diff --git a/include/exterr.h b/include/exterr.h
new file mode 100644
--- /dev/null
+++ b/include/exterr.h
@@ -0,0 +1,11 @@
+#ifndef _EXTERR_H_
+#define	_EXTERR_H_
+
+#include <sys/cdefs.h>
+#include <sys/exterr_cat.h>
+
+__BEGIN_DECLS
+int uexterr_gettext(char *buf, size_t bufsz);
+__END_DECLS
+
+#endif
diff --git a/lib/libc/gen/Makefile.inc b/lib/libc/gen/Makefile.inc
--- a/lib/libc/gen/Makefile.inc
+++ b/lib/libc/gen/Makefile.inc
@@ -159,6 +159,8 @@
 	ttyname.c \
 	ttyslot.c \
 	ualarm.c \
+	uexterr_format.c \
+	uexterr_gettext.c \
 	ulimit.c \
 	uname.c \
 	unvis-compat.c \
diff --git a/lib/libc/gen/Symbol.map b/lib/libc/gen/Symbol.map
--- a/lib/libc/gen/Symbol.map
+++ b/lib/libc/gen/Symbol.map
@@ -462,6 +462,7 @@
 	psiginfo;
 	rtld_get_var;
 	rtld_set_var;
+	uexterr_gettext;
 };
 
 FBSDprivate_1.0 {
@@ -592,4 +593,5 @@
 	__fillcontextx2;
 	__getcontextx_size;
 	__makecontext;
+	__uexterr_format;
 };
diff --git a/lib/libc/gen/libc_interposing_table.c b/lib/libc/gen/libc_interposing_table.c
--- a/lib/libc/gen/libc_interposing_table.c
+++ b/lib/libc/gen/libc_interposing_table.c
@@ -43,6 +43,7 @@
 	SLOT(spinunlock, __libc_spinunlock_stub),
 	SLOT(map_stacks_exec, __libc_map_stacks_exec),
 	SLOT(distribute_static_tls, __libc_distribute_static_tls),
+	SLOT(uexterr_gettext, __libc_uexterr_gettext),
 };
 #undef SLOT
 
diff --git a/lib/libc/gen/uexterr_format.c b/lib/libc/gen/uexterr_format.c
new file mode 100644
--- /dev/null
+++ b/lib/libc/gen/uexterr_format.c
@@ -0,0 +1,12 @@
+#include <sys/types.h>
+#include <sys/exterrvar.h>
+#include <exterr.h>
+#include <string.h>
+
+int
+__uexterr_format(const struct uexterror *ue, char *buf, size_t bufsz)
+{
+	/* XXXKIB */
+	strlcpy(buf, ue->msg, bufsz);
+	return (0);
+}
diff --git a/lib/libc/gen/uexterr_get.c b/lib/libc/gen/uexterr_get.c
new file mode 100644
--- /dev/null
+++ b/lib/libc/gen/uexterr_get.c
@@ -0,0 +1,10 @@
+#include <sys/types.h>
+#include <sys/exterr.h>
+#include <string.h>
+
+int
+__uexterr_format(const struct uexterror *ue, char *buf, size_t bufsz)
+{
+	strlcpy(buf, ue->msg, bufsz);
+	return (0);
+}
diff --git a/lib/libc/gen/uexterr_gettext.c b/lib/libc/gen/uexterr_gettext.c
new file mode 100644
--- /dev/null
+++ b/lib/libc/gen/uexterr_gettext.c
@@ -0,0 +1,29 @@
+#include <sys/types.h>
+#include <sys/exterrvar.h>
+#include <exterr.h>
+#include <string.h>
+#include "libc_private.h"
+
+static struct uexterror uexterr = {
+	.ver = UEXTERROR_VER,
+};
+
+static void uexterr_ctr(void) __attribute__((constructor));
+static void
+uexterr_ctr(void)
+{
+	exterrctl(EXTERRCTL_ENABLE, 0, &uexterr);
+}
+
+int
+__libc_uexterr_gettext(char *buf, size_t bufsz)
+{
+	return (__uexterr_format(&uexterr, buf, bufsz));
+}
+
+int
+uexterr_gettext(char *buf, size_t bufsz)
+{
+	return (((int (*)(char *, size_t))
+	    __libc_interposing[INTERPOS_uexterr_gettext])(buf, bufsz));
+}
diff --git a/lib/libc/include/libc_private.h b/lib/libc/include/libc_private.h
--- a/lib/libc/include/libc_private.h
+++ b/lib/libc/include/libc_private.h
@@ -251,6 +251,7 @@
 	INTERPOS_clock_nanosleep,
 	INTERPOS_distribute_static_tls,
 	INTERPOS_pdfork,
+	INTERPOS_uexterr_gettext,
 	INTERPOS_MAX
 };
 
