--- sys/fs/nfs/nfsport.h.bc	2021-04-06 00:52:37.070211000 -0700
+++ sys/fs/nfs/nfsport.h	2021-03-23 16:22:37.000000000 -0700
@@ -412,10 +412,13 @@
 #define	NFSPROC_RMEXTATTR	63
 #define	NFSPROC_LISTEXTATTR	64
 
+/* BindConnectionToSession, done by the krpc for a new connection. */
+#define	NFSPROC_BINDCONNTOSESS	65
+
 /*
  * Must be defined as one higher than the last NFSv4.2 Proc# above.
  */
-#define	NFSV42_NPROCS		65
+#define	NFSV42_NPROCS		66
 
 #endif	/* NFS_V3NPROCS */
 
@@ -444,7 +447,7 @@ struct nfsstatsv1 {
 	uint64_t	readlink_bios;
 	uint64_t	biocache_readdirs;
 	uint64_t	readdir_bios;
-	uint64_t	rpccnt[NFSV42_NPROCS + 15];
+	uint64_t	rpccnt[NFSV42_NPROCS + 14];
 	uint64_t	rpcretries;
 	uint64_t	srvrpccnt[NFSV42_NOPS + NFSV4OP_FAKENOPS + 15];
 	uint64_t	reserved_0;
@@ -509,7 +512,7 @@ struct nfsstatsov1 {
 	uint64_t	readlink_bios;
 	uint64_t	biocache_readdirs;
 	uint64_t	readdir_bios;
-	uint64_t	rpccnt[NFSV42_NPROCS + 4];
+	uint64_t	rpccnt[NFSV42_NPROCS + 3];
 	uint64_t	rpcretries;
 	uint64_t	srvrpccnt[NFSV42_PURENOPS + NFSV4OP_FAKENOPS];
 	uint64_t	reserved_0;
@@ -1006,6 +1009,7 @@ bool ncl_pager_setsize(struct vnode *vp, u_quad_t *nsi
  * "out by one" without disastrous consequences.
  */
 #define	NFSINCRGLOBAL(a)	((a)++)
+#define	NFSDECRGLOBAL(a)	((a)--)
 
 /*
  * Assorted funky stuff to make things work under Darwin8.
--- sys/fs/nfs/nfsproto.h.bc	2021-03-23 12:04:08.210255000 -0700
+++ sys/fs/nfs/nfsproto.h	2021-03-23 16:23:09.000000000 -0700
@@ -394,10 +394,13 @@
 #define	NFSPROC_RMEXTATTR	63
 #define	NFSPROC_LISTEXTATTR	64
 
+/* BindConnectionToSession, done by the krpc for a new connection. */
+#define	NFSPROC_BINDCONNTOSESS	65
+
 /*
  * Must be defined as one higher than the last NFSv4.2 Proc# above.
  */
-#define	NFSV42_NPROCS		65
+#define	NFSV42_NPROCS		66
 
 #endif	/* NFS_V3NPROCS */
 
--- sys/fs/nfs/nfs_var.h.bc	2021-03-23 12:04:21.163112000 -0700
+++ sys/fs/nfs/nfs_var.h	2021-04-03 10:54:08.575100000 -0700
@@ -561,6 +561,7 @@ int nfsrpc_listextattr(vnode_t, uint64_t *, struct uio
     struct nfsvattr *, int *, struct ucred *, NFSPROC_T *);
 int nfsrpc_rmextattr(vnode_t, const char *, struct nfsvattr *, int *,
     struct ucred *, NFSPROC_T *);
+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/nfs/nfs_commonsubs.c.bc	2021-03-23 12:04:41.516877000 -0700
+++ sys/fs/nfs/nfs_commonsubs.c	2021-04-05 07:04:40.804714000 -0700
@@ -218,7 +218,7 @@ static struct nfsrv_lughash	*nfsgroupnamehash;
 static int nfs_bigreply[NFSV42_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,
-    1, 0, 0, 1 };
+    1, 0, 0, 1, 0 };
 
 /* local functions */
 static int nfsrv_skipace(struct nfsrv_descript *nd, int *acesizep);
@@ -301,6 +301,7 @@ static struct {
 	{ NFSV4OP_SETXATTR, 2, "Setxattr", 8, },
 	{ NFSV4OP_REMOVEXATTR, 2, "Rmxattr", 7, },
 	{ NFSV4OP_LISTXATTRS, 2, "Listxattr", 9, },
+	{ NFSV4OP_BINDCONNTOSESS, 1, "BindConSess", 11, },
 };
 
 /*
@@ -309,7 +310,7 @@ static struct {
 static int nfs_bigrequest[NFSV42_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, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0
 };
 
