--- sys/fs/nfs/nfsport.h.sav	2021-04-08 14:44:23.331512000 -0700
+++ sys/fs/nfs/nfsport.h	2021-04-08 14:50:19.337092000 -0700
@@ -357,10 +357,13 @@
 #define	NFSPROC_OPENLAYGET	54
 #define	NFSPROC_CREATELAYGET	55
 
+/* BindConnectionToSession, done by the krpc for a new connection. */
+#define	NFSPROC_BINDCONNTOSESS	56
+
 /*
  * Must be defined as one higher than the last NFSv4.1 Proc# above.
  */
-#define	NFSV41_NPROCS		56
+#define	NFSV41_NPROCS		57
 
 #endif	/* NFS_V3NPROCS */
 
@@ -389,7 +392,7 @@ struct nfsstatsv1 {
 	uint64_t	readlink_bios;
 	uint64_t	biocache_readdirs;
 	uint64_t	readdir_bios;
-	uint64_t	rpccnt[NFSV41_NPROCS + 13];
+	uint64_t	rpccnt[NFSV41_NPROCS + 12];
 	uint64_t	rpcretries;
 	uint64_t	srvrpccnt[NFSV42_NOPS + NFSV4OP_FAKENOPS];
 	uint64_t	srvrpc_errs;
--- sys/fs/nfs/nfsproto.h.sav	2021-04-08 14:50:36.325656000 -0700
+++ sys/fs/nfs/nfsproto.h	2021-04-08 14:51:35.235980000 -0700
@@ -359,10 +359,13 @@
 #define	NFSPROC_OPENLAYGET	54
 #define	NFSPROC_CREATELAYGET	55
 
+/* BindConnectionToSession, done by the krpc for a new connection. */
+#define	NFSPROC_BINDCONNTOSESS	56
+
 /*
  * Must be defined as one higher than the last NFSv4.1 Proc# above.
  */
-#define	NFSV41_NPROCS		56
+#define	NFSV41_NPROCS		57
 
 #endif	/* NFS_V3NPROCS */
 
--- sys/fs/nfs/nfs_commonsubs.c.sav	2021-04-08 14:52:36.933768000 -0700
+++ sys/fs/nfs/nfs_commonsubs.c	2021-04-08 16:56:27.955021000 -0700
@@ -192,7 +192,7 @@ static struct nfsrv_lughash	*nfsgroupnamehash;
  */
 static int nfs_bigreply[NFSV41_NPROCS] = { 0, 0, 0, 1, 0, 1, 1, 0, 0, 0, 0,
     0, 0, 0, 0, 0, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0 };
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0 };
 
 /* local functions */
 static int nfsrv_skipace(struct nfsrv_descript *nd, int *acesizep);
@@ -267,6 +267,7 @@ static struct {
 	{ NFSV4OP_COMMIT, 1, "CommitDS", 8, },
 	{ NFSV4OP_OPEN, 3, "OpenLayoutGet", 13, },
 	{ NFSV4OP_OPEN, 8, "CreateLayGet", 12, },
+	{ NFSV4OP_BINDCONNTOSESS, 1, "BindConSess", 11, },
 };
 
 /*
@@ -275,7 +276,7 @@ static struct {
 static int nfs_bigrequest[NFSV41_NPROCS] = {
 	0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
 	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-	0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0
+	0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0
 };
 
 /*
--- sys/fs/nfs/nfscl.h.sav	2021-04-08 15:05:27.154268000 -0700
+++ sys/fs/nfs/nfscl.h	2021-04-08 15:06:57.802860000 -0700
@@ -81,4 +81,9 @@ struct nfsv4node {
 			printf(__VA_ARGS__);				\
 	} while (0)
 
+struct nfscl_reconarg {
+	int	minorvers;
+	uint8_t	sessionid[NFSX_V4SESSIONID];
+};
+
 #endif	/* _NFS_NFSCL_H */
--- sys/fs/nfs/nfs_var.h.sav	2021-04-08 15:05:43.243693000 -0700
+++ sys/fs/nfs/nfs_var.h	2021-04-08 15:06:25.758858000 -0700
@@ -535,6 +535,7 @@ int nfscl_doiods(vnode_t, struct uio *, int *, int *, 
 int nfscl_findlayoutforio(struct nfscllayout *, uint64_t, uint32_t,
     struct nfsclflayout **);
 void nfscl_freenfsclds(struct nfsclds *);
+void nfsrpc_bindconnsess(CLIENT *, void *, struct ucred *);
 
