diff --git a/lib/libjail/jail.c b/lib/libjail/jail.c
--- a/lib/libjail/jail.c
+++ b/lib/libjail/jail.c
@@ -75,8 +75,9 @@
 jail_setv(int flags, ...)
 {
 	va_list ap, tap;
-	struct jailparam *jp;
-	const char *name, *value;
+	struct jailparam *jp, *jp_desc;
+	const char *name;
+	char *value, *desc_value;
 	int njp, jid;
 
 	/* Create the parameter list and import the parameters. */
@@ -86,15 +87,24 @@
 		(void)va_arg(tap, char *);
 	va_end(tap);
 	jp = alloca(njp * sizeof(struct jailparam));
-	for (njp = 0; (name = va_arg(ap, char *)) != NULL;) {
+	jp_desc = NULL;
+	desc_value = NULL;
+	for (njp = 0; (name = va_arg(ap, char *)) != NULL; njp++) {
 		value = va_arg(ap, char *);
 		if (jailparam_init(jp + njp, name) < 0)
 			goto error;
-		if (jailparam_import(jp + njp++, value) < 0)
+		if (jailparam_import(jp + njp, value) < 0)
 			goto error;
+		if (!strcmp(name, "desc")
+		    && (flags & (JAIL_GET_DESC | JAIL_OWN_DESC))) {
+			jp_desc = jp + njp;
+			desc_value = value;
+		}
 	}
 	va_end(ap);
 	jid = jailparam_set(jp, njp, flags);
+	if (jid > 0 && jp_desc != NULL)
+		sprintf(desc_value, "%d", *(int *)jp_desc->jp_value);
 	jailparam_free(jp, njp);
 	return (jid);
 
@@ -112,9 +122,10 @@
 jail_getv(int flags, ...)
 {
 	va_list ap, tap;
-	struct jailparam *jp, *jp_lastjid, *jp_jid, *jp_name, *jp_key;
+	struct jailparam *jp, *jp_desc, *jp_lastjid, *jp_jid, *jp_name, *jp_key;
 	char *valarg, *value;
-	const char *name, *key_value, *lastjid_value, *jid_value, *name_value;
+	const char *name, *key_value, *desc_value, *lastjid_value, *jid_value;
+	const char *name_value;
 	int njp, i, jid;
 
 	/* Create the parameter list and find the key. */
@@ -126,15 +137,19 @@
 
 	jp = alloca(njp * sizeof(struct jailparam));
 	va_copy(tap, ap);
-	jp_lastjid = jp_jid = jp_name = NULL;
-	lastjid_value = jid_value = name_value = NULL;
+	jp_desc = jp_lastjid = jp_jid = jp_name = NULL;
+	desc_value = lastjid_value = jid_value = name_value = NULL;
 	for (njp = 0; (name = va_arg(tap, char *)) != NULL; njp++) {
 		value = va_arg(tap, char *);
 		if (jailparam_init(jp + njp, name) < 0) {
 			va_end(tap);
 			goto error;
 		}
-		if (!strcmp(jp[njp].jp_name, "lastjid")) {
+		if (!strcmp(jp[njp].jp_name, "desc")
+		    && (flags & (JAIL_USE_DESC | JAIL_AT_DESC))) {
+			jp_desc = jp + njp;
+			desc_value = value;
+		} else if (!strcmp(jp[njp].jp_name, "lastjid")) {
 			jp_lastjid = jp + njp;
 			lastjid_value = value;
 		} else if (!strcmp(jp[njp].jp_name, "jid")) {
@@ -147,7 +162,10 @@
 	}
 	va_end(tap);
 	/* Import the key parameter. */
-	if (jp_lastjid != NULL) {
+	if (jp_desc != NULL && (flags & JAIL_USE_DESC)) {
+		jp_key = jp_desc;
+		key_value = desc_value;
+	} else if (jp_lastjid != NULL) {
 		jp_key = jp_lastjid;
 		key_value = lastjid_value;
 	} else if (jp_jid != NULL && strtol(jid_value, NULL, 10) != 0) {
@@ -163,6 +181,9 @@
 	}
 	if (jailparam_import(jp_key, key_value) < 0)
 		goto error;
+	if (jp_desc != NULL && jp_desc != jp_key
+	    && jailparam_import(jp_desc, desc_value) < 0)
+		goto error;
 	/* Get the jail and export the parameters. */
 	jid = jailparam_get(jp, njp, flags);
 	if (jid < 0)
@@ -571,7 +592,7 @@
 jailparam_get(struct jailparam *jp, unsigned njp, int flags)
 {
 	struct iovec *jiov;
-	struct jailparam *jp_lastjid, *jp_jid, *jp_name, *jp_key;
+	struct jailparam *jp_desc, *jp_lastjid, *jp_jid, *jp_name, *jp_key;
 	int i, ai, ki, jid, arrays, sanity;
 	unsigned j;
 
@@ -580,10 +601,13 @@
 	 * Find the key and any array parameters.
 	 */
 	jiov = alloca(sizeof(struct iovec) * 2 * (njp + 1));
-	jp_lastjid = jp_jid = jp_name = NULL;
+	jp_desc = jp_lastjid = jp_jid = jp_name = NULL;
 	arrays = 0;
 	for (ai = j = 0; j < njp; j++) {
-		if (!strcmp(jp[j].jp_name, "lastjid"))
+		if (!strcmp(jp[j].jp_name, "desc")
+		    && (flags & (JAIL_USE_DESC | JAIL_AT_DESC)))
+			jp_desc = jp + j;
+		else if (!strcmp(jp[j].jp_name, "lastjid"))
 			jp_lastjid = jp + j;
 		else if (!strcmp(jp[j].jp_name, "jid"))
 			jp_jid = jp + j;
@@ -599,7 +623,9 @@
 			ai++;
 		}
 	}
-	jp_key = jp_lastjid ? jp_lastjid :
+	jp_key = jp_desc && jp_desc->jp_valuelen == sizeof(int) &&
+	    jp_desc->jp_value && (flags & JAIL_USE_DESC) ? jp_desc :
+	    jp_lastjid ? jp_lastjid :
 	    jp_jid && jp_jid->jp_valuelen == sizeof(int) &&
 	    jp_jid->jp_value && *(int *)jp_jid->jp_value ? jp_jid : jp_name;
 	if (jp_key == NULL || jp_key->jp_value == NULL) {
@@ -622,6 +648,14 @@
 	jiov[ki].iov_len = JAIL_ERRMSGLEN;
 	ki++;
 	jail_errmsg[0] = 0;
+	if (jp_desc != NULL && jp_desc != jp_key) {
+		jiov[ki].iov_base = jp_desc->jp_name;
+		jiov[ki].iov_len = strlen(jp_desc->jp_name) + 1;
+		ki++;
+		jiov[ki].iov_base = jp_desc->jp_value;
+		jiov[ki].iov_len = jp_desc->jp_valuelen;
+		ki++;
+	}
 	if (arrays && jail_get(jiov, ki, flags) < 0) {
 		if (!jail_errmsg[0])
 			snprintf(jail_errmsg, sizeof(jail_errmsg),
@@ -649,7 +683,7 @@
 			jiov[ai].iov_base = jp[j].jp_value;
 			memset(jiov[ai].iov_base, 0, jiov[ai].iov_len);
 			ai++;
-		} else if (jp + j != jp_key) {
+		} else if (jp + j != jp_key && jp + j != jp_desc) {
 			jiov[i].iov_base = jp[j].jp_name;
 			jiov[i].iov_len = strlen(jp[j].jp_name) + 1;
 			i++;
diff --git a/lib/libsys/Symbol.sys.map b/lib/libsys/Symbol.sys.map
--- a/lib/libsys/Symbol.sys.map
+++ b/lib/libsys/Symbol.sys.map
@@ -383,6 +383,8 @@
 	getrlimitusage;
 	inotify_add_watch_at;
 	inotify_rm_watch;
+	jail_attach_jd;
+	jail_remove_jd;
 	kcmp;
 	setcred;
 };
diff --git a/lib/libsys/_libsys.h b/lib/libsys/_libsys.h
--- a/lib/libsys/_libsys.h
+++ b/lib/libsys/_libsys.h
@@ -468,6 +468,8 @@
 typedef int (__sys_exterrctl_t)(u_int, u_int, void *);
 typedef int (__sys_inotify_add_watch_at_t)(int, int, const char *, uint32_t);
 typedef int (__sys_inotify_rm_watch_t)(int, int);
+typedef int (__sys_jail_attach_jd_t)(int);
+typedef int (__sys_jail_remove_jd_t)(int);
 
 void __sys_exit(int rval);
 int __sys_fork(void);
@@ -872,6 +874,8 @@
 int __sys_exterrctl(u_int op, u_int flags, void * ptr);
 int __sys_inotify_add_watch_at(int fd, int dfd, const char * path, uint32_t mask);
 int __sys_inotify_rm_watch(int fd, int wd);
+int __sys_jail_attach_jd(int fd);
+int __sys_jail_remove_jd(int fd);
 __END_DECLS
 