@@ -380,4 +381,8 @@
 int __strerror_rl(int errnum, char *strerrbuf, size_t buflen,
 	    struct _xlocale *locale);
 
+struct uexterror;
+int __uexterr_format(const struct uexterror *ue, char *buf, size_t bufsz);
+int __libc_uexterr_gettext(char *buf, size_t bufsz);
+
 #endif /* _LIBC_PRIVATE_H_ */
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
@@ -378,6 +378,7 @@
 };
 
 FBSD_1.8 {
+	exterrctl;
 	fchroot;
 	getrlimitusage;
 	kcmp;
diff --git a/lib/libsys/_libsys.h b/lib/libsys/_libsys.h
--- a/lib/libsys/_libsys.h
+++ b/lib/libsys/_libsys.h
@@ -465,6 +465,7 @@
 typedef int (__sys_getrlimitusage_t)(u_int, int, rlim_t *);
 typedef int (__sys_fchroot_t)(int);
 typedef int (__sys_setcred_t)(u_int, const struct setcred *, size_t);
+typedef int (__sys_exterrctl_t)(u_int, u_int, void *);
 
 void __sys_exit(int rval);
 int __sys_fork(void);
@@ -866,6 +867,7 @@
 int __sys_getrlimitusage(u_int which, int flags, rlim_t * res);
 int __sys_fchroot(int fd);
 int __sys_setcred(u_int flags, const struct setcred * wcred, size_t size);
+int __sys_exterrctl(u_int op, u_int flags, void * ptr);
 __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
@@ -807,4 +807,6 @@
 	__sys_fchroot;
 	_setcred;
 	__sys_setcred;
+	_exterrctl;
+	__sys_exterrctl;
 };
diff --git a/lib/libthr/thread/thr_create.c b/lib/libthr/thread/thr_create.c
--- a/lib/libthr/thread/thr_create.c
+++ b/lib/libthr/thread/thr_create.c
@@ -31,6 +31,7 @@
 #include <sys/types.h>
 #include <sys/rtprio.h>
 #include <sys/signalvar.h>
+#include <sys/exterrvar.h>
 #include <errno.h>
 #include <link.h>
 #include <stdlib.h>
@@ -285,6 +286,9 @@
 		curthread->attr.stacksize_attr;
 #endif
 
+	curthread->uexterr.ver = UEXTERROR_VER;
+	exterrctl(EXTERRCTL_ENABLE, 0, &curthread->uexterr);
+
 	/* Run the current thread's start routine with argument: */
 	_pthread_exit(curthread->start_routine(curthread->arg));
 
diff --git a/lib/libthr/thread/thr_private.h b/lib/libthr/thread/thr_private.h
--- a/lib/libthr/thread/thr_private.h
+++ b/lib/libthr/thread/thr_private.h
@@ -40,6 +40,7 @@
 #include <sys/queue.h>
 #include <sys/param.h>
 #include <sys/cpuset.h>
+#include <sys/exterrvar.h>
 #include <machine/atomic.h>
 #include <errno.h>
 #include <limits.h>
@@ -576,6 +577,8 @@
 
 	/* pthread_set/get_name_np */
 	char			*name;
+
+	struct uexterror	uexterr;
 };
 
