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
@@ -1289,7 +1289,7 @@
 /*
  * Returns whether gid designates a supplementary group in cred.
  */
-static bool
+bool
 group_is_supplementary(const gid_t gid, const struct ucred *const cred)
 {
 
diff --git a/sys/sys/ucred.h b/sys/sys/ucred.h
--- a/sys/sys/ucred.h
+++ b/sys/sys/ucred.h
@@ -155,6 +155,17 @@
 void	cru2x(struct ucred *cr, struct xucred *xcr);
 void	cru2xt(struct thread *td, struct xucred *xcr);
 void	crsetgroups(struct ucred *cr, int n, gid_t *groups);
+
+/*
+ * Returns whether gid designates a primary group in cred.
+ */
+static inline bool
+group_is_primary(const gid_t gid, const struct ucred *const cred)
+{
+	return (gid == cred->cr_groups[0] || gid == cred->cr_rgid ||
+	    gid == cred->cr_svgid);
+}
+bool	group_is_supplementary(const gid_t gid, const struct ucred *const cred);
 bool	groupmember(gid_t gid, const struct ucred *cred);
 bool	realgroupmember(gid_t gid, const struct ucred *cred);
 #endif /* _KERNEL */