diff --git a/src/process/posix_spawnattr_setflags.c b/src/process/posix_spawnattr_setflags.c
index f750c040..68780992 100644
--- a/src/process/posix_spawnattr_setflags.c
+++ b/src/process/posix_spawnattr_setflags.c
@@ -1,7 +1,18 @@
 #include <spawn.h>
+#include <errno.h>
 
 int posix_spawnattr_setflags(posix_spawnattr_t *attr, short flags)
 {
+	const unsigned all_flags =
+		POSIX_SPAWN_RESETIDS |
+		POSIX_SPAWN_SETPGROUP |
+		POSIX_SPAWN_SETSIGDEF |
+		POSIX_SPAWN_SETSIGMASK |
+		POSIX_SPAWN_SETSCHEDPARAM |
+		POSIX_SPAWN_SETSCHEDULER |
+		POSIX_SPAWN_USEVFORK |
+		POSIX_SPAWN_SETSID;
+	if (flags & ~all_flags) return EINVAL;
 	attr->__flags = flags;
 	return 0;
 }