diff --git a/lib/libc/gen/err.3 b/lib/libc/gen/err.3
--- a/lib/libc/gen/err.3
+++ b/lib/libc/gen/err.3
@@ -114,6 +114,22 @@
 argument is
 .Dv NULL .
 .Pp
+If the kernel returned and extended error string in addition to the
+.Va errno
+code, the
+.Fn err
+function prints the string with interpolated values for parameters,
+as provided to the corresponding invocation of
+.Xr EXTERROR 9 .
+If the extended error string was not provided, but extended error
+information was, or even if string was provided and the
+.Ev EXTERROR_VERBOSE
+environment variable is present, an additional report is printed.
+The report includes at least the category of the error, the name of
+the source file (if known by the used version of libc),
+the source line number, and parameters.
+The format of the printed string is not contractual and might be changed.
+.Pp
 In the case of the
 .Fn errc ,
 .Fn verrc ,
diff --git a/lib/libc/gen/err.c b/lib/libc/gen/err.c
--- a/lib/libc/gen/err.c
+++ b/lib/libc/gen/err.c
@@ -120,7 +120,7 @@
 	}
 	fprintf(err_file, "%s", strerror(code));
 	if (doexterr && extstatus == 0 && exterr[0] != '\0')
-		fprintf(err_file, " (extended error %s)", exterr);
+		fprintf(err_file, " (%s)", exterr);
 	fprintf(err_file, "\n");
 }
 
diff --git a/lib/libc/gen/exterr_cat_filenames.h b/lib/libc/gen/exterr_cat_filenames.h
new file mode 100644
--- /dev/null
+++ b/lib/libc/gen/exterr_cat_filenames.h
@@ -0,0 +1,17 @@
+/*
+ * Automatically @generated, use
+ * tools/build/make_libc_exterr_cat_filenames.sh
+ */
+	[EXTERR_CAT_FUSE_DEVICE] = "fs/fuse/fuse_device.c",
+	[EXTERR_CAT_FUSE_VFS] = "fs/fuse/fuse_vfsops.c",
+	[EXTERR_CAT_FUSE_VNOPS] = "fs/fuse/fuse_vnops.c",
+	[EXTERR_CAT_GEOM] = "geom/geom_subr.c",
+	[EXTERR_CAT_GEOMVFS] = "geom/geom_vfs.c",
+	[EXTERR_CAT_FILEDESC] = "kern/kern_descrip.c",
+	[EXTERR_CAT_INOTIFY] = "kern/vfs_inotify.c",
+	[EXTERR_CAT_GENIO] = "kern/sys_generic.c",
+	[EXTERR_CAT_VFSBIO] = "kern/vfs_bio.c",
+	[EXTERR_CAT_VFSSYSCALL] = "kern/vfs_syscalls.c",
+	[EXTERR_CAT_BRIDGE] = "net/if_bridge.c",
+	[EXTERR_CAT_SWAP] = "vm/swap_pager.c",
+	[EXTERR_CAT_MMAP] = "vm/vm_mmap.c",
diff --git a/lib/libc/gen/uexterr_format.c b/lib/libc/gen/uexterr_format.c
--- a/lib/libc/gen/uexterr_format.c
+++ b/lib/libc/gen/uexterr_format.c
@@ -8,28 +8,85 @@
  * under sponsorship from the FreeBSD Foundation.
  */
 