 /* nfs_clstate.c */
 int nfscl_open(vnode_t, u_int8_t *, int, u_int32_t, int,
--- sys/fs/nfsclient/nfs_clrpcops.c.sav	2021-04-08 15:14:26.058756000 -0700
+++ sys/fs/nfsclient/nfs_clrpcops.c	2021-04-10 16:23:06.269087000 -0700
@@ -924,6 +924,8 @@ nfsrpc_setclient(struct nfsmount *nmp, struct nfsclcli
 	struct nfsclds *dsp;
 	struct in6_addr a6;
 	struct nfsclsession *tsep;
+	struct rpc_reconupcall recon;
+	struct nfscl_reconarg *rcp;
 
 	if (nfsboottime.tv_sec == 0)
 		NFSSETBOOTTIME(nfsboottime);
@@ -942,6 +944,23 @@ nfsrpc_setclient(struct nfsmount *nmp, struct nfsclcli
 			    &nmp->nm_sockreq,
 			    dsp->nfsclds_sess.nfsess_sequenceid, 1, cred, p);
 		if (error == 0) {
+			/*
+			 * If the session supports a backchannel, set up
+			 * the BindConnectionToSession call in the krpc
+			 * so that it is done on a reconnection.
+			 */
+			if (nfscl_enablecallb != 0 && nfs_numnfscbd > 0) {
+				rcp = mem_alloc(sizeof(*rcp));
+				rcp->minorvers = nmp->nm_minorvers;
+				memcpy(rcp->sessionid,
+				    dsp->nfsclds_sess.nfsess_sessionid,
+				    NFSX_V4SESSIONID);
+				recon.call = nfsrpc_bindconnsess;
+				recon.arg = rcp;
+				CLNT_CONTROL(nmp->nm_client, CLSET_RECONUPCALL,
+				    &recon);
+			}
+
 			NFSLOCKMNT(nmp);
 			/*
 			 * The old sessions cannot be safely free'd
@@ -5854,6 +5873,7 @@ nfsm_copym(struct mbuf *m, int off, int xfer)
 	return (m2);
 }
 
+
 /*
  * Find a file layout that will handle the first bytes of the requested
  * range and return the information from it needed to the I/O operation.
@@ -7678,5 +7698,76 @@ out:
 			*islockedp = 1;
 	}
 	return (laystat);
+}
+
+/*
+ * Do the NFSv4.1 Bind Connection to Session.
+ * Called from the reconnect layer of the krpc (sys/rpc/clnt_rc.c).
+ */
+void
+nfsrpc_bindconnsess(CLIENT *cl, void *arg, struct ucred *cr)
+{
+	struct nfscl_reconarg *rcp = (struct nfscl_reconarg *)arg;
+	uint32_t res, *tl;
+	struct nfsrv_descript nfsd;
+	struct nfsrv_descript *nd = &nfsd;
+	struct rpc_callextra ext;
+	struct timeval utimeout;
+	enum clnt_stat stat;
+	int error;
+
+	nfscl_reqstart(nd, NFSPROC_BINDCONNTOSESS, NULL, NULL, 0, NULL, NULL,
+	    NFS_VER4, rcp->minorvers);
+	NFSM_BUILD(tl, uint32_t *, NFSX_V4SESSIONID + 2 * NFSX_UNSIGNED);
+	memcpy(tl, rcp->sessionid, NFSX_V4SESSIONID);
+	tl += NFSX_V4SESSIONID / NFSX_UNSIGNED;
+	*tl++ = txdr_unsigned(NFSCDFC4_FORE_OR_BOTH);
+	*tl = newnfs_false;
+
+	memset(&ext, 0, sizeof(ext));
+	utimeout.tv_sec = 30;
+	utimeout.tv_usec = 0;
+	ext.rc_auth = authunix_create(cr);
+	nd->nd_mrep = NULL;
+	stat = CLNT_CALL_MBUF(cl, &ext, NFSV4PROC_COMPOUND, nd->nd_mreq,
+	    &nd->nd_mrep, utimeout);
+	AUTH_DESTROY(ext.rc_auth);
+	if (stat != RPC_SUCCESS) {
+		printf("nfsrpc_bindconnsess: call failed stat=%d\n", stat);
+		return;
+	}
+	if (nd->nd_mrep == NULL) {
+		printf("nfsrpc_bindconnsess: no reply args\n");
+		return;
+	}
+	error = 0;
+	newnfs_realign(&nd->nd_mrep, M_WAITOK);
+	nd->nd_md = nd->nd_mrep;
+	nd->nd_dpos = mtod(nd->nd_md, char *);
+	NFSM_DISSECT(tl, uint32_t *, 2 * NFSX_UNSIGNED);
+	nd->nd_repstat = fxdr_unsigned(uint32_t, *tl++);
+	if (nd->nd_repstat == NFSERR_OK) {
+		res = fxdr_unsigned(uint32_t, *tl);
+		if (res > 0 && (error = nfsm_advance(nd, NFSM_RNDUP(res),
+		    -1)) != 0)
+			goto nfsmout;
+		NFSM_DISSECT(tl, uint32_t *, NFSX_V4SESSIONID +
+		    4 * NFSX_UNSIGNED);
+		tl += 3;
+		if (!NFSBCMP(tl, rcp->sessionid, NFSX_V4SESSIONID)) {
+			tl += NFSX_V4SESSIONID / NFSX_UNSIGNED;
+			res = fxdr_unsigned(uint32_t, *tl);
+			if (res != NFSCDFS4_BOTH)
+				printf("nfsrpc_bindconnsess: did not "
+				    "return FS4_BOTH\n");
+		} else
+			printf("nfsrpc_bindconnsess: not same "
+			    "sessionid\n");
+	} else if (nd->nd_repstat != NFSERR_BADSESSION)
+		printf("nfsrpc_bindconnsess: returned %d\n", nd->nd_repstat);
+nfsmout:
+	if (error != 0)
+		printf("nfsrpc_bindconnsess: reply bad xdr\n");
+	m_freem(nd->nd_mrep);
 }
 