 #endif /* __LIBSYS_H_ */
diff --git a/lib/libsys/syscalls.map b/lib/libsys/syscalls.map
--- a/lib/libsys/syscalls.map
+++ b/lib/libsys/syscalls.map
@@ -813,4 +813,8 @@
 	__sys_inotify_add_watch_at;
 	_inotify_rm_watch;
 	__sys_inotify_rm_watch;
+	_jail_attach_jd;
+	__sys_jail_attach_jd;
+	_jail_remove_jd;
+	__sys_jail_remove_jd;
 };
diff --git a/sys/compat/freebsd32/freebsd32_syscall.h b/sys/compat/freebsd32/freebsd32_syscall.h
--- a/sys/compat/freebsd32/freebsd32_syscall.h
+++ b/sys/compat/freebsd32/freebsd32_syscall.h
@@ -513,4 +513,6 @@
 #define	FREEBSD32_SYS_exterrctl	592
 #define	FREEBSD32_SYS_inotify_add_watch_at	593
 #define	FREEBSD32_SYS_inotify_rm_watch	594
-#define	FREEBSD32_SYS_MAXSYSCALL	595
+#define	FREEBSD32_SYS_jail_attach_jd	595
+#define	FREEBSD32_SYS_jail_remove_jd	596
+#define	FREEBSD32_SYS_MAXSYSCALL	597
diff --git a/sys/compat/freebsd32/freebsd32_syscalls.c b/sys/compat/freebsd32/freebsd32_syscalls.c
--- a/sys/compat/freebsd32/freebsd32_syscalls.c
+++ b/sys/compat/freebsd32/freebsd32_syscalls.c
@@ -600,4 +600,6 @@
 	"exterrctl",			/* 592 = exterrctl */
 	"inotify_add_watch_at",			/* 593 = inotify_add_watch_at */
 	"inotify_rm_watch",			/* 594 = inotify_rm_watch */
+	"jail_attach_jd",			/* 595 = jail_attach_jd */
+	"jail_remove_jd",			/* 596 = jail_remove_jd */
 };
diff --git a/sys/compat/freebsd32/freebsd32_sysent.c b/sys/compat/freebsd32/freebsd32_sysent.c
--- a/sys/compat/freebsd32/freebsd32_sysent.c
+++ b/sys/compat/freebsd32/freebsd32_sysent.c
@@ -662,4 +662,6 @@
 	{ .sy_narg = AS(exterrctl_args), .sy_call = (sy_call_t *)sys_exterrctl, .sy_auevent = AUE_NULL, .sy_flags = SYF_CAPENABLED, .sy_thrcnt = SY_THR_STATIC },	/* 592 = exterrctl */
 	{ .sy_narg = AS(inotify_add_watch_at_args), .sy_call = (sy_call_t *)sys_inotify_add_watch_at, .sy_auevent = AUE_INOTIFY, .sy_flags = SYF_CAPENABLED, .sy_thrcnt = SY_THR_STATIC },	/* 593 = inotify_add_watch_at */
 	{ .sy_narg = AS(inotify_rm_watch_args), .sy_call = (sy_call_t *)sys_inotify_rm_watch, .sy_auevent = AUE_INOTIFY, .sy_flags = SYF_CAPENABLED, .sy_thrcnt = SY_THR_STATIC },	/* 594 = inotify_rm_watch */
+	{ .sy_narg = AS(jail_attach_jd_args), .sy_call = (sy_call_t *)sys_jail_attach_jd, .sy_auevent = AUE_JAIL_ATTACH, .sy_flags = 0, .sy_thrcnt = SY_THR_STATIC },	/* 595 = jail_attach_jd */
+	{ .sy_narg = AS(jail_remove_jd_args), .sy_call = (sy_call_t *)sys_jail_remove_jd, .sy_auevent = AUE_JAIL_REMOVE, .sy_flags = 0, .sy_thrcnt = SY_THR_STATIC },	/* 596 = jail_remove_jd */
 };
diff --git a/sys/compat/freebsd32/freebsd32_systrace_args.c b/sys/compat/freebsd32/freebsd32_systrace_args.c
--- a/sys/compat/freebsd32/freebsd32_systrace_args.c
+++ b/sys/compat/freebsd32/freebsd32_systrace_args.c
@@ -3413,6 +3413,20 @@
 		*n_args = 2;
 		break;
 	}
+	/* jail_attach_jd */
+	case 595: {
+		struct jail_attach_jd_args *p = params;
+		iarg[a++] = p->fd; /* int */
+		*n_args = 1;
+		break;
+	}
+	/* jail_remove_jd */
+	case 596: {
+		struct jail_remove_jd_args *p = params;
+		iarg[a++] = p->fd; /* int */
+		*n_args = 1;
+		break;
+	}
 	default:
 		*n_args = 0;
 		break;
@@ -9222,6 +9236,26 @@
 			break;
 		};
 		break;
+	/* jail_attach_jd */
+	case 595:
+		switch (ndx) {
+		case 0:
+			p = "int";
+			break;
+		default:
+			break;
+		};
+		break;
+	/* jail_remove_jd */
+	case 596:
+		switch (ndx) {
+		case 0:
+			p = "int";
+			break;
+		default:
+			break;
+		};
+		break;
 	default:
 		break;
 	};
@@ -11130,6 +11164,16 @@
 		if (ndx == 0 || ndx == 1)
 			p = "int";
 		break;
+	/* jail_attach_jd */
+	case 595:
+		if (ndx == 0 || ndx == 1)
+			p = "int";
+		break;
+	/* jail_remove_jd */
+	case 596:
+		if (ndx == 0 || ndx == 1)
+			p = "int";
+		break;
 	default:
 		break;
 	};
diff --git a/sys/conf/files b/sys/conf/files
--- a/sys/conf/files
+++ b/sys/conf/files
@@ -3804,6 +3804,7 @@
 kern/kern_idle.c		standard
 kern/kern_intr.c		standard
 kern/kern_jail.c		standard
+kern/kern_jaildesc.c		standard
 kern/kern_jailmeta.c		standard
 kern/kern_kcov.c		optional kcov			\
 	compile-with "${NOSAN_C} ${MSAN_CFLAGS}"
diff --git a/sys/kern/init_sysent.c b/sys/kern/init_sysent.c
--- a/sys/kern/init_sysent.c
+++ b/sys/kern/init_sysent.c
@@ -661,4 +661,6 @@
 	{ .sy_narg = AS(exterrctl_args), .sy_call = (sy_call_t *)sys_exterrctl, .sy_auevent = AUE_NULL, .sy_flags = SYF_CAPENABLED, .sy_thrcnt = SY_THR_STATIC },	/* 592 = exterrctl */
 	{ .sy_narg = AS(inotify_add_watch_at_args), .sy_call = (sy_call_t *)sys_inotify_add_watch_at, .sy_auevent = AUE_INOTIFY, .sy_flags = SYF_CAPENABLED, .sy_thrcnt = SY_THR_STATIC },	/* 593 = inotify_add_watch_at */
 	{ .sy_narg = AS(inotify_rm_watch_args), .sy_call = (sy_call_t *)sys_inotify_rm_watch, .sy_auevent = AUE_INOTIFY, .sy_flags = SYF_CAPENABLED, .sy_thrcnt = SY_THR_STATIC },	/* 594 = inotify_rm_watch */
+	{ .sy_narg = AS(jail_attach_jd_args), .sy_call = (sy_call_t *)sys_jail_attach_jd, .sy_auevent = AUE_JAIL_ATTACH, .sy_flags = 0, .sy_thrcnt = SY_THR_STATIC },	/* 595 = jail_attach_jd */
+	{ .sy_narg = AS(jail_remove_jd_args), .sy_call = (sy_call_t *)sys_jail_remove_jd, .sy_auevent = AUE_JAIL_REMOVE, .sy_flags = 0, .sy_thrcnt = SY_THR_STATIC },	/* 596 = jail_remove_jd */
 };
diff --git a/sys/kern/kern_descrip.c b/sys/kern/kern_descrip.c
--- a/sys/kern/kern_descrip.c
+++ b/sys/kern/kern_descrip.c
@@ -5250,6 +5250,8 @@
 		return ("eventfd");
 	case DTYPE_TIMERFD:
 		return ("timerfd");
+	case DTYPE_JAILDESC:
+		return ("jail");
 	default:
 		return ("unkn");
 	}
diff --git a/sys/kern/kern_event.c b/sys/kern/kern_event.c
--- a/sys/kern/kern_event.c
+++ b/sys/kern/kern_event.c
@@ -365,6 +365,7 @@
 	[~EVFILT_USER] = { &user_filtops, 1 },
 	[~EVFILT_SENDFILE] = { &null_filtops },
 	[~EVFILT_EMPTY] = { &file_filtops, 1 },
+	[~EVFILT_JAILDESC] = { &file_filtops, 1 },
 };
 