-#include <sys/types.h>
+#include <sys/param.h>
 #include <sys/exterrvar.h>
 #include <exterr.h>
+#include <stdbool.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
+
+static const char * const cat_to_filenames[] = {
+#include "exterr_cat_filenames.h"
+};
+
+static const char *
+cat_to_filename(int category)
+{
+	if (category < 0 || category >= nitems(cat_to_filenames) ||
+	    cat_to_filenames[category] == NULL)
+		return ("unknown");
+	return (cat_to_filenames[category]);
+}
+
+static const char exterror_verbose_name[] = "EXTERROR_VERBOSE";
+enum exterr_verbose_state {
+	EXTERR_VERBOSE_UNKNOWN = 100,
+	EXTERR_VERBOSE_DEFAULT,
+	EXTERR_VERBOSE_ALLOW,
+};
+static enum exterr_verbose_state exterror_verbose = EXTERR_VERBOSE_UNKNOWN;
+
+static void
+exterr_verbose_init(void)
+{
+	/*
+	 * No need to care about thread-safety, the result is
+	 * idempotent.
+	 */
+	if (exterror_verbose != EXTERR_VERBOSE_UNKNOWN)
+		return;
+	if (issetugid()) {
+		exterror_verbose = EXTERR_VERBOSE_DEFAULT;
+	} else if (getenv(exterror_verbose_name) != NULL) {
+		exterror_verbose = EXTERR_VERBOSE_ALLOW;
+	} else {
+		exterror_verbose = EXTERR_VERBOSE_DEFAULT;
+	}
+}
 
 int
 __uexterr_format(const struct uexterror *ue, char *buf, size_t bufsz)
 {
+	bool has_msg;
+
 	if (bufsz > UEXTERROR_MAXLEN)
 		bufsz = UEXTERROR_MAXLEN;
 	if (ue->error == 0) {
 		strlcpy(buf, "", bufsz);
 		return (0);
 	}
-	if (ue->msg[0] == '\0') {
-		snprintf(buf, bufsz,
-		    "errno %d category %u (src line %u) p1 %#jx p2 %#jx",
-		    ue->error, ue->cat, ue->src_line,
-		    (uintmax_t)ue->p1, (uintmax_t)ue->p2);
+	exterr_verbose_init();
+	has_msg = ue->msg[0] != '\0';
+
+	if (has_msg) {
+		snprintf(buf, bufsz, ue->msg, (uintmax_t)ue->p1,
+		    (uintmax_t)ue->p2);
 	} else {
-		strlcpy(buf, ue->msg, bufsz);
+		strlcpy(buf, "", bufsz);
+	}
+
+	if (exterror_verbose == EXTERR_VERBOSE_ALLOW || !has_msg) {
+		char lbuf[128];
+
+		snprintf(lbuf, sizeof(lbuf),
+		    "errno %d category %u (src sys/%s:%u) p1 %#jx p2 %#jx",
+		    ue->error, ue->cat, cat_to_filename(ue->cat),
+		    ue->src_line, (uintmax_t)ue->p1, (uintmax_t)ue->p2);
+		if (has_msg)
+			strlcat(buf, " ", bufsz);
+		strlcat(buf, lbuf, bufsz);
 	}
 	return (0);
 }
diff --git a/share/man/man7/environ.7 b/share/man/man7/environ.7
--- a/share/man/man7/environ.7
+++ b/share/man/man7/environ.7
@@ -114,6 +114,18 @@
 .Xr ex 1
 and
 .Xr vi 1 .
+.It Ev EXTERROR_VERBOSE
+Request the
+.Xr err 3
+and
+.Xr uexterr_gettext
+functions to unconditionally report additional information,
+mostly useful for the (kernel) developer to diagnose the issue.
+See
+.Xr err 3
+and
+.Xr exterror 9
+for more details.
 .It Ev HOME
 A user's login directory, set by
 .Xr login 1
@@ -298,6 +310,7 @@
 .Xr cd 1 ,
 .Xr csh 1 ,
 .Xr env 1 ,
+.Xr err 3 ,
 .Xr ex 1 ,
 .Xr login 1 ,
 .Xr printenv 1 ,
@@ -311,7 +324,8 @@
 .Xr system 3 ,
 .Xr termcap 3 ,
 .Xr termcap 5 ,
-.Xr simd 7
+.Xr simd 7 ,
+.Xr exterror 9
 .Sh HISTORY
 The
 .Nm
diff --git a/share/man/man9/exterror.9 b/share/man/man9/exterror.9
--- a/share/man/man9/exterror.9
+++ b/share/man/man9/exterror.9
@@ -90,6 +90,16 @@
 .Fn EXTERROR
 macro can take two optional 64-bit integer arguments,
 whose meaning is specific to the subsystem.
+The format string may include up to two printf-like format
+specifiers to insert the optional argument values in the
+user output, which is done in userspace.
+.Pp
+The format specifier must be for an integer type, and include the
+.Dq j
+format modifier to accept only the types
+.Vt intmax_t
+or
+.Vt uintmax_t .
 .El
 .Pp
 The strings passed as the second argument are only retained
diff --git a/sys/fs/fuse/fuse_device.c b/sys/fs/fuse/fuse_device.c
--- a/sys/fs/fuse/fuse_device.c
+++ b/sys/fs/fuse/fuse_device.c
@@ -82,7 +82,7 @@
 #include <sys/sysctl.h>
 #include <sys/poll.h>
 #include <sys/selinfo.h>
-#define EXTERR_CATEGORY EXTERR_CAT_FUSE
+#define EXTERR_CATEGORY EXTERR_CAT_FUSE_DEVICE
 #include <sys/exterrvar.h>
 