 #define THR_SHOULD_GC(thrd) 						\
diff --git a/lib/libthr/thread/thr_syscalls.c b/lib/libthr/thread/thr_syscalls.c
--- a/lib/libthr/thread/thr_syscalls.c
+++ b/lib/libthr/thread/thr_syscalls.c
@@ -66,9 +66,9 @@
  */
 
 #include "namespace.h"
-#include <sys/types.h>
-#include <sys/mman.h>
 #include <sys/param.h>
+#include <sys/exterrvar.h>
+#include <sys/mman.h>
 #include <sys/select.h>
 #include <sys/signalvar.h>
 #include <sys/socket.h>
@@ -620,6 +620,15 @@
 	return (ret);
 }
 
+static int
+__thr_uexterr_gettext(char *buf, size_t bufsz)
+{
+	struct pthread *curthread;
+
+	curthread = _get_curthread();
+	return (__uexterr_format(&curthread->uexterr, buf, bufsz));
+}
+
 void
 __thr_interpose_libc(void)
 {
@@ -675,6 +684,7 @@
 	SLOT(fdatasync);
 	SLOT(clock_nanosleep);
 	SLOT(pdfork);
+	SLOT(uexterr_gettext);
 #undef SLOT
 	*(__libc_interposing_slot(
 	    INTERPOS__pthread_mutex_init_calloc_cb)) =
diff --git a/sys/amd64/conf/GENERIC b/sys/amd64/conf/GENERIC
--- a/sys/amd64/conf/GENERIC
+++ b/sys/amd64/conf/GENERIC
@@ -26,6 +26,7 @@
 options 	SCHED_ULE		# ULE scheduler
 options 	NUMA			# Non-Uniform Memory Architecture support
 options 	PREEMPTION		# Enable kernel thread preemption
+options 	BLOW_KERNEL_WITH_EXTERR
 options 	VIMAGE			# Subsystem virtualization, e.g. VNET
 options 	INET			# InterNETworking
 options 	INET6			# IPv6 communications protocols
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
@@ -510,4 +510,5 @@
 #define	FREEBSD32_SYS_getrlimitusage	589
 #define	FREEBSD32_SYS_fchroot	590
 #define	FREEBSD32_SYS_freebsd32_setcred	591
-#define	FREEBSD32_SYS_MAXSYSCALL	592
+#define	FREEBSD32_SYS_exterrctl	592
+#define	FREEBSD32_SYS_MAXSYSCALL	593
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
@@ -597,4 +597,5 @@
 	"getrlimitusage",			/* 589 = getrlimitusage */
 	"fchroot",			/* 590 = fchroot */
 	"freebsd32_setcred",			/* 591 = freebsd32_setcred */
+	"exterrctl",			/* 592 = exterrctl */
 };
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
@@ -659,4 +659,5 @@
 	{ .sy_narg = AS(getrlimitusage_args), .sy_call = (sy_call_t *)sys_getrlimitusage, .sy_auevent = AUE_NULL, .sy_flags = SYF_CAPENABLED, .sy_thrcnt = SY_THR_STATIC },	/* 589 = getrlimitusage */
 	{ .sy_narg = AS(fchroot_args), .sy_call = (sy_call_t *)sys_fchroot, .sy_auevent = AUE_NULL, .sy_flags = 0, .sy_thrcnt = SY_THR_STATIC },	/* 590 = fchroot */
 	{ .sy_narg = AS(freebsd32_setcred_args), .sy_call = (sy_call_t *)freebsd32_setcred, .sy_auevent = AUE_SETCRED, .sy_flags = SYF_CAPENABLED, .sy_thrcnt = SY_THR_STATIC },	/* 591 = freebsd32_setcred */
+	{ .sy_narg = AS(exterrctl_args), .sy_call = (sy_call_t *)sys_exterrctl, .sy_auevent = AUE_NULL, .sy_flags = 0, .sy_thrcnt = SY_THR_STATIC },	/* 592 = exterrctl */
 };
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
@@ -3386,6 +3386,15 @@
 		*n_args = 3;
 		break;
 	}
+	/* exterrctl */
+	case 592: {
+		struct exterrctl_args *p = params;
+		uarg[a++] = p->op; /* u_int */