--- sys/rpc/clnt.h.sav	2021-04-08 15:21:58.584734000 -0700
+++ sys/rpc/clnt.h	2021-04-08 15:24:19.660301000 -0700
@@ -357,6 +357,12 @@ enum clnt_stat clnt_call_private(CLIENT *, struct rpc_
 #define CLSET_PRIVPORT		27	/* set privileged source port flag */
 #define CLGET_PRIVPORT		28	/* get privileged source port flag */
 #define CLSET_BACKCHANNEL	29	/* set backchannel for socket */
+/* Structure used as the argument for CLSET_RECONUPCALL. */
+struct rpc_reconupcall {
+	void	(*call)(CLIENT *, void *, struct ucred *);
+	void	*arg;
+};
+#define	CLSET_RECONUPCALL	30	/* Reconnect upcall */
 #endif
 
 
--- sys/rpc/krpc.h.sav	2021-04-08 15:22:10.346699000 -0700
+++ sys/rpc/krpc.h	2021-04-08 15:26:15.497461000 -0700
@@ -78,6 +78,9 @@ struct rc_data {
 	CLIENT*			rc_client; /* underlying RPC client */
 	struct rpc_err		rc_err;
 	void			*rc_backchannel;
+	void			(*rc_reconcall)(CLIENT *, void *,
+				    struct ucred *); /* reconection upcall */
+	void			*rc_reconarg;	/* upcall arg */
 };
 
 struct ct_data {
--- sys/rpc/clnt_rc.c.sav	2021-04-08 15:22:25.663647000 -0700
+++ sys/rpc/clnt_rc.c	2021-04-08 15:42:21.700777000 -0700
@@ -108,6 +108,8 @@ clnt_reconnect_create(
 	rc->rc_closed = FALSE;
 	rc->rc_ucred = crdup(curthread->td_ucred);
 	rc->rc_client = NULL;
+	rc->rc_reconcall = NULL;
+	rc->rc_reconarg = NULL;
 
 	cl->cl_refs = 1;
 	cl->cl_ops = &clnt_reconnect_ops;
@@ -193,6 +195,9 @@ clnt_reconnect_connect(CLIENT *cl)
 		newclient = clnt_vc_create(so,
 		    (struct sockaddr *) &rc->rc_addr, rc->rc_prog, rc->rc_vers,
 		    rc->rc_sendsz, rc->rc_recvsz, rc->rc_intr);
+		if (newclient != NULL && rc->rc_reconcall != NULL)
+			(*rc->rc_reconcall)(newclient, rc->rc_reconarg,
+			    rc->rc_ucred);
 	}
 	td->td_ucred = oldcred;
 
@@ -385,6 +390,7 @@ clnt_reconnect_control(CLIENT *cl, u_int request, void
 {
 	struct rc_data *rc = (struct rc_data *)cl->cl_private;
 	SVCXPRT *xprt;
+	struct rpc_reconupcall *upcp;
 
 	if (info == NULL) {
 		return (FALSE);
@@ -472,6 +478,12 @@ clnt_reconnect_control(CLIENT *cl, u_int request, void
 		rc->rc_backchannel = info;
 		break;
 
+	case CLSET_RECONUPCALL:
+		upcp = (struct rpc_reconupcall *)info;
+		rc->rc_reconcall = upcp->call;
+		rc->rc_reconarg = upcp->arg;
+		break;
+
 	default:
 		return (FALSE);
 	}
@@ -514,11 +526,14 @@ clnt_reconnect_destroy(CLIENT *cl)
 		CLNT_DESTROY(rc->rc_client);
 	if (rc->rc_backchannel) {
 		xprt = (SVCXPRT *)rc->rc_backchannel;
+		KASSERT(xprt->xp_socket == NULL,
+		    ("clnt_reconnect_destroy: xp_socket not NULL"));
 		xprt_unregister(xprt);
 		SVC_RELEASE(xprt);
 	}
 	crfree(rc->rc_ucred);
 	mtx_destroy(&rc->rc_lock);
+	mem_free(rc->rc_reconarg, 0);
 	mem_free(rc, sizeof(*rc));
 	mem_free(cl, sizeof (CLIENT));
 }