 /*
diff --git a/sys/kern/kern_jail.c b/sys/kern/kern_jail.c
--- a/sys/kern/kern_jail.c
+++ b/sys/kern/kern_jail.c
@@ -39,6 +39,7 @@
 #include <sys/kernel.h>
 #include <sys/systm.h>
 #include <sys/errno.h>
+#include <sys/file.h>
 #include <sys/sysproto.h>
 #include <sys/malloc.h>
 #include <sys/osd.h>
@@ -48,6 +49,7 @@
 #include <sys/taskqueue.h>
 #include <sys/fcntl.h>
 #include <sys/jail.h>
+#include <sys/jaildesc.h>
 #include <sys/linker.h>
 #include <sys/lock.h>
 #include <sys/mman.h>
@@ -979,6 +981,8 @@
 int
 kern_jail_set(struct thread *td, struct uio *optuio, int flags)
 {
+	struct file *jfp_out;
+	struct jaildesc *desc_in;
 	struct nameidata nd;
 #ifdef INET
 	struct prison_ip *ip4;
@@ -989,6 +993,7 @@
 	struct vfsopt *opt;
 	struct vfsoptlist *opts;
 	struct prison *pr, *deadpr, *dinspr, *inspr, *mypr, *ppr, *tpr;
+	struct ucred *jdcred;
 	struct vnode *root;
 	char *domain, *errmsg, *host, *name, *namelc, *p, *path, *uuid;
 	char *g_path, *osrelstr;
@@ -1002,7 +1007,7 @@
 	int created, cuflags, descend, drflags, enforce;
 	int error, errmsg_len, errmsg_pos;
 	int gotchildmax, gotenforce, gothid, gotrsnum, gotslevel;
-	int deadid, jid, jsys, len, level;
+	int deadid, jfd_in, jfd_out, jfd_pos, jid, jsys, len, level;
 	int childmax, osreldt, rsnum, slevel;
 #ifdef INET
 	int ip4s;
@@ -1017,17 +1022,26 @@
 	unsigned tallow;
 	char numbuf[12];
 
-	error = priv_check(td, PRIV_JAIL_SET);
-	if (!error && (flags & JAIL_ATTACH))
-		error = priv_check(td, PRIV_JAIL_ATTACH);
-	if (error)
-		return (error);
 	mypr = td->td_ucred->cr_prison;
-	if ((flags & JAIL_CREATE) && mypr->pr_childmax == 0)
+	if (((flags & (JAIL_CREATE | JAIL_AT_DESC)) == JAIL_CREATE)
+	    && mypr->pr_childmax == 0)
 		return (EPERM);
 	if (flags & ~JAIL_SET_MASK)
 		return (EINVAL);
+	if ((flags & (JAIL_USE_DESC | JAIL_AT_DESC))
+	    == (JAIL_USE_DESC | JAIL_AT_DESC))
+		return (EINVAL);
+	prison_hold(mypr);
 
+#ifdef INET
+	ip4 = NULL;
+#endif
+#ifdef INET6
+	ip6 = NULL;
+#endif
+	g_path = NULL;
+	jfp_out = NULL;
+	jfd_out = -1;
 	/*
 	 * Check all the parameters before committing to anything.  Not all
 	 * errors can be caught early, but we may as well try.  Also, this
@@ -1040,14 +1054,7 @@
 	 */
 	error = vfs_buildopts(optuio, &opts);
 	if (error)
-		return (error);
-#ifdef INET
-	ip4 = NULL;
-#endif
-#ifdef INET6
-	ip6 = NULL;
-#endif
-	g_path = NULL;
+		goto done_free;
 
 	cuflags = flags & (JAIL_CREATE | JAIL_UPDATE);
 	if (!cuflags) {
@@ -1056,6 +1063,72 @@
 		goto done_errmsg;
 	}
 
+	error = vfs_copyopt(opts, "desc", &jfd_in, sizeof(jfd_in));
+	if (error == ENOENT) {
+		if (flags & (JAIL_USE_DESC | JAIL_AT_DESC | JAIL_GET_DESC |
+		    JAIL_OWN_DESC)) {
+			vfs_opterror(opts, "missing desc");
+			goto done_errmsg;
+		}
+		jfd_in = -1;
+	} else if (error != 0)
+		goto done_free;
+	else {
+		if (!(flags & (JAIL_USE_DESC | JAIL_AT_DESC | JAIL_GET_DESC |
+		    JAIL_OWN_DESC))) {
+			vfs_opterror(opts, "unexpected desc");
+			goto done_errmsg;
+		}
+		if (flags & JAIL_AT_DESC) {
+			/*
+			 * Look up and create jails based on the
+			 * descriptor's prison.
+			 */
+			prison_free(mypr);
+			error = jaildesc_find(td, jfd_in, &desc_in, &mypr,
+			    NULL);
+			if (error != 0) {
+				vfs_opterror(opts, error == ENOENT
+				    ? "descriptor to dead jail"
+				    : "not a jail descriptor");
+				goto done_errmsg;
+			}
+			/*
+			 * Check file permissions using the current
+			 * credentials, and operation permissions
+			 * using the descriptor's credentials.
+			 */
+			error = vaccess(VREG, desc_in->jd_mode, desc_in->jd_uid,
+			    desc_in->jd_gid, VEXEC, td->td_ucred);
+			JAILDESC_UNLOCK(desc_in);
+			if (error != 0)
+				goto done_free;
+			if ((flags & JAIL_CREATE) && mypr->pr_childmax == 0) {
+				error = EPERM;
+				goto done_free;
+			}
+		}
+		if (flags & (JAIL_GET_DESC | JAIL_OWN_DESC)) {
+			/* Allocate a jail descriptor to return later. */
+			error = jaildesc_alloc(td, &jfp_out, &jfd_out,
+				flags & JAIL_OWN_DESC);
+			if (error)
+				goto done_free;
+		}
+	}
+
+	/*
+	 * Delay the permission check if using a jail descriptor,
+	 * until we get the descriptor's credentials.
+	 */
+	if (!(flags & JAIL_USE_DESC)) {
+		error = priv_check(td, PRIV_JAIL_SET);
+		if (error == 0 && (flags & JAIL_ATTACH))
+			error = priv_check(td, PRIV_JAIL_ATTACH);
+		if (error)
+			goto done_free;
+	}
+
 	error = vfs_copyopt(opts, "jid", &jid, sizeof(jid));
 	if (error == ENOENT)
 		jid = 0;
@@ -1430,7 +1503,57 @@
 		error = EAGAIN;
 		goto done_deref;
 	}
-	if (jid != 0) {
+	if (flags & JAIL_USE_DESC) {
+		/* Get the jail from its descriptor. */
+		error = jaildesc_find(td, jfd_in, &desc_in, &pr, &jdcred);
+		if (error) {
+			vfs_opterror(opts, error == ENOENT
+			    ? "descriptor to dead jail"
+			    : "not a jail descriptor");
+			goto done_deref;
+		}
+		drflags |= PD_DEREF;
+		/*
+		 * Check file permissions using the current credentials,
+		 * and operation permissions using the descriptor's
+		 * credentials.
+		 */
+		error = vaccess(VREG, desc_in->jd_mode, desc_in->jd_uid,
+		    desc_in->jd_gid, VWRITE, td->td_ucred);
+		if (error == 0 && (flags & JAIL_ATTACH))
+			error = vaccess(VREG, desc_in->jd_mode, desc_in->jd_uid,
+			    desc_in->jd_gid, VEXEC, td->td_ucred);
+		JAILDESC_UNLOCK(desc_in);
+		if (error == 0)
+			error = priv_check_cred(jdcred, PRIV_JAIL_SET);
+		if (error == 0 && (flags & JAIL_ATTACH))
+			error = priv_check_cred(jdcred, PRIV_JAIL_ATTACH);
+		crfree(jdcred);
+		if (error)
+			goto done_deref;
+		mtx_lock(&pr->pr_mtx);
+		drflags |= PD_LOCKED;
+		if (cuflags == JAIL_CREATE) {
+			error = EEXIST;
+			vfs_opterror(opts, "jail %d already exists",
+			    pr->pr_id);
+			goto done_deref;
+		}
+		if (!prison_isalive(pr)) {
+			/* While a jid can be resurrected, the prison
+			 * itself cannot.
+			 */
+			error = ENOENT;
+			vfs_opterror(opts, "jail %d is dying", pr->pr_id);
+			goto done_deref;
+		}
+		if (jid != 0 && jid != pr->pr_id) {
+			error = EINVAL;
+			vfs_opterror(opts, "cannot change jid");
+			goto done_deref;
+		}
+		jid = pr->pr_id;
+	} else if (jid != 0) {
 		if (jid < 0) {
 			error = EINVAL;
 			vfs_opterror(opts, "negative jid");
@@ -1564,7 +1687,7 @@
 			}
 		}
 	}
-	/* Update: must provide a jid or name. */
+	/* Update: must provide a desc, jid, or name. */
 	else if (cuflags == JAIL_UPDATE && pr == NULL) {
 		error = ENOENT;
 		vfs_opterror(opts, "update specified no jail");
@@ -1716,8 +1839,10 @@
 		 * Grab a reference for existing prisons, to ensure they
 		 * continue to exist for the duration of the call.
 		 */
-		prison_hold(pr);
-		drflags |= PD_DEREF;
+		if (!(drflags & PD_DEREF)) {
+			prison_hold(pr);
+			drflags |= PD_DEREF;
+		}
 #if defined(VIMAGE) && (defined(INET) || defined(INET6))
 		if ((pr->pr_flags & PR_VNET) &&
 		    (ch_flags & (PR_IP4_USER | PR_IP6_USER))) {
@@ -2108,7 +2233,9 @@
 	if (created) {
 		drflags = prison_lock_xlock(pr, drflags);
 		pr->pr_state = PRISON_STATE_ALIVE;
-	}
+		jaildesc_note(ppr, NOTE_JAIL_CREATE, pr->pr_id);
+	} else
+		jaildesc_note(pr, NOTE_JAIL_SET, 0);
 
