con,
            proto, flags, (long)(tspnow.tv_sec - con->c_atime),
            npf_state_etime(npf_getkernctx(), &con->c_state, proto));
        npf_connkey_print(fw);
        npf_connkey_print(bk);
        npf_state_dump(&con->c_state);
        if (con->c_nat) {
                npf_nat_dump(con->c_nat);
        }
}

#endif