 #include "fuse.h"
diff --git a/sys/fs/fuse/fuse_vfsops.c b/sys/fs/fuse/fuse_vfsops.c
--- a/sys/fs/fuse/fuse_vfsops.c
+++ b/sys/fs/fuse/fuse_vfsops.c
@@ -81,7 +81,7 @@
 #include <sys/mount.h>
 #include <sys/sysctl.h>
 #include <sys/fcntl.h>
-#define EXTERR_CATEGORY EXTERR_CAT_FUSE
+#define EXTERR_CATEGORY EXTERR_CAT_FUSE_VFS
 #include <sys/exterrvar.h>
 
 #include "fuse.h"
diff --git a/sys/fs/fuse/fuse_vnops.c b/sys/fs/fuse/fuse_vnops.c
--- a/sys/fs/fuse/fuse_vnops.c
+++ b/sys/fs/fuse/fuse_vnops.c
@@ -89,7 +89,7 @@
 #include <sys/buf.h>
 #include <sys/sysctl.h>
 #include <sys/vmmeter.h>
-#define EXTERR_CATEGORY EXTERR_CAT_FUSE
+#define EXTERR_CATEGORY EXTERR_CAT_FUSE_VNOPS
 #include <sys/exterrvar.h>
 
 #include <vm/vm.h>
diff --git a/sys/sys/exterr_cat.h b/sys/sys/exterr_cat.h
--- a/sys/sys/exterr_cat.h
+++ b/sys/sys/exterr_cat.h
@@ -8,6 +8,17 @@
  * under sponsorship from the FreeBSD Foundation.
  */
 
+/*
+ * The category identifiers for the extended errors.
+ * The ids participate in ABI between kernel and libc, so they must
+ * never be reused or changed.  Only new ids can be added.
+ *
+ * After adding a new category id, run
+ * tools/build/make_libc_exterr_cat_filenames.sh
+ * from the top of the source tree, and commit updated file
+ * lib/libc/gen/exterr_cat_filenames.h
+ */
+
 #ifndef _SYS_EXTERR_CAT_H_
 #define	_SYS_EXTERR_CAT_H_
 
@@ -15,7 +26,7 @@
 #define	EXTERR_CAT_FILEDESC	2
 #define	EXTERR_KTRACE		3	/* To allow inclusion of this
 					   file into kern_ktrace.c */
-#define	EXTERR_CAT_FUSE		4
+#define	EXTERR_CAT_FUSE_VNOPS	4
 #define	EXTERR_CAT_INOTIFY	5
 #define	EXTERR_CAT_GENIO	6
 #define	EXTERR_CAT_BRIDGE	7
@@ -24,6 +35,8 @@
 #define	EXTERR_CAT_VFSBIO	10
 #define	EXTERR_CAT_GEOMVFS	11
 #define	EXTERR_CAT_GEOM		12
+#define	EXTERR_CAT_FUSE_VFS	13
+#define	EXTERR_CAT_FUSE_DEVICE	14
 