 	/* Attach this process to the prison if requested. */
 	if (flags & JAIL_ATTACH) {
@@ -2140,6 +2267,31 @@
 		printf("Warning jail jid=%d: mountd/nfsd requires a separate"
 		   " file system\n", pr->pr_id);
 
+	/*
+	 * Now that the prison is fully created without error, set the
+	 * jail descriptor if one was requested.  This is the only
+	 * parameter that is returned to the caller (except the error
+	 * message).
+	 */
+	if (jfd_out >= 0) {
+		if (!(drflags & PD_LOCKED)) {
+			mtx_lock(&pr->pr_mtx);
+			drflags |= PD_LOCKED;
+		}
+		jfd_pos = 2 * vfs_getopt_pos(opts, "desc") + 1;
+		if (optuio->uio_segflg == UIO_SYSSPACE)
+			*(int*)optuio->uio_iov[jfd_pos].iov_base = jfd_out;
+		else
+			(void)copyout(&jfd_out,
+			    optuio->uio_iov[jfd_pos].iov_base, sizeof(jfd_out));
+		jaildesc_set_prison(jfp_out, pr);
+	}
+	/* Let kinfo listeners know about the prison. */
+	if (created)
+		jaildesc_note(ppr, NOTE_JAIL_CREATE, pr->pr_id);
+	else
+		jaildesc_note(pr, NOTE_JAIL_SET, 0);
+
 	drflags &= ~PD_KILL;
 	td->td_retval[0] = pr->pr_id;
 
@@ -2170,15 +2322,21 @@
 		}
 	}
  done_free:
+	/* Clean up other resources. */
 #ifdef INET
 	prison_ip_free(ip4);
 #endif
 #ifdef INET6
 	prison_ip_free(ip6);
 #endif
+	if (jfp_out != NULL)
+		fdrop(jfp_out, td);
+	if (error && jfd_out >= 0)
+		(void)kern_close(td, jfd_out);
 	if (g_path != NULL)
 		free(g_path, M_TEMP);
 	vfs_freeopts(opts);
+	prison_free(mypr);
 	return (error);
 }
 
@@ -2323,16 +2481,22 @@
 kern_jail_get(struct thread *td, struct uio *optuio, int flags)
 {
 	struct bool_flags *bf;
+	struct file *jfp_out;
+	struct jaildesc *desc_in;
 	struct jailsys_flags *jsf;
 	struct prison *pr, *mypr;
 	struct vfsopt *opt;
 	struct vfsoptlist *opts;
 	char *errmsg, *name;
 	int drflags, error, errmsg_len, errmsg_pos, i, jid, len, pos;
+	int jfd_in, jfd_out;
 	unsigned f;
 
 	if (flags & ~JAIL_GET_MASK)
 		return (EINVAL);
+	if ((flags & (JAIL_USE_DESC | JAIL_AT_DESC))
+	    == (JAIL_USE_DESC | JAIL_AT_DESC))
+		return (EINVAL);
 
 	/* Get the parameter list. */
 	error = vfs_buildopts(optuio, &opts);
@@ -2340,13 +2504,81 @@
 		return (error);
 	errmsg_pos = vfs_getopt_pos(opts, "errmsg");
 	mypr = td->td_ucred->cr_prison;
+	prison_hold(mypr);
 	pr = NULL;
+	jfp_out = NULL;
+	jfd_out = -1;
 
 	/*
-	 * Find the prison specified by one of: lastjid, jid, name.
+	 * Find the prison specified by one of: desc, lastjid, jid, name.
 	 */
 	sx_slock(&allprison_lock);
 	drflags = PD_LIST_SLOCKED;
+
+	error = vfs_copyopt(opts, "desc", &jfd_in, sizeof(jfd_in));
+	if (error == ENOENT) {
+		if (flags & (JAIL_AT_DESC | JAIL_GET_DESC | JAIL_OWN_DESC)) {
+			vfs_opterror(opts, "missing desc");
+			goto done;
+		}
+	} else if (error == 0) {
+		if (!(flags & (JAIL_USE_DESC | JAIL_AT_DESC | JAIL_GET_DESC |
+		    JAIL_OWN_DESC))) {
+			vfs_opterror(opts, "unexpected desc");
+			goto done;
+		}
+		if (flags & JAIL_USE_DESC) {
+			/* Get the jail from its descriptor. */
+			error = jaildesc_find(td, jfd_in, &desc_in, &pr, NULL);
+			if (error) {
+				vfs_opterror(opts, error == ENOENT
+				    ? "descriptor to dead jail"
+				    : "not a jail descriptor");
+				goto done;
+			}
+			drflags |= PD_DEREF;
+			error = vaccess(VREG, desc_in->jd_mode, desc_in->jd_uid,
+			    desc_in->jd_gid, VREAD, td->td_ucred);
+			JAILDESC_UNLOCK(desc_in);
+			if (error != 0)
+				goto done;
+			mtx_lock(&pr->pr_mtx);
+			drflags |= PD_LOCKED;
+			if (!(prison_isalive(pr) || (flags & JAIL_DYING))) {
+				error = ENOENT;
+				vfs_opterror(opts, "jail %d is dying",
+				    pr->pr_id);
+				goto done;
+			}
+			goto found_prison;
+		}
+		if (flags & JAIL_AT_DESC) {
+			/* Look up jails based on the descriptor's prison. */
+			prison_free(mypr);
+			error = jaildesc_find(td, jfd_in, &desc_in, &mypr,
+			    NULL);
+			if (error != 0) {
+				vfs_opterror(opts, error == ENOENT
+				    ? "descriptor to dead jail"
+				    : "not a jail descriptor");
+				goto done;
+			}
+			error = vaccess(VREG, desc_in->jd_mode, desc_in->jd_uid,
+			    desc_in->jd_gid, VEXEC, td->td_ucred);
+			JAILDESC_UNLOCK(desc_in);
+			if (error != 0)
+				goto done;
+		}
+		if (flags & (JAIL_GET_DESC | JAIL_OWN_DESC)) {
+			/* Allocate a jail descriptor to return later. */
+			error = jaildesc_alloc(td, &jfp_out, &jfd_out,
+				flags & JAIL_OWN_DESC);
+			if (error)
+				goto done;
+		}
+	} else
+		goto done;
+
 	error = vfs_copyopt(opts, "lastjid", &jid, sizeof(jid));
 	if (error == 0) {
 		TAILQ_FOREACH(pr, &allprison, pr_list) {
@@ -2415,9 +2647,17 @@
 
  found_prison:
 	/* Get the parameters of the prison. */
-	prison_hold(pr);
-	drflags |= PD_DEREF;
+	if (!(drflags & PD_DEREF)) {
+		prison_hold(pr);
+		drflags |= PD_DEREF;
+	}
 	td->td_retval[0] = pr->pr_id;
+	if (jfd_out >= 0) {
+		error = vfs_setopt(opts, "desc", &jfd_out, sizeof(jfd_out));
+		if (error != 0 && error != ENOENT)
+			goto done;
+		jaildesc_set_prison(jfp_out, pr);
+	}
 	error = vfs_setopt(opts, "jid", &pr->pr_id, sizeof(pr->pr_id));
 	if (error != 0 && error != ENOENT)
 		goto done;
@@ -2597,6 +2837,13 @@
 		prison_deref(pr, drflags);
 	else if (drflags & PD_LIST_SLOCKED)
 		sx_sunlock(&allprison_lock);
+	else if (drflags & PD_LIST_XLOCKED)
+		sx_xunlock(&allprison_lock);
+	/* Clean up other resources. */
+	if (jfp_out != NULL)
+		(void)fdrop(jfp_out, td);
+	if (error && jfd_out >= 0)
+		(void)kern_close(td, jfd_out);
 	if (error && errmsg_pos >= 0) {
 		/* Write the error message back to userspace. */
 		vfs_getopt(opts, "errmsg", (void **)&errmsg, &errmsg_len);
@@ -2613,6 +2860,7 @@
 		}
 	}
 	vfs_freeopts(opts);
+	prison_free(mypr);
 	return (error);
 }
 
