diff --git a/etc/mtree/BSD.tests.dist b/etc/mtree/BSD.tests.dist
--- a/etc/mtree/BSD.tests.dist
+++ b/etc/mtree/BSD.tests.dist
@@ -404,6 +404,8 @@
         ..
         libsbuf
         ..
+        libsysdecode
+        ..
         libthr
             dlopen
             ..
diff --git a/lib/libsysdecode/Makefile b/lib/libsysdecode/Makefile
--- a/lib/libsysdecode/Makefile
+++ b/lib/libsysdecode/Makefile
@@ -141,4 +141,7 @@
 
 beforedepend: ioctl.c tables.h
 
+HAS_TESTS=
+SUBDIR.${MK_TESTS}+= tests
+
 .include <bsd.lib.mk>
diff --git a/lib/libsysdecode/tests/Makefile b/lib/libsysdecode/tests/Makefile
new file mode 100644
--- /dev/null
+++ b/lib/libsysdecode/tests/Makefile
@@ -0,0 +1,5 @@
+ATF_TESTS_C+= sysdecode_test
+
+LIBADD= sysdecode
+
+.include <bsd.test.mk>
diff --git a/lib/libsysdecode/tests/sysdecode_test.c b/lib/libsysdecode/tests/sysdecode_test.c
new file mode 100644
--- /dev/null
+++ b/lib/libsysdecode/tests/sysdecode_test.c
@@ -0,0 +1,152 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+ *
+ * Copyright (c) 2022 The FreeBSD Foundation
+ *
+ * This software was developed by Mark Johnston under sponsorship from
+ * the FreeBSD Foundation.
+ *
+ * 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/capsicum.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#include <atf-c.h>
+#include <sysdecode.h>
+
+/*
+ * Take a comma-separated list of capability rights and verify that all rights
+ * are present in the specified table, and that all rights in the table are
+ * present in the list.
+ */
+static void
+check_sysdecode_cap_rights(FILE *fp, char **bufp, size_t *szp,
+    cap_rights_t *rightsp, const char *tab[])
+{
+	const char *next, *tok;
+	char *buf;
+	int i;
+
+	sysdecode_cap_rights(fp, rightsp);
+
+	ATF_REQUIRE(fflush(fp) == 0);
+	(*bufp)[*szp] = '\0';
+
+	buf = strdup(*bufp);
+	for (tok = buf; (next = strsep(&buf, ",")), tok != NULL; tok = next) {
+		for (i = 0; tab[i] != NULL; i++) {
+			if (strcmp(tok, tab[i]) == 0)
+				break;
+		}
+		ATF_REQUIRE_MSG(tab[i] != NULL,
+		    "did not find '%s' in table", tok);
+	}
+	free(buf);
+
+	for (i = 0; tab[i] != NULL; i++) {
+		buf = strdup(*bufp);
+		for (tok = buf; (next = strsep(&buf, ",")), tok != NULL;
+		    tok = next) {
+			if (strcmp(tok, tab[i]) == 0)
+				break;
+		}
+		free(buf);
+		ATF_REQUIRE_MSG(tok != NULL,
+		    "did not find '%s' in output stream", tab[i]);
+	}
+
+	ATF_REQUIRE(fseek(fp, 0, SEEK_SET) == 0);
+}
+
+/*
+ * Regression tests for sysdecode_cap_rights(3).
+ */
+ATF_TC_WITHOUT_HEAD(cap_rights);
+ATF_TC_BODY(cap_rights, tc)
+{
+	char *buf;
+	FILE *fp;
+	size_t sz;
+	cap_rights_t rights;
+
+	fp = open_memstream(&buf, &sz);
+	ATF_REQUIRE(fp != NULL);
+
+	/*
+	 * libsysdecode emits a pseudo-right, CAP_NONE, when no rights are
+	 * present.
+	 */
+	check_sysdecode_cap_rights(fp, &buf, &sz,
+	    cap_rights_init(&rights),
+	    (const char *[]){ "CAP_NONE", NULL, });
+
+	check_sysdecode_cap_rights(fp, &buf, &sz,
+	    cap_rights_init(&rights, CAP_READ, CAP_SEEK),
+	    (const char *[]){ "CAP_PREAD", NULL, });
+
+	check_sysdecode_cap_rights(fp, &buf, &sz,
+	    cap_rights_init(&rights, CAP_READ, CAP_MMAP, CAP_SEEK_TELL),
+	    (const char *[]){ "CAP_READ", "CAP_MMAP", "CAP_SEEK_TELL", NULL, });
+
+	check_sysdecode_cap_rights(fp, &buf, &sz,
+	    cap_rights_init(&rights, CAP_MMAP, CAP_READ, CAP_WRITE, CAP_SEEK),
+	    (const char *[]){ "CAP_MMAP_RW", NULL, });
+
+	check_sysdecode_cap_rights(fp, &buf, &sz,
+	    cap_rights_init(&rights, CAP_READ, CAP_MMAP_X),
+	    (const char *[]){ "CAP_MMAP_RX", NULL, });
+
+	/* Aliases map back to the main definition. */
+	check_sysdecode_cap_rights(fp, &buf, &sz,
+	    cap_rights_init(&rights, CAP_RECV, CAP_SEND),
+	    (const char *[]){ "CAP_READ", "CAP_WRITE", NULL, });
+
+	/* This set straddles both indices. */
+	check_sysdecode_cap_rights(fp, &buf, &sz,
+	    cap_rights_init(&rights, CAP_READ, CAP_KQUEUE),
+	    (const char *[]){ "CAP_READ", "CAP_KQUEUE", NULL, });
+
+	/* Create a rights set with an unnamed flag. */
+	cap_rights_init(&rights, CAP_SEEK);
+	cap_rights_clear(&rights, CAP_SEEK_TELL);
+	check_sysdecode_cap_rights(fp, &buf, &sz,
+	    &rights,
+	    (const char *[]){ "CAP_NONE", "unknown rights", NULL, });
+
+	cap_rights_init(&rights, CAP_SEEK, CAP_KQUEUE_CHANGE);
+	cap_rights_clear(&rights, CAP_SEEK_TELL);
+	check_sysdecode_cap_rights(fp, &buf, &sz,
+	    &rights,
+	    (const char *[]){ "CAP_KQUEUE_CHANGE", "unknown rights", NULL, });
+
+	ATF_REQUIRE(fclose(fp) == 0);
+	free(buf);
+}
+
+ATF_TP_ADD_TCS(tp)
+{
+	ATF_TP_ADD_TC(tp, cap_rights);
+
+	return (atf_no_error());
+}