diff --git a/lib/libc/gen/posix_spawn.c b/lib/libc/gen/posix_spawn.c
--- a/lib/libc/gen/posix_spawn.c
+++ b/lib/libc/gen/posix_spawn.c
@@ -629,6 +629,10 @@
 int
 posix_spawnattr_setflags(posix_spawnattr_t *sa, short flags)
 {
+	if ((flags & ~(POSIX_SPAWN_RESETIDS | POSIX_SPAWN_SETPGROUP |
+	    POSIX_SPAWN_SETSCHEDPARAM | POSIX_SPAWN_SETSCHEDULER |
+	    POSIX_SPAWN_SETSIGDEF | POSIX_SPAWN_SETSIGMASK)) != 0)
+		return (EINVAL);
 	(*sa)->sa_flags = flags;
 	return (0);
 }
diff --git a/lib/libc/gen/posix_spawnattr_getflags.3 b/lib/libc/gen/posix_spawnattr_getflags.3
--- a/lib/libc/gen/posix_spawnattr_getflags.3
+++ b/lib/libc/gen/posix_spawnattr_getflags.3
@@ -83,9 +83,12 @@
 .Sh RETURN VALUES
 The
 .Fn posix_spawnattr_getflags
-and
+function returns zero.
+The
 .Fn posix_spawnattr_setflags
-functions return zero.
+function returns zero on success, and
+.Er EINVAL
+on failure due to invalid flag specified.
 .Sh SEE ALSO
 .Xr posix_spawn 3 ,
 .Xr posix_spawnattr_destroy 3 ,