@@ -2637,14 +2885,54 @@
 		sx_xunlock(&allprison_lock);
 		return (EINVAL);
 	}
+	prison_hold(pr);
+	prison_remove(pr);
+	return (0);
+}
+
+/*
+ * struct jail_remove_jd_args {
+ *	int fd;
+ * };
+ */
+int
+sys_jail_remove_jd(struct thread *td, struct jail_remove_jd_args *uap)
+{
+	struct prison *pr;
+	struct ucred *jdcred;
+	int error;
+
+	error = jaildesc_find(td, uap->fd, NULL, &pr, &jdcred);
+	if (error)
+		return (error);
+	error = priv_check_cred(jdcred, PRIV_JAIL_REMOVE);
+	crfree(jdcred);
+	if (error) {
+		prison_free(pr);
+		return (error);
+	}
+	sx_xlock(&allprison_lock);
+	mtx_lock(&pr->pr_mtx);
+	prison_remove(pr);
+	return (0);
+}
+
+/*
+ * Begin the removal process for a prison.  The allprison lock should
+ * be held exclusively, and the prison should be both locked and held.
+ */
+void
+prison_remove(struct prison *pr)
+{
+	sx_assert(&allprison_lock, SA_XLOCKED);
+	mtx_assert(&pr->pr_mtx, MA_OWNED);
 	if (!prison_isalive(pr)) {
 		/* Silently ignore already-dying prisons. */
 		mtx_unlock(&pr->pr_mtx);
 		sx_xunlock(&allprison_lock);
-		return (0);
+		return;
 	}
-	prison_deref(pr, PD_KILL | PD_LOCKED | PD_LIST_XLOCKED);
-	return (0);
+	prison_deref(pr, PD_KILL | PD_DEREF | PD_LOCKED | PD_LIST_XLOCKED);
 }
 
 /*
@@ -2679,6 +2967,53 @@
 	return (do_jail_attach(td, pr, PD_LOCKED | PD_LIST_SLOCKED));
 }
 
+/*
+ * struct jail_attach_jd_args {
+ *	int fd;
+ * };
+ */
+int
+sys_jail_attach_jd(struct thread *td, struct jail_attach_jd_args *uap)
+{
+	struct jaildesc *jd;
+	struct prison *pr;
+	struct ucred *jdcred;
+	int drflags, error;
+
+	sx_slock(&allprison_lock);
+	drflags = PD_LIST_SLOCKED;
+	error = jaildesc_find(td, uap->fd, &jd, &pr, &jdcred);
+	if (error)
+		goto fail;
+	drflags |= PD_DEREF;
+	/*
+	 * Check file permissions using the current credentials, and
+	 * operation permissions using the descriptor's credentials.
+	 */
+	error = vaccess(VREG, jd->jd_mode, jd->jd_uid, jd->jd_gid, VEXEC,
+	    td->td_ucred);
+	JAILDESC_UNLOCK(jd);
+	if (error == 0)
+		error = priv_check_cred(jdcred, PRIV_JAIL_ATTACH);
+	crfree(jdcred);
+	if (error)
+		goto fail;
+	mtx_lock(&pr->pr_mtx);
+	drflags |= PD_LOCKED;
+
+	/* Do not allow a process to attach to a prison that is not alive. */
+	if (!prison_isalive(pr)) {
+		error = EINVAL;
+		goto fail;
+	}
+
+	return (do_jail_attach(td, pr, drflags));
+
+ fail:
+	prison_deref(pr, drflags);
+	return (error);
+}
+
 static int
 do_jail_attach(struct thread *td, struct prison *pr, int drflags)
 {
@@ -2697,9 +3032,12 @@
 	 * a process root from one prison, but attached to the jail
 	 * of another.
 	 */
-	prison_hold(pr);
+	if (!(drflags & PD_DEREF)) {
+		prison_hold(pr);
+		drflags |= PD_DEREF;
+	}
 	refcount_acquire(&pr->pr_uref);
-	drflags |= PD_DEREF | PD_DEUREF;
+	drflags |= PD_DEUREF;
 	mtx_unlock(&pr->pr_mtx);
 	drflags &= ~PD_LOCKED;
 
@@ -2709,6 +3047,7 @@
 		prison_deref(pr, drflags);
 		return (error);
 	}
+	jaildesc_note(pr, NOTE_JAIL_ATTACH, td->td_proc->p_pid);
 	sx_unlock(&allprison_lock);
 	drflags &= ~(PD_LIST_SLOCKED | PD_LIST_XLOCKED);
 
@@ -3176,6 +3515,7 @@
 					    refcount_load(&prison0.pr_uref) > 0,
 					    ("prison0 pr_uref=0"));
 					pr->pr_state = PRISON_STATE_DYING;
+					jaildesc_remove_prison(pr);
 					mtx_unlock(&pr->pr_mtx);
 					flags &= ~PD_LOCKED;
 					prison_cleanup(pr);
@@ -3323,6 +3663,7 @@
 			continue;
 		prison_cleanup(cpr);
 		mtx_lock(&cpr->pr_mtx);
+		jaildesc_remove_prison(cpr);
 		cpr->pr_flags &= ~PR_REMOVE;
 		if (cpr->pr_flags & PR_PERSIST) {
 			cpr->pr_flags &= ~PR_PERSIST;
@@ -3359,6 +3700,7 @@
 