+		uarg[a++] = p->flags; /* u_int */
+		uarg[a++] = (intptr_t)p->ptr; /* void * */
+		*n_args = 3;
+		break;
+	}
 	default:
 		*n_args = 0;
 		break;
@@ -9147,6 +9156,22 @@
 			break;
 		};
 		break;
+	/* exterrctl */
+	case 592:
+		switch (ndx) {
+		case 0:
+			p = "u_int";
+			break;
+		case 1:
+			p = "u_int";
+			break;
+		case 2:
+			p = "userland void *";
+			break;
+		default:
+			break;
+		};
+		break;
 	default:
 		break;
 	};
@@ -11040,6 +11065,11 @@
 		if (ndx == 0 || ndx == 1)
 			p = "int";
 		break;
+	/* exterrctl */
+	case 592:
+		if (ndx == 0 || ndx == 1)
+			p = "int";
+		break;
 	default:
 		break;
 	};
diff --git a/sys/conf/options b/sys/conf/options
--- a/sys/conf/options
+++ b/sys/conf/options
@@ -53,6 +53,7 @@
 DDB_CTF		opt_ddb.h
 DDB_NUMSYM	opt_ddb.h
 EARLY_PRINTF	opt_global.h
+BLOW_KERNEL_WITH_EXTERR	opt_global.h
 FULL_BUF_TRACKING	opt_global.h
 GDB
 KDB		opt_global.h
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
@@ -658,4 +658,5 @@
 	{ .sy_narg = AS(getrlimitusage_args), .sy_call = (sy_call_t *)sys_getrlimitusage, .sy_auevent = AUE_NULL, .sy_flags = SYF_CAPENABLED, .sy_thrcnt = SY_THR_STATIC },	/* 589 = getrlimitusage */
 	{ .sy_narg = AS(fchroot_args), .sy_call = (sy_call_t *)sys_fchroot, .sy_auevent = AUE_NULL, .sy_flags = 0, .sy_thrcnt = SY_THR_STATIC },	/* 590 = fchroot */
 	{ .sy_narg = AS(setcred_args), .sy_call = (sy_call_t *)sys_setcred, .sy_auevent = AUE_SETCRED, .sy_flags = SYF_CAPENABLED, .sy_thrcnt = SY_THR_STATIC },	/* 591 = setcred */
+	{ .sy_narg = AS(exterrctl_args), .sy_call = (sy_call_t *)sys_exterrctl, .sy_auevent = AUE_NULL, .sy_flags = 0, .sy_thrcnt = SY_THR_STATIC },	/* 592 = exterrctl */
 };
diff --git a/sys/kern/kern_thread.c b/sys/kern/kern_thread.c
--- a/sys/kern/kern_thread.c
+++ b/sys/kern/kern_thread.c
@@ -86,9 +86,9 @@
     "struct thread KBI td_flags");
 _Static_assert(offsetof(struct thread, td_pflags) == 0x114,
     "struct thread KBI td_pflags");
-_Static_assert(offsetof(struct thread, td_frame) == 0x4b8,
+_Static_assert(offsetof(struct thread, td_frame) == 0x4e8,
     "struct thread KBI td_frame");
-_Static_assert(offsetof(struct thread, td_emuldata) == 0x6c0,
+_Static_assert(offsetof(struct thread, td_emuldata) == 0x6f0,
     "struct thread KBI td_emuldata");
 _Static_assert(offsetof(struct proc, p_flag) == 0xb8,
     "struct proc KBI p_flag");
diff --git a/sys/kern/subr_syscall.c b/sys/kern/subr_syscall.c
--- a/sys/kern/subr_syscall.c
+++ b/sys/kern/subr_syscall.c
@@ -74,6 +74,8 @@
 			td->td_dbgflags |= TDB_SCE;
 		PROC_UNLOCK(p);
 	}
+	if ((td->td_pflags2 & TDP2_UEXTERR) != 0)
+		td->td_pflags2 &= ~TDP2_EXTERR;
 	error = (p->p_sysent->sv_fetch_syscall_args)(td);
 	se = sa->callp;
 #ifdef KTRACE
@@ -207,6 +209,10 @@
 		PROC_UNLOCK(p);
 	}
 	(p->p_sysent->sv_set_syscall_retval)(td, error);
+	if ((td->td_pflags2 & (TDP2_EXTERR | TDP2_UEXTERR)) ==
+	    (TDP2_EXTERR | TDP2_UEXTERR)) {
+		exterr_copyout(td);
+	}
 }
 