 /*
--- sys/fs/nfs/nfscl.h.bc	2021-03-24 08:44:36.568914000 -0700
+++ sys/fs/nfs/nfscl.h	2021-03-24 13:04:14.000000000 -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/nfsclient/nfs_clrpcops.c.bc	2021-04-06 01:06:06.586782000 -0700
+++ sys/fs/nfsclient/nfs_clrpcops.c	2021-03-28 07:21:33.759492000 -0700
@@ -946,6 +946,8 @@ nfsrpc_setclient(struct nfsmount *nmp, struct nfsclcli
 	struct nfsclds *dsp, *odsp;
 	struct in6_addr a6;
 	struct nfsclsession *tsep;
+	struct rpc_reconupcall recon;
+	struct nfscl_reconarg *rcp;
 
 	if (nfsboottime.tv_sec == 0)
 		NFSSETBOOTTIME(nfsboottime);
@@ -1019,6 +1021,23 @@ nfsrpc_setclient(struct nfsmount *nmp, struct nfsclcli
 			NFSCL_DEBUG(1, "aft createsess=%d\n", error);
 		}
 		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
@@ -8709,3 +8728,75 @@ nfsm_split(struct mbuf *mp, uint64_t xfer)
 	m->m_next = NULL;
 	return (m2);
 }
+
+/*
+ * 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
+		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/krpc.h.bc	2021-03-23 12:02:30.993335000 -0700
+++ sys/rpc/krpc.h	2021-03-24 14:17:05.000000000 -0700
@@ -81,6 +81,9 @@ struct rc_data {
 	void			*rc_backchannel;
 	bool			rc_tls; /* Enable TLS on connection */
 	char			*rc_tlscertname;
+	void			(*rc_reconcall)(CLIENT *, void *,
+				    struct ucred *); /* reconection upcall */
+	void			*rc_reconarg;	/* upcall arg */
 };
 
 /* Bits for ct_rcvstate. */
--- sys/rpc/clnt.h.bc	2021-03-23 12:02:43.013133000 -0700
+++ sys/rpc/clnt.h	2021-03-25 05:42:01.000000000 -0700
@@ -360,6 +360,12 @@ enum clnt_stat clnt_call_private(CLIENT *, struct rpc_
 #define	CLSET_TLS		30	/* set TLS for socket */
 #define	CLSET_BLOCKRCV		31	/* Temporarily block reception */
 #define	CLSET_TLSCERTNAME	32	/* TLS certificate file name */
+/* Structure used as the argument for CLSET_RECONUPCALL. */
+struct rpc_reconupcall {
+	void	(*call)(CLIENT *, void *, struct ucred *);
+	void	*arg;
+};
+#define	CLSET_RECONUPCALL	33	/* Reconnect upcall */
 #endif
 
 
--- sys/rpc/clnt_rc.c.bc	2021-03-23 12:03:02.451346000 -0700
+++ sys/rpc/clnt_rc.c	2021-03-25 00:00:16.684092000 -0700
@@ -111,6 +111,8 @@ clnt_reconnect_create(
 	rc->rc_client = NULL;
 	rc->rc_tls = false;
 	rc->rc_tlscertname = NULL;
+	rc->rc_reconcall = NULL;
+	rc->rc_reconarg = NULL;
 
 	cl->cl_refs = 1;
 	cl->cl_ops = &clnt_reconnect_ops;
@@ -213,6 +215,9 @@ clnt_reconnect_connect(CLIENT *cl)
 				goto out;
 			}
 		}
+		if (newclient != NULL && rc->rc_reconcall != NULL)
+			(*rc->rc_reconcall)(newclient, rc->rc_reconarg,
+			    rc->rc_ucred);
 	}
 	td->td_ucred = oldcred;
 
@@ -408,6 +413,7 @@ clnt_reconnect_control(CLIENT *cl, u_int request, void
 	struct rc_data *rc = (struct rc_data *)cl->cl_private;
 	SVCXPRT *xprt;
 	size_t slen;
+	struct rpc_reconupcall *upcp;
 
 	if (info == NULL) {
 		return (FALSE);
@@ -513,6 +519,12 @@ clnt_reconnect_control(CLIENT *cl, u_int request, void
 		strlcpy(rc->rc_tlscertname, info, slen);
 		break;
 
+	case CLSET_RECONUPCALL:
+		upcp = (struct rpc_reconupcall *)info;
+		rc->rc_reconcall = upcp->call;
+		rc->rc_reconarg = upcp->arg;
+		break;
+
 	default:
 		return (FALSE);
 	}
@@ -555,12 +567,15 @@ 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_tlscertname, 0);	/* 0 ok, since arg. ignored. */
+	mem_free(rc->rc_reconarg, 0);
 	mem_free(rc, sizeof(*rc));
 	mem_free(cl, sizeof (CLIENT));
 }