 	prison_cleanup(pr);
 	mtx_lock(&pr->pr_mtx);
+	jaildesc_remove_prison(pr);
 	if (pr->pr_flags & PR_PERSIST) {
 		pr->pr_flags &= ~PR_PERSIST;
 		prison_proc_free_not_last(pr);
@@ -4610,6 +4952,7 @@
  * jail creation time but cannot be changed in an existing jail.
  */
 SYSCTL_JAIL_PARAM(, jid, CTLTYPE_INT | CTLFLAG_RDTUN, "I", "Jail ID");
+SYSCTL_JAIL_PARAM(, desc, CTLTYPE_INT | CTLFLAG_RW, "I", "Jail descriptor");
 SYSCTL_JAIL_PARAM(, parent, CTLTYPE_INT | CTLFLAG_RD, "I", "Jail parent ID");
 SYSCTL_JAIL_PARAM_STRING(, name, CTLFLAG_RW, MAXHOSTNAMELEN, "Jail name");
 SYSCTL_JAIL_PARAM_STRING(, path, CTLFLAG_RDTUN, MAXPATHLEN, "Jail root path");
diff --git a/sys/kern/kern_jaildesc.c b/sys/kern/kern_jaildesc.c
--- a/sys/kern/kern_jaildesc.c
+++ b/sys/kern/kern_jaildesc.c
@@ -0,0 +1,459 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2024 James Gritton.
+ * All rights reserved.
+ *
+ * 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 AUTHOR 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 AUTHOR 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.
+ */
+
+#include <sys/param.h>
+#include <sys/fcntl.h>
+#include <sys/file.h>
+#include <sys/filedesc.h>
+#include <sys/kernel.h>
+#include <sys/jail.h>
+#include <sys/jaildesc.h>
+#include <sys/lock.h>
+#include <sys/malloc.h>
+#include <sys/mutex.h>
+#include <sys/poll.h>
+#include <sys/priv.h>
+#include <sys/stat.h>
+#include <sys/sysproto.h>
+#include <sys/systm.h>
+#include <sys/ucred.h>
+#include <sys/vnode.h>
+
+MALLOC_DEFINE(M_JAILDESC, "jaildesc", "jail descriptors");
+
+static fo_poll_t	jaildesc_poll;
+static fo_kqfilter_t	jaildesc_kqfilter;
+static fo_stat_t	jaildesc_stat;
+static fo_close_t	jaildesc_close;
+static fo_chmod_t	jaildesc_chmod;
+static fo_chown_t	jaildesc_chown;
+static fo_fill_kinfo_t	jaildesc_fill_kinfo;
+static fo_cmp_t		jaildesc_cmp;
+
+static struct fileops jaildesc_ops = {
+	.fo_read = invfo_rdwr,
+	.fo_write = invfo_rdwr,
+	.fo_truncate = invfo_truncate,
+	.fo_ioctl = invfo_ioctl,
+	.fo_poll = jaildesc_poll,
+	.fo_kqfilter = jaildesc_kqfilter,
+	.fo_stat = jaildesc_stat,
+	.fo_close = jaildesc_close,
+	.fo_chmod = jaildesc_chmod,
+	.fo_chown = jaildesc_chown,
+	.fo_sendfile = invfo_sendfile,
+	.fo_fill_kinfo = jaildesc_fill_kinfo,
+	.fo_cmp = jaildesc_cmp,
+	.fo_flags = DFLAG_PASSABLE,
+};
+
+/*
+ * Given a jail descriptor number, return the jaildesc, its prison,
+ * and its credential.  The jaildesc will be returned locked, and
+ * prison and the credential will be returned held.
+ */
+int
+jaildesc_find(struct thread *td, int fd, struct jaildesc **jdp,
+    struct prison **prp, struct ucred **ucredp)
+{
+	struct file *fp;
+	struct jaildesc *jd;
+	struct prison *pr;
+	int error;
+
+	error = fget(td, fd, &cap_no_rights, &fp);
+	if (error != 0)
+		return (error);
+	if (fp->f_type != DTYPE_JAILDESC) {
+		error = EBADF;
+		goto out;
+	}
+	jd = fp->f_data;
+	JAILDESC_LOCK(jd);
+	pr = jd->jd_prison;
+	if (pr == NULL || !prison_isvalid(pr)) {
+		error = ENOENT;
+		JAILDESC_UNLOCK(jd);
+		goto out;
+	}
+	prison_hold(pr);
+	*prp = pr;
+	if (jdp != NULL)
+		*jdp = jd;
+	else
+		JAILDESC_UNLOCK(jd);
+	if (ucredp != NULL)
+		*ucredp = crhold(fp->f_cred);
+ out:
+	fdrop(fp, td);
+	return (error);
+}
+
+/*
+ * Allocate a new jail decriptor, not yet associated with a prison.
+ * Return the file pointer (with a reference held) and the descriptor
+ * number.
+ */
+int
+jaildesc_alloc(struct thread *td, struct file **fpp, int *fdp, int owning)
+{
+	struct file *fp;
+	struct jaildesc *jd;
+	int error;
+	mode_t mode;
+
+	if (owning) {
+		error = priv_check(td, PRIV_JAIL_REMOVE);
+		if (error != 0)
+			return (error);
+		mode = S_ISTXT;
+	} else
+		mode = 0;
+	jd = malloc(sizeof(*jd), M_JAILDESC, M_WAITOK | M_ZERO);
+	error = falloc_caps(td, &fp, fdp, 0, NULL);
+	finit(fp, priv_check_cred(fp->f_cred, PRIV_JAIL_SET) == 0
+	    ? FREAD | FWRITE : FREAD, DTYPE_JAILDESC, jd, &jaildesc_ops);
+	if (error != 0) {
+		free(jd, M_JAILDESC);
+		return (error);
+	}
+	JAILDESC_LOCK_INIT(jd);
+	knlist_init_mtx(&jd->jd_selinfo.si_note, &jd->jd_lock);
+	jd->jd_uid = fp->f_cred->cr_uid;
+	jd->jd_gid = fp->f_cred->cr_gid;
+	jd->jd_mode = S_IFREG | S_IRUSR | S_IRGRP | S_IROTH | mode
+	    | (priv_check(td, PRIV_JAIL_SET) == 0 ? S_IWUSR | S_IXUSR : 0)
+	    | (priv_check(td, PRIV_JAIL_ATTACH) == 0 ? S_IXUSR : 0);
+	*fpp = fp;
+	return (0);
+}
+
+/*
+ * Assocate a jail descriptor with its prison.
+ */
+void
+jaildesc_set_prison(struct file *fp, struct prison *pr)
+{
+	struct jaildesc *jd;
+
+	mtx_assert(&pr->pr_mtx, MA_OWNED);
+	jd = fp->f_data;
+	JAILDESC_LOCK(jd);
+	jd->jd_prison = pr;
+	LIST_INSERT_HEAD(&pr->pr_descs, jd, jd_list);
+	prison_hold(pr);
+	JAILDESC_UNLOCK(jd);
+}
+
+/*
+ * Note prison removal, and detach the all jail descriptors from a prison.
+ */
+void
+jaildesc_remove_prison(struct prison *pr)
+{
+	struct jaildesc *jd;
+
+	mtx_assert(&pr->pr_mtx, MA_OWNED);
+	jaildesc_note(pr, NOTE_JAIL_REMOVE, 0);
+	while ((jd = LIST_FIRST(&pr->pr_descs))) {
+		JAILDESC_LOCK(jd);
+		LIST_REMOVE(jd, jd_list);
+		jd->jd_prison = NULL;
+		JAILDESC_UNLOCK(jd);
+		prison_free(pr);
+	}
+}
+
+/*
+ * Pass a note to all listening kqueues.
+ */
+void
+jaildesc_note(struct prison *pr, long hint, int64_t data)
+{
+	struct jaildesc *jd;
+	int prison_locked;
+
+	if (!LIST_EMPTY(&pr->pr_descs)) {
+		prison_locked = mtx_owned(&pr->pr_mtx);
+		if (!prison_locked)
+			prison_lock(pr);
+		LIST_FOREACH(jd, &pr->pr_descs, jd_list) {
+			JAILDESC_LOCK(jd);
+			jd->jd_event_data = data;
+			if (hint == NOTE_JAIL_REMOVE) {
+				jd->jd_flags |= JDF_REMOVED;
+				if (jd->jd_flags & JDF_SELECTED) {
+					jd->jd_flags &= ~JDF_SELECTED;
+					selwakeup(&jd->jd_selinfo);
+				}
+			}
+			KNOTE_LOCKED(&jd->jd_selinfo.si_note, hint);
+			JAILDESC_UNLOCK(jd);
+		}
+		if (!prison_locked)
+			prison_unlock(pr);
+	}
+}
+
+static int
+jaildesc_close(struct file *fp, struct thread *td)
+{
+	struct jaildesc *jd;
+	struct prison *pr;
+
+	jd = fp->f_data;
+	fp->f_data = NULL;
+	if (jd != NULL) {
+		JAILDESC_LOCK(jd);
+		pr = jd->jd_prison;
+		if (pr != NULL) {
+			/*
+			 * Free or remove the associated prison.
+			 * This requires a second check after re-
+			 * ordering locks.  This jaildesc can remain
+			 * unlocked once we have a prison reference,
+			 * because that prison is the only place that
+			 * still points back to it.
+			 */
+			prison_hold(pr);
+			JAILDESC_UNLOCK(jd);
+			if (jd->jd_mode & S_ISTXT) {
+				sx_xlock(&allprison_lock);
+				prison_lock(pr);
+				if (jd->jd_prison != NULL) {
+					/*
+					 * Unlink the prison, but don't free
+					 * it; that will be done as part of
+					 * of prison_remove.
+					 */
+					LIST_REMOVE(jd, jd_list);
+					prison_remove(pr);
+				} else {
+					prison_unlock(pr);
+					sx_xunlock(&allprison_lock);
+				}
+			} else {
+				prison_lock(pr);
+				if (jd->jd_prison != NULL) {
+					LIST_REMOVE(jd, jd_list);
+					prison_free(pr);
+				}
+				prison_unlock(pr);
+			}
+			prison_free(pr);
+		}
+		knlist_destroy(&jd->jd_selinfo.si_note);
+		JAILDESC_LOCK_DESTROY(jd);
+		free(jd, M_JAILDESC);
+	}
+	finit(fp, 0, DTYPE_NONE, NULL, &badfileops);
+	return (0);
+}
+
+static int
+jaildesc_poll(struct file *fp, int events, struct ucred *active_cred,
+    struct thread *td)
+{
+	struct jaildesc *jd;
+	int revents;
+
+	revents = 0;
+	jd = fp->f_data;
+	JAILDESC_LOCK(jd);
+	if (jd->jd_flags & JDF_REMOVED)
+		revents |= POLLHUP;
+	if (revents == 0) {
+		selrecord(td, &jd->jd_selinfo);
+		jd->jd_flags |= JDF_SELECTED;
+	}
+	JAILDESC_UNLOCK(jd);
+	return (revents);
+}
+
+static void
+jaildesc_kqops_detach(struct knote *kn)
+{
+	struct jaildesc *jd;
+
+	jd = kn->kn_fp->f_data;
+	knlist_remove(&jd->jd_selinfo.si_note, kn, 0);
+}
+
+static int
+jaildesc_kqops_event(struct knote *kn, long hint)
+{
+	struct jaildesc *jd;
+	u_int event;
+
+	jd = kn->kn_fp->f_data;
+	kn->kn_data = jd->jd_event_data;
+	if (hint == 0) {
+		/*
+		 * Initial test after registration. Generate a
+		 *  NOTE_JAIL_REMOVE in case the prison already died
+		 * before registration.
+		 */
+		event = jd->jd_flags & JDF_REMOVED ? NOTE_JAIL_REMOVE : 0;
+	} else
+		event = (u_int)hint;
+
+	/* If the user is interested in this event, record it. */
+	if (kn->kn_sfflags & event)
+		kn->kn_fflags |= event;
+
+	/* Prison is gone, so flag the event as finished. */
+	if (event == NOTE_JAIL_REMOVE) {
+		kn->kn_flags |= EV_EOF | EV_ONESHOT;
+		if (kn->kn_fflags == 0)
+			kn->kn_flags |= EV_DROP;
+		return (1);
+	}
+
+	return (kn->kn_fflags != 0);
+}
+
+static const struct filterops jaildesc_kqops = {
+	.f_isfd = 1,
+	.f_detach = jaildesc_kqops_detach,
+	.f_event = jaildesc_kqops_event,
+};
+
+static int
+jaildesc_kqfilter(struct file *fp, struct knote *kn)
+{
+	struct jaildesc *jd;
+
+	jd = fp->f_data;
+	switch (kn->kn_filter) {
+	case EVFILT_JAILDESC:
+		kn->kn_fop = &jaildesc_kqops;
+		kn->kn_flags |= EV_CLEAR;
+		knlist_add(&jd->jd_selinfo.si_note, kn, 0);
+		return (0);
+	default:
+		return (EINVAL);
+	}
+}
+
+static int
+jaildesc_stat(struct file *fp, struct stat *sb, struct ucred *active_cred)
+{
+	struct jaildesc *jd;
+
+	bzero(sb, sizeof(struct stat));
+	jd = fp->f_data;
+	JAILDESC_LOCK(jd);
+	if (jd->jd_prison != NULL) {
+		sb->st_ino = jd->jd_prison ? jd->jd_prison->pr_id : 0;
+		sb->st_uid = jd->jd_uid;
+		sb->st_gid = jd->jd_gid;
+		sb->st_mode = jd->jd_mode;
+	} else
+		sb->st_mode = S_IFREG;
+	JAILDESC_UNLOCK(jd);
+	return (0);
+}
+
+static int
+jaildesc_chmod(struct file *fp, mode_t mode, struct ucred *active_cred,
+    struct thread *td)
+{
+	struct jaildesc *jd;
+	int error;
+
+	/* Reject permissions that the creator doesn't have. */
+	if (((mode & (S_IWUSR | S_IWGRP | S_IWOTH))
+	    && priv_check_cred(fp->f_cred, PRIV_JAIL_SET) != 0)
+	    || ((mode & (S_IXUSR | S_IXGRP | S_IXOTH))
+	    && priv_check_cred(fp->f_cred, PRIV_JAIL_ATTACH) != 0
+	    && priv_check_cred(fp->f_cred, PRIV_JAIL_SET) != 0)
+	    || ((mode & S_ISTXT)
+	    && priv_check_cred(fp->f_cred, PRIV_JAIL_REMOVE) != 0))
+		return (EPERM);
+	if (mode & (S_ISUID | S_ISGID))
+		return (EINVAL);
+	jd = fp->f_data;
+	JAILDESC_LOCK(jd);
+	error = vaccess(VREG, jd->jd_mode, jd->jd_uid, jd->jd_gid, VADMIN,
+		active_cred);
+	if (error == 0)
+		jd->jd_mode = S_IFREG | (mode & ALLPERMS);
+	JAILDESC_UNLOCK(jd);
+	return (error);
+}
+
+static int
+jaildesc_chown(struct file *fp, uid_t uid, gid_t gid, struct ucred *active_cred,
+    struct thread *td)
+{
+	struct jaildesc *jd;
+	int error;
+
+	error = 0;
+	jd = fp->f_data;
+	JAILDESC_LOCK(jd);
+	if (uid == (uid_t)-1)
+		uid = jd->jd_uid;
+	if (gid == (gid_t)-1)
+		gid = jd->jd_gid;
+	if ((uid != jd->jd_uid && uid != active_cred->cr_uid) ||
+	    (gid != jd->jd_gid && !groupmember(gid, active_cred)))
+		error = priv_check_cred(active_cred, PRIV_VFS_CHOWN);
+	if (error == 0) {
+		jd->jd_uid = uid;
+		jd->jd_gid = gid;
+	}
+	JAILDESC_UNLOCK(jd);
+	return (error);
+}
+
+static int
+jaildesc_fill_kinfo(struct file *fp, struct kinfo_file *kif,
+    struct filedesc *fdp)
+{
+	return (EINVAL);
+}
+
+static int
+jaildesc_cmp(struct file *fp1, struct file *fp2, struct thread *td)
+{
+	struct jaildesc *jd1, *jd2;
+	int jid1, jid2;
+
+	if (fp2->f_type != DTYPE_JAILDESC)
+		return (3);
+	jd1 = fp1->f_data;
+	JAILDESC_LOCK(jd1);
+	jid1 = jd1->jd_prison ? (uintptr_t)jd1->jd_prison->pr_id : 0;
+	JAILDESC_UNLOCK(jd1);
+	jd2 = fp2->f_data;
+	JAILDESC_LOCK(jd2);
+	jid2 = jd2->jd_prison ? (uintptr_t)jd2->jd_prison->pr_id : 0;
+	JAILDESC_UNLOCK(jd2);
+	return (kcmp_cmp(jid1, jid2));
+}
diff --git a/sys/kern/syscalls.c b/sys/kern/syscalls.c
--- a/sys/kern/syscalls.c
+++ b/sys/kern/syscalls.c
@@ -600,4 +600,6 @@
 	"exterrctl",			/* 592 = exterrctl */
 	"inotify_add_watch_at",			/* 593 = inotify_add_watch_at */
 	"inotify_rm_watch",			/* 594 = inotify_rm_watch */
+	"jail_attach_jd",			/* 595 = jail_attach_jd */
+	"jail_remove_jd",			/* 596 = jail_remove_jd */
 };
diff --git a/sys/kern/syscalls.master b/sys/kern/syscalls.master
--- a/sys/kern/syscalls.master
+++ b/sys/kern/syscalls.master
@@ -3370,5 +3370,15 @@
 		    int wd
 		);
 	}
+595	AUE_JAIL_ATTACH	STD {
+		int jail_attach_jd(
+		    int fd
+		);
+	}
+596	AUE_JAIL_REMOVE	STD {
+		int jail_remove_jd(
+		    int fd
+		);
+	}
 