 static inline void
diff --git a/sys/kern/sys_generic.c b/sys/kern/sys_generic.c
--- a/sys/kern/sys_generic.c
+++ b/sys/kern/sys_generic.c
@@ -34,10 +34,10 @@
  * SUCH DAMAGE.
  */
 
-#include <sys/cdefs.h>
 #include "opt_capsicum.h"
 #include "opt_ktrace.h"
 
+#define	EXTERR_CATEGORY	EXTERR_CAT_FILEDESC
 #include <sys/param.h>
 #include <sys/systm.h>
 #include <sys/sysproto.h>
@@ -46,6 +46,7 @@
 #include <sys/filio.h>
 #include <sys/fcntl.h>
 #include <sys/file.h>
+#include <sys/exterrvar.h>
 #include <sys/lock.h>
 #include <sys/proc.h>
 #include <sys/signalvar.h>
@@ -2200,3 +2201,70 @@
 		return (3);
 	return (kcmp_cmp((uintptr_t)fp1->f_data, (uintptr_t)fp2->f_data));
 }
+
+void
+exterr_copyout(struct thread *td)
+{
+	struct uexterror ue;
+	int error;
+
+	MPASS((td->td_pflags2 & (TDP2_EXTERR | TDP2_UEXTERR)) ==
+	    (TDP2_EXTERR | TDP2_UEXTERR));
+
+	memset(&ue, 0, sizeof(ue));
+	ue.error = td->td_kexterr.error;
+	ue.cat = td->td_kexterr.cat;
+	ue.src_line = td->td_kexterr.src_line;
+	ue.p1 = td->td_kexterr.p1;
+	ue.p2 = td->td_kexterr.p2;
+	if (td->td_kexterr.msg != NULL)
+		strlcpy(ue.msg, td->td_kexterr.msg, sizeof(ue.msg));
+
+	error = copyout((char *)&ue + __offsetof(struct uexterror, error),
+	    (char *)td->td_exterr_ptr + __offsetof(struct uexterror, error),
+	    sizeof(ue) - __offsetof(struct uexterror, error));
+	if (error != 0)
+		td->td_pflags2 &= ~TDP2_UEXTERR;
+}
+
+int
+sys_exterrctl(struct thread *td, struct exterrctl_args *uap)
+{
+	uint32_t ver;
+	int error;
+
+	switch (uap->op) {
+	case EXTERRCTL_ENABLE:
+		if ((td->td_pflags2 & TDP2_EXTERR) != 0)
+			return (EBUSY);
+		error = copyin(uap->ptr, &ver, sizeof(ver));
+		if (error != 0)
+			return (error);
+		if (ver != UEXTERROR_VER)
+			return (EINVAL);
+		td->td_pflags2 |= TDP2_EXTERR;
+		td->td_exterr_ptr = uap->ptr;
+		return (0);
+	case EXTERRCTL_DISABLE:
+		if ((td->td_pflags2 & TDP2_EXTERR) == 0)
+			return (EINVAL);
+		td->td_pflags2 &= ~TDP2_EXTERR;
+		return (0);
+	case EXTERRCTL_GETPTR:
+		if ((td->td_pflags2 & TDP2_EXTERR) == 0)
+			return (EINVAL);
+#ifdef COMPAT_FREEBSD32
+		if (SV_PROC_FLAG(td->td_proc, SV_ILP32) != 0) {
+			uint32_t ptr;
