diff --git a/sys/kern/kern_prot.c b/sys/kern/kern_prot.c
--- a/sys/kern/kern_prot.c
+++ b/sys/kern/kern_prot.c
@@ -1316,6 +1316,19 @@
 	return (supplementary_group_member(gid, cred));
 }
 
+/*
+ * Check if gid is a member of the real group set (i.e., real and supplementary
+ * groups).
+ */
+int
+realgroupmember(gid_t gid, struct ucred *cred)
+{
+	if (gid == cred->cr_rgid)
+		return (1);
+
+	return (supplementary_group_member(gid, cred));
+}
+
 /*
  * Test the active securelevel against a given level.  securelevel_gt()
  * implements (securelevel > level).  securelevel_ge() implements
diff --git a/sys/sys/ucred.h b/sys/sys/ucred.h
--- a/sys/sys/ucred.h
+++ b/sys/sys/ucred.h
@@ -159,6 +159,7 @@
 void	cru2xt(struct thread *td, struct xucred *xcr);
 void	crsetgroups(struct ucred *cr, int n, gid_t *groups);
 int	groupmember(gid_t gid, struct ucred *cred);
+int	realgroupmember(gid_t gid, struct ucred *cred);
 #endif /* _KERNEL */
 
 #endif /* !_SYS_UCRED_H_ */