 ; vim: syntax=off
diff --git a/sys/kern/systrace_args.c b/sys/kern/systrace_args.c
--- a/sys/kern/systrace_args.c
+++ b/sys/kern/systrace_args.c
@@ -3500,6 +3500,20 @@
 		*n_args = 2;
 		break;
 	}
+	/* jail_attach_jd */
+	case 595: {
+		struct jail_attach_jd_args *p = params;
+		iarg[a++] = p->fd; /* int */
+		*n_args = 1;
+		break;
+	}
+	/* jail_remove_jd */
+	case 596: {
+		struct jail_remove_jd_args *p = params;
+		iarg[a++] = p->fd; /* int */
+		*n_args = 1;
+		break;
+	}
 	default:
 		*n_args = 0;
 		break;
@@ -9367,6 +9381,26 @@
 			break;
 		};
 		break;
+	/* jail_attach_jd */
+	case 595:
+		switch (ndx) {
+		case 0:
+			p = "int";
+			break;
+		default:
+			break;
+		};
+		break;
+	/* jail_remove_jd */
+	case 596:
+		switch (ndx) {
+		case 0:
+			p = "int";
+			break;
+		default:
+			break;
+		};
+		break;
 	default:
 		break;
 	};
@@ -11365,6 +11399,16 @@
 		if (ndx == 0 || ndx == 1)
 			p = "int";
 		break;
+	/* jail_attach_jd */
+	case 595:
+		if (ndx == 0 || ndx == 1)
+			p = "int";
+		break;
+	/* jail_remove_jd */
+	case 596:
+		if (ndx == 0 || ndx == 1)
+			p = "int";
+		break;
 	default:
 		break;
 	};
diff --git a/sys/sys/event.h b/sys/sys/event.h
--- a/sys/sys/event.h
+++ b/sys/sys/event.h
@@ -45,7 +45,8 @@
 #define EVFILT_USER		(-11)	/* User events */
 #define EVFILT_SENDFILE		(-12)	/* attached to sendfile requests */
 #define EVFILT_EMPTY		(-13)	/* empty send socket buf */
-#define EVFILT_SYSCOUNT		13
+#define EVFILT_JAILDESC		(-14)	/* attached to jail descriptors */
+#define EVFILT_SYSCOUNT		14
 
 #if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901L
 #define	EV_SET(kevp_, a, b, c, d, e, f) do {	\
@@ -216,6 +217,12 @@
 #define NOTE_NSECONDS		0x00000008	/* data is nanoseconds */
 #define	NOTE_ABSTIME		0x00000010	/* timeout is absolute */
 
+/* additional flags for EVFILT_JAIL */
+#define	NOTE_JAIL_SET		0x00000001	/* jail was modified */
+#define	NOTE_JAIL_ATTACH	0x00000002	/* jail was attached to */
+#define	NOTE_JAIL_REMOVE	0x00000004	/* jail was removed */
+#define	NOTE_JAIL_CREATE	0x00000008	/* a child jail was created */
+
 /* Flags for kqueuex(2) */
 #define	KQUEUE_CLOEXEC	0x00000001	/* close on exec */
 
diff --git a/sys/sys/file.h b/sys/sys/file.h
--- a/sys/sys/file.h
+++ b/sys/sys/file.h
@@ -72,6 +72,7 @@
 #define	DTYPE_EVENTFD	13	/* eventfd */
 #define	DTYPE_TIMERFD	14	/* timerfd */
 #define	DTYPE_INOTIFY	15	/* inotify descriptor */
+#define	DTYPE_JAILDESC	16	/* jail descriptor */
 
 #ifdef _KERNEL
 
diff --git a/sys/sys/jail.h b/sys/sys/jail.h
--- a/sys/sys/jail.h
+++ b/sys/sys/jail.h
@@ -99,8 +99,12 @@
 #define	JAIL_UPDATE	0x02	/* Update parameters of existing jail */
 #define	JAIL_ATTACH	0x04	/* Attach to jail upon creation */
 #define	JAIL_DYING	0x08	/* Allow getting a dying jail */
-#define	JAIL_SET_MASK	0x0f	/* JAIL_DYING is deprecated/ignored here */
-#define	JAIL_GET_MASK	0x08
+#define JAIL_USE_DESC	0x10	/* Get/set jail in descriptor */
+#define JAIL_AT_DESC	0x20	/* Find/add jail under descriptor */
+#define	JAIL_GET_DESC	0x40	/* Return a new jail descriptor */
+#define	JAIL_OWN_DESC	0x80	/* Return a new owning jail descriptor */
+#define	JAIL_SET_MASK	0xff	/* JAIL_DYING is deprecated/ignored here */
+#define	JAIL_GET_MASK	0xf8
 