+
+			ptr = (uintptr_t)td->td_exterr_ptr;
+			error = copyout(&ptr, uap->ptr, sizeof(ptr));
+		} else
+#endif
+			error = copyout(td->td_exterr_ptr, uap->ptr,
+			    sizeof(void *));
+		return (error);
+	default:
+		return (EINVAL);
+	}
+}
diff --git a/sys/kern/syscalls.c b/sys/kern/syscalls.c
--- a/sys/kern/syscalls.c
+++ b/sys/kern/syscalls.c
@@ -597,4 +597,5 @@
 	"getrlimitusage",			/* 589 = getrlimitusage */
 	"fchroot",			/* 590 = fchroot */
 	"setcred",			/* 591 = setcred */
+	"exterrctl",			/* 592 = exterrctl */
 };
diff --git a/sys/kern/syscalls.master b/sys/kern/syscalls.master
--- a/sys/kern/syscalls.master
+++ b/sys/kern/syscalls.master
@@ -3349,5 +3349,11 @@
 		    size_t size
 		);
 	}
-
+592	AUE_NULL	STD {
+		int exterrctl(
+		    u_int op,
+		    u_int flags,
+		    _Inout_updates_bytes_opt_(4) void *ptr
+		);
+	}
 ; 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
@@ -3473,6 +3473,15 @@
 		*n_args = 3;
 		break;
 	}
+	/* exterrctl */
+	case 592: {
+		struct exterrctl_args *p = params;
+		uarg[a++] = p->op; /* u_int */
+		uarg[a++] = p->flags; /* u_int */
+		uarg[a++] = (intptr_t)p->ptr; /* void * */
+		*n_args = 3;
+		break;
+	}
 	default:
 		*n_args = 0;
 		break;
@@ -9292,6 +9301,22 @@
 			break;
 		};
 		break;
+	/* exterrctl */
+	case 592:
+		switch (ndx) {
+		case 0:
+			p = "u_int";
+			break;
+		case 1:
+			p = "u_int";
+			break;
+		case 2:
+			p = "userland void *";
+			break;
+		default:
+			break;
+		};
+		break;
 	default:
 		break;
 	};
@@ -11275,6 +11300,11 @@
 		if (ndx == 0 || ndx == 1)
 			p = "int";
 		break;
+	/* exterrctl */
+	case 592:
+		if (ndx == 0 || ndx == 1)
+			p = "int";
+		break;
 	default:
 		break;
 	};
diff --git a/sys/sys/_exterr.h b/sys/sys/_exterr.h
new file mode 100644
--- /dev/null
+++ b/sys/sys/_exterr.h
@@ -0,0 +1,15 @@
+#ifndef _SYS__EXTERR_H_
+#define	_SYS__EXTERR_H_
+
+#include <sys/_types.h>
+
+struct kexterr {
+	int error;
+	const char *msg;
+	__uint64_t p1;
+	__uint64_t p2;
+	unsigned cat;
+	unsigned src_line;
+};
+
+#endif
diff --git a/sys/sys/exterr_cat.h b/sys/sys/exterr_cat.h
new file mode 100644
--- /dev/null
+++ b/sys/sys/exterr_cat.h