 #endif
 
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
@@ -197,12 +197,14 @@
 	check_fp_fn = mrp->mr_check_fp_fn;
 
 	if ((prot & ~(_PROT_ALL | PROT_MAX(_PROT_ALL))) != 0) {
-		return (EXTERROR(EINVAL, "unknown PROT bits"));
+		return (EXTERROR(EINVAL, "unknown PROT bits %#jx", prot));
 	}
 	max_prot = PROT_MAX_EXTRACT(prot);
 	prot = PROT_EXTRACT(prot);
 	if (max_prot != 0 && (max_prot & prot) != prot) {
-		return (EXTERROR(ENOTSUP, "prot is not subset of max_prot"));
+		return (EXTERROR(ENOTSUP,
+		    "prot %#jx is not subset of max_prot %#jx",
+		    prot, max_prot));
 	}
 
 	p = td->td_proc;
@@ -236,7 +238,7 @@
 		if ((len == 0 && p->p_osrel >= P_OSREL_MAP_ANON) ||
 		    ((flags & MAP_ANON) != 0 && (fd != -1 || pos != 0))) {
 			return (EXTERROR(EINVAL,
-			    "offset not zero/fd not -1 for MAP_ANON",
+			    "offset %#jd not zero/fd %#jd not -1 for MAP_ANON",
 			    fd, pos));
 		}
 	} else {
@@ -247,8 +249,8 @@
 	if (flags & MAP_STACK) {
 		if ((fd != -1) || ((prot & (PROT_READ | PROT_WRITE)) !=
 		    (PROT_READ | PROT_WRITE))) {
-			return (EXTERROR(EINVAL, "MAP_STACK with prot < rw",
-			    prot));
+			return (EXTERROR(EINVAL,
+			    "MAP_STACK with prot %#jx < rw", prot));
 		}
 		flags |= MAP_ANON;
 		pos = 0;
@@ -257,18 +259,21 @@
 	    MAP_STACK | MAP_NOSYNC | MAP_ANON | MAP_EXCL | MAP_NOCORE |
 	    MAP_PREFAULT_READ | MAP_GUARD | MAP_32BIT |
 	    MAP_ALIGNMENT_MASK)) != 0) {
-		return (EXTERROR(EINVAL, "reserved flag set"));
+		return (EXTERROR(EINVAL, "reserved flag set (flags %#jx)",
+		    flags));
 	}
 	if ((flags & (MAP_EXCL | MAP_FIXED)) == MAP_EXCL) {
-		return (EXTERROR(EINVAL, "EXCL without FIXED"));
+		return (EXTERROR(EINVAL, "EXCL without FIXED (flags %#jx)",
+		    flags));
 	}
 	if ((flags & (MAP_SHARED | MAP_PRIVATE)) == (MAP_SHARED |
 	    MAP_PRIVATE)) {
-		return (EXTERROR(EINVAL, "both SHARED and PRIVATE set"));
+		return (EXTERROR(EINVAL,
+		    "both SHARED and PRIVATE set (flags %#jx)", flags));
 	}
 	if (prot != PROT_NONE &&
 	    (prot & ~(PROT_READ | PROT_WRITE | PROT_EXEC)) != 0) {
-		return (EXTERROR(EINVAL, "invalid prot", prot));
+		return (EXTERROR(EINVAL, "invalid prot %#jx", prot));
 	}
 	if ((flags & MAP_GUARD) != 0 && (prot != PROT_NONE || fd != -1 ||
 	    pos != 0 || (flags & ~(MAP_FIXED | MAP_GUARD | MAP_EXCL |
@@ -295,7 +300,7 @@
 	if (align != 0 && align != MAP_ALIGNED_SUPER &&
 	    (align >> MAP_ALIGNMENT_SHIFT >= sizeof(void *) * NBBY ||
 	    align >> MAP_ALIGNMENT_SHIFT < PAGE_SHIFT)) {
-		return (EXTERROR(EINVAL, "bad alignment", align));
+		return (EXTERROR(EINVAL, "bad alignment %#jx", align));
 	}
 
 	/*
@@ -310,8 +315,8 @@
 		 */
 		addr -= pageoff;
 		if ((addr & PAGE_MASK) != 0) {
-			return (EXTERROR(EINVAL, "fixed mapping not aligned",
-			    addr));
+			return (EXTERROR(EINVAL,
+			    "fixed mapping at %#jx not aligned", addr));
 		}
 