 #define	JAIL_SYS_DISABLE	0
 #define	JAIL_SYS_NEW		1
@@ -115,7 +119,9 @@
 int jail_set(struct iovec *, unsigned int, int);
 int jail_get(struct iovec *, unsigned int, int);
 int jail_attach(int);
+int jail_attach_jd(int);
 int jail_remove(int);
+int jail_remove_jd(int);
 __END_DECLS
 
 #else /* _KERNEL */
@@ -144,6 +150,7 @@
 #define	JAIL_META_PRIVATE	"meta"
 #define	JAIL_META_SHARED	"env"
 
+struct jaildesc;
 struct racct;
 struct prison_racct;
 
@@ -189,7 +196,8 @@
 	struct vnode	*pr_root;			/* (c) vnode to rdir */
 	struct prison_ip  *pr_addrs[PR_FAMILY_MAX];	/* (p,n) IPs of jail */
 	struct prison_racct *pr_prison_racct;		/* (c) racct jail proxy */
-	void		*pr_sparep[3];
+	LIST_HEAD(, jaildesc) pr_descs;			/* (a) attached descriptors */
+	void		*pr_sparep[2];
 	int		 pr_childcount;			/* (a) number of child jails */
 	int		 pr_childmax;			/* (p) maximum child jails */
 	unsigned	 pr_allow;			/* (p) PR_ALLOW_* flags */
@@ -452,6 +460,7 @@
 void prison_proc_link(struct prison *, struct proc *);
 void prison_proc_unlink(struct prison *, struct proc *);
 void prison_proc_iterate(struct prison *, void (*)(struct proc *, void *), void *);
+void prison_remove(struct prison *);
 void prison_set_allow(struct ucred *cred, unsigned flag, int enable);
 bool prison_ischild(struct prison *, struct prison *);
 bool prison_isalive(const struct prison *);
diff --git a/sys/sys/jaildesc.h b/sys/sys/jaildesc.h
--- a/sys/sys/jaildesc.h
+++ b/sys/sys/jaildesc.h
@@ -0,0 +1,90 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2024 James Gritton.
+ * All rights reserved.
+ *
+ * This software was developed at the University of Cambridge Computer
+ * Laboratory with support from a grant from Google, Inc.
+ *
+ * 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 AUTHOR 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 AUTHOR 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.
+ */
+
+#ifndef _SYS_JAILDESC_H_
+#define	_SYS_JAILDESC_H_
+
+#ifdef _KERNEL
+
+#include <sys/selinfo.h>
+#include <sys/queue.h>
+#include <sys/_lock.h>
+#include <sys/_mutex.h>
+#include <sys/_types.h>
+
+struct prison;
+
+/*-
+ * struct jaildesc describes a jail descriptor, which points to a struct
+ * prison.  struct prison in turn has a linked list of struct jaildesc.
+ *
+ * Locking key:
+ *   (c) set on creation, remains unchanged
+ *   (d) jd_lock
+ *   (p) jd_prison->pr_mtx
+ */
+struct jaildesc {
+	LIST_ENTRY(jaildesc) jd_list;	/* (d,p) this prison's descs */
+	struct prison	*jd_prison;	/* (d) the prison */
+	struct mtx	 jd_lock;
+	struct selinfo	 jd_selinfo;	/* (d) event notification */
+	int64_t		 jd_event_data;	/* (d) data to pass in kevent */
+	uid_t		 jd_uid;	/* (d) nominal file owner */
+	gid_t		 jd_gid;	/* (d) nominal file group */
+	mode_t		 jd_mode;	/* (d) descriptor permissions */
+	unsigned	 jd_flags;	/* (d) JDF_* flags */
+};
+
+/*
+ * Locking macros for the jaildesc.
+ */
+#define	JAILDESC_LOCK_DESTROY(jd)	mtx_destroy(&(jd)->jd_lock)
+#define	JAILDESC_LOCK_INIT(jd)		mtx_init(&(jd)->jd_lock, "jaildesc", \
+					    NULL, MTX_DEF)
+#define	JAILDESC_LOCK(jd)		mtx_lock(&(jd)->jd_lock)
+#define	JAILDESC_UNLOCK(jd)		mtx_unlock(&(jd)->jd_lock)
+
+/*
+ * Flags for the jd_flags field
+ */
+#define	JDF_SELECTED	0x00000001	/* issue selwakeup() */
+#define	JDF_REMOVED	0x00000002	/* jail was removed */
+
+int jaildesc_find(struct thread *td, int fd, struct jaildesc **jdp,
+    struct prison **prp, struct ucred **ucredp);
+int jaildesc_alloc(struct thread *td, struct file **fpp, int *fdp, int owning);
+void jaildesc_set_prison(struct file *jd, struct prison *pr);
+void jaildesc_remove_prison(struct prison *pr);
+void jaildesc_note(struct prison *pr, long hint, int64_t data);
+
+#endif /* _KERNEL */
+
+#endif /* !_SYS_JAILDESC_H_ */
diff --git a/sys/sys/syscall.h b/sys/sys/syscall.h
--- a/sys/sys/syscall.h
+++ b/sys/sys/syscall.h
@@ -531,4 +531,6 @@
 #define	SYS_exterrctl	592
 #define	SYS_inotify_add_watch_at	593
 #define	SYS_inotify_rm_watch	594
-#define	SYS_MAXSYSCALL	595
+#define	SYS_jail_attach_jd	595
+#define	SYS_jail_remove_jd	596
+#define	SYS_MAXSYSCALL	597
diff --git a/sys/sys/syscall.mk b/sys/sys/syscall.mk
--- a/sys/sys/syscall.mk
+++ b/sys/sys/syscall.mk
@@ -436,4 +436,6 @@
 	setcred.o \
 	exterrctl.o \
 	inotify_add_watch_at.o \
-	inotify_rm_watch.o
+	inotify_rm_watch.o \
+	jail_attach_jd.o \
+	jail_remove_jd.o
diff --git a/sys/sys/sysproto.h b/sys/sys/sysproto.h
--- a/sys/sys/sysproto.h
+++ b/sys/sys/sysproto.h
@@ -1901,6 +1901,12 @@
 	char fd_l_[PADL_(int)]; int fd; char fd_r_[PADR_(int)];
 	char wd_l_[PADL_(int)]; int wd; char wd_r_[PADR_(int)];
 };
+struct jail_attach_jd_args {
+	char fd_l_[PADL_(int)]; int fd; char fd_r_[PADR_(int)];
+};
+struct jail_remove_jd_args {
+	char fd_l_[PADL_(int)]; int fd; char fd_r_[PADR_(int)];
+};
 int	sys_exit(struct thread *, struct exit_args *);
 int	sys_fork(struct thread *, struct fork_args *);
 int	sys_read(struct thread *, struct read_args *);
@@ -2305,6 +2311,8 @@
 int	sys_exterrctl(struct thread *, struct exterrctl_args *);
 int	sys_inotify_add_watch_at(struct thread *, struct inotify_add_watch_at_args *);
 int	sys_inotify_rm_watch(struct thread *, struct inotify_rm_watch_args *);
+int	sys_jail_attach_jd(struct thread *, struct jail_attach_jd_args *);
+int	sys_jail_remove_jd(struct thread *, struct jail_remove_jd_args *);
 
 #ifdef COMPAT_43
 
@@ -3289,6 +3297,8 @@
 #define	SYS_AUE_exterrctl	AUE_NULL
 #define	SYS_AUE_inotify_add_watch_at	AUE_INOTIFY
 #define	SYS_AUE_inotify_rm_watch	AUE_INOTIFY
+#define	SYS_AUE_jail_attach_jd	AUE_JAIL_ATTACH
+#define	SYS_AUE_jail_remove_jd	AUE_JAIL_REMOVE
 
 #undef PAD_
 #undef PADL_
diff --git a/sys/sys/user.h b/sys/sys/user.h
--- a/sys/sys/user.h
+++ b/sys/sys/user.h
@@ -266,6 +266,7 @@
 #define	KF_TYPE_EVENTFD	13
 #define	KF_TYPE_TIMERFD	14
 #define	KF_TYPE_INOTIFY	15
+#define	KF_TYPE_JAILDESC	16
 #define	KF_TYPE_UNKNOWN	255
 
 #define	KF_VTYPE_VNON	0
@@ -452,6 +453,9 @@
 				uint32_t	kf_timerfd_flags;
 				uint64_t	kf_timerfd_addr;
 			} kf_timerfd;
+			struct {
+				int32_t		kf_jid;
+			} kf_jail;
 			struct {
 				uint64_t	kf_kqueue_addr;
 				int32_t		kf_kqueue_count;