@@ -0,0 +1,8 @@
+#ifndef _SYS_EXTERR_CAT_H_
+#define	_SYS_EXTERR_CAT_H_
+
+#define	EXTERR_CAT_MMAP		1
+#define	EXTERR_CAT_FILEDESC	2
+
+#endif
+
diff --git a/sys/sys/exterrvar.h b/sys/sys/exterrvar.h
new file mode 100644
--- /dev/null
+++ b/sys/sys/exterrvar.h
@@ -0,0 +1,58 @@
+#ifndef _SYS_EXTERRVAR_H_
+#define	_SYS_EXTERRVAR_H_
+
+#include <sys/_exterr.h>
+#include <sys/exterr_cat.h>
+#include <sys/types.h>
+
+struct uexterror {
+	uint32_t ver;
+	uint32_t error;
+	uint32_t cat;
+	uint32_t src_line;
+	uint64_t p1;
+	uint64_t p2;
+	uint64_t rsrv1[4];
+	char msg[128];
+};
+
+#define	UEXTERROR_VER		0x10010001
+
+#define	EXTERRCTL_ENABLE	1
+#define	EXTERRCTL_DISABLE	2
+#define	EXTERRCTL_GETPTR	3
+
+#ifdef _KERNEL
+
+#ifndef EXTERR_CATEGORY
+#error "Specify error category before including sys/exterr.h"
+#endif
+
+#ifdef	BLOW_KERNEL_WITH_EXTERR
+#define	SET_ERROR_MSG(mmsg)	_Td->td_kexterr.msg = mmsg
+#else
+#define	SET_ERROR_MSG(mmsg)	_Td->td_kexterr.msg = NULL
+#endif
+
+#define	SET_ERROR2(eerror, mmsg, pp1, pp2) do {	\
+	struct thread *_Td = curthread;		\
+	_Td->td_pflags2 |= TDP2_EXTERR;		\
+	_Td->td_kexterr.error = eerror;		\
+	_Td->td_kexterr.cat = EXTERR_CATEGORY;	\
+	SET_ERROR_MSG(mmsg);			\
+	_Td->td_kexterr.p1 = pp1;		\
+	_Td->td_kexterr.p2 = pp2;		\
+	_Td->td_kexterr.src_line = __LINE__;	\
+} while (0)
+#define	SET_ERROR0(eerror, mmsg)	SET_ERROR2(eerror, mmsg, 0, 0)
+#define	SET_ERROR1(eerror, mmsg, pp1)	SET_ERROR2(eerror, mmsg, pp1, 0)
+
+#else	/* _KERNEL */
+
+__BEGIN_DECLS
+int exterrctl(u_int op, u_int flags, void *ptr);
+__END_DECLS
+
+#endif	/* _KERNEL */
+
+#endif
diff --git a/sys/sys/proc.h b/sys/sys/proc.h
--- a/sys/sys/proc.h
+++ b/sys/sys/proc.h
@@ -42,6 +42,7 @@
 #ifdef _KERNEL
 #include <sys/_eventhandler.h>
 #endif
+#include <sys/_exterr.h>
 #include <sys/condvar.h>
 #ifndef _KERNEL
 #include <sys/filedesc.h>
@@ -322,6 +323,8 @@
 	size_t		td_vslock_sz;	/* (k) amount of vslock-ed space */
 	struct kcov_info *td_kcov_info;	/* (*) Kernel code coverage data */
 	long		td_ucredref;	/* (k) references on td_realucred */
+	void		*td_exterr_ptr;
+	struct kexterr	td_kexterr;
 #define	td_endzero td_sigmask
 