 		/* Address range must be all in user VM space. */
@@ -321,7 +326,8 @@
 		}
 		if (flags & MAP_32BIT && addr + size > MAP_32BIT_MAX_ADDR) {
 			return (EXTERROR(EINVAL,
-			    "fixed 32bit mapping does not fit into 4G"));
+		    "fixed 32bit mapping of [%#jx %#jx] does not fit into 4G",
+			    addr, addr + size));
 		}
 	} else if (flags & MAP_32BIT) {
 		/*
@@ -1495,7 +1501,7 @@
 		    handle, &foff, &object, &writecounted);
 		break;
 	default:
-		error = EXTERROR(EINVAL, "unsupported backing obj type",
+		error = EXTERROR(EINVAL, "unsupported backing obj type %jd",
 		    handle_type);
 		break;
 	}
@@ -1578,7 +1584,7 @@
 	 * exec).
 	 */
 	if ((foff & PAGE_MASK) != 0) {
-		return (EXTERROR(EINVAL, "offset not page-aligned", foff));
+		return (EXTERROR(EINVAL, "offset %#jx not page-aligned", foff));
 	}
 
 	if ((flags & MAP_FIXED) == 0) {
@@ -1587,7 +1593,8 @@
 	} else {
 		if (*addr != trunc_page(*addr)) {
 			return (EXTERROR(EINVAL,
-			    "non-fixed mapping address not aligned", *addr));
+			    "non-fixed mapping address %#jx not aligned",
+			    *addr));
 		}
 		fitit = false;
 	}
@@ -1599,7 +1606,7 @@
 		}
 		if (foff != 0) {
 			return (EXTERROR(EINVAL,
-			    "anon mapping with non-zero offset"));
+			    "anon mapping with non-zero offset %#jx", foff));
 		}
 		docow = 0;
 	} else if (flags & MAP_PREFAULT_READ)
@@ -1702,6 +1709,6 @@
 	}
 	if ((curthread->td_pflags2 & (TDP2_UEXTERR | TDP2_EXTERR)) ==
 	    TDP2_UEXTERR)
-		EXTERROR(error, "mach error", rv);
+		EXTERROR(error, "mach error %jd", rv);
 	return (error);
 }
diff --git a/tests/sys/kern/exterr_test.c b/tests/sys/kern/exterr_test.c
--- a/tests/sys/kern/exterr_test.c
+++ b/tests/sys/kern/exterr_test.c
@@ -51,7 +51,7 @@
 	ATF_CHECK_EQ(0, r);
 	printf("Extended error: %s\n", exterr);
 	/* Note: error string may need to be updated due to kernel changes */
-	ATF_CHECK(strstr(exterr, "prot is not subset of max_prot") != 0);
+	ATF_CHECK(strstr(exterr, " is not subset of ") != 0);
 }
 
 ATF_TC(gettext_noextended);
diff --git a/tools/build/make_libc_exterr_cat_filenames.sh b/tools/build/make_libc_exterr_cat_filenames.sh
new file mode 100755
--- /dev/null
+++ b/tools/build/make_libc_exterr_cat_filenames.sh
@@ -0,0 +1,22 @@
+#!/bin/sh
+set -e
+
+check="lib/libc/gen/uexterr_format.c"
+target="lib/libc/gen/exterr_cat_filenames.h"
+
+if [ \! -f "${check}" ] ; then
+    echo "Script must be run from the top of the full source tree"
+    exit 1
+fi
+
+echo "/*" >"${target}"
+printf " * Automatically %sgenerated, use\\n" \@ >>"${target}"
+echo " * tools/build/make_libc_exterr_cat_filenames.sh" >>"${target}"
+echo " */" >>"${target}"
+
+(find sys -type f -name '*.c' | \
+    xargs grep -E '^#define[[:space:]]+EXTERR_CATEGORY[[:space:]]+EXTERR_CAT_' | \
+    sed -E 's/[[:space:]]+/:/g' | \
+    awk -F ':' '{filename = $1; sub(/^sys\//, "", filename);
+        printf("\t[%s] = \"%s\",\n", $4, filename)}') \
+    >>"${target}"