 /* Copied during fork1(), thread_create(), or kthread_add(). */
@@ -569,6 +572,8 @@
 #define	TDP2_COMPAT32RB	0x00000002 /* compat32 ABI for robust lists */
 #define	TDP2_ACCT	0x00000004 /* Doing accounting */
 #define	TDP2_SAN_QUIET	0x00000008 /* Disable warnings from K(A|M)SAN */
+#define	TDP2_EXTERR	0x00000010 /* Kernel reported ext error */
+#define	TDP2_UEXTERR	0x00000020 /* User set ext error reporting ptr */
 
 /*
  * Reasons that the current thread can not be run yet.
diff --git a/sys/sys/syscall.h b/sys/sys/syscall.h
--- a/sys/sys/syscall.h
+++ b/sys/sys/syscall.h
@@ -528,4 +528,5 @@
 #define	SYS_getrlimitusage	589
 #define	SYS_fchroot	590
 #define	SYS_setcred	591
-#define	SYS_MAXSYSCALL	592
+#define	SYS_exterrctl	592
+#define	SYS_MAXSYSCALL	593
diff --git a/sys/sys/syscall.mk b/sys/sys/syscall.mk
--- a/sys/sys/syscall.mk
+++ b/sys/sys/syscall.mk
@@ -433,4 +433,5 @@
 	kcmp.o \
 	getrlimitusage.o \
 	fchroot.o \
-	setcred.o
+	setcred.o \
+	exterrctl.o
diff --git a/sys/sys/sysproto.h b/sys/sys/sysproto.h
--- a/sys/sys/sysproto.h
+++ b/sys/sys/sysproto.h
@@ -1886,6 +1886,11 @@
 	char wcred_l_[PADL_(const struct setcred *)]; const struct setcred * wcred; char wcred_r_[PADR_(const struct setcred *)];
 	char size_l_[PADL_(size_t)]; size_t size; char size_r_[PADR_(size_t)];
 };
+struct exterrctl_args {
+	char op_l_[PADL_(u_int)]; u_int op; char op_r_[PADR_(u_int)];
+	char flags_l_[PADL_(u_int)]; u_int flags; char flags_r_[PADR_(u_int)];
+	char ptr_l_[PADL_(void *)]; void * ptr; char ptr_r_[PADR_(void *)];
+};
 int	sys_exit(struct thread *, struct exit_args *);
 int	sys_fork(struct thread *, struct fork_args *);
 int	sys_read(struct thread *, struct read_args *);
@@ -2287,6 +2292,7 @@
 int	sys_getrlimitusage(struct thread *, struct getrlimitusage_args *);
 int	sys_fchroot(struct thread *, struct fchroot_args *);
 int	sys_setcred(struct thread *, struct setcred_args *);
+int	sys_exterrctl(struct thread *, struct exterrctl_args *);
 
 #ifdef COMPAT_43
 
@@ -3268,6 +3274,7 @@
 #define	SYS_AUE_getrlimitusage	AUE_NULL
 #define	SYS_AUE_fchroot	AUE_NULL
 #define	SYS_AUE_setcred	AUE_SETCRED
+#define	SYS_AUE_exterrctl	AUE_NULL
 
 #undef PAD_
 #undef PADL_
diff --git a/sys/sys/uio.h b/sys/sys/uio.h
--- a/sys/sys/uio.h
+++ b/sys/sys/uio.h
@@ -84,6 +84,7 @@
 int	copyinuio(const struct iovec *iovp, u_int iovcnt, struct uio **uiop);
 int	copyout_map(struct thread *td, vm_offset_t *addr, size_t sz);
 int	copyout_unmap(struct thread *td, vm_offset_t addr, size_t sz);
+void	exterr_copyout(struct thread *td);
 int	physcopyin(void *src, vm_paddr_t dst, size_t len);
 int	physcopyout(vm_paddr_t src, void *dst, size_t len);
 int	physcopyin_vlist(struct bus_dma_segment *src, off_t offset,
diff --git a/sys/vm/vm_mmap.c b/sys/vm/vm_mmap.c
--- a/sys/vm/vm_mmap.c
+++ b/sys/vm/vm_mmap.c
@@ -40,13 +40,14 @@
  * Mapped file (mmap) interface to VM
  */
 
-#include <sys/cdefs.h>
 #include "opt_hwpmc_hooks.h"
 #include "opt_vm.h"
 
+#define	EXTERR_CATEGORY	EXTERR_CAT_MMAP
 #include <sys/param.h>
 #include <sys/systm.h>
 #include <sys/capsicum.h>
+#include <sys/exterrvar.h>
 #include <sys/kernel.h>
 #include <sys/lock.h>
 #include <sys/mutex.h>
@@ -190,12 +191,16 @@
 	pos = mrp->mr_pos;
 	check_fp_fn = mrp->mr_check_fp_fn;
 
-	if ((prot & ~(_PROT_ALL | PROT_MAX(_PROT_ALL))) != 0)
+	if ((prot & ~(_PROT_ALL | PROT_MAX(_PROT_ALL))) != 0) {
+		SET_ERROR0(EINVAL, "unknown PROT bits");
 		return (EINVAL);
+	}
 	max_prot = PROT_MAX_EXTRACT(prot);
 	prot = PROT_EXTRACT(prot);
-	if (max_prot != 0 && (max_prot & prot) != prot)
+	if (max_prot != 0 && (max_prot & prot) != prot) {
+		SET_ERROR0(ENOTSUP, "prot is not subset of max_prot");
 		return (ENOTSUP);
+	}
 
 	p = td->td_proc;