diff --git a/sys/kern/uipc_sockbuf.c b/sys/kern/uipc_sockbuf.c
--- a/sys/kern/uipc_sockbuf.c
+++ b/sys/kern/uipc_sockbuf.c
@@ -66,9 +66,10 @@
  * Primitive routines for operating on socket buffers
  */
 
+#define	BUF_MAX_ADJ(_sz)	(((u_quad_t)(_sz)) * MCLBYTES / (MSIZE + MCLBYTES))
+
 u_long	sb_max = SB_MAX;
-u_long sb_max_adj =
-       (quad_t)SB_MAX * MCLBYTES / (MSIZE + MCLBYTES); /* adjusted sb_max */
+u_long sb_max_adj = BUF_MAX_ADJ(SB_MAX);
 
 static	u_long sb_efficiency = 8;	/* parameter for sbreserve() */
 
@@ -611,7 +612,7 @@
 	if (tmp_sb_max < MSIZE + MCLBYTES)
 		return (EINVAL);
 	sb_max = tmp_sb_max;
-	sb_max_adj = (u_quad_t)sb_max * MCLBYTES / (MSIZE + MCLBYTES);
+	sb_max_adj = BUF_MAX_ADJ(sb_max);
 	return (0);
 }
 
@@ -620,8 +621,8 @@
  * become limiting if buffering efficiency is near the normal case.
  */
 bool
-sbreserve_locked(struct socket *so, sb_which which, u_long cc,
-    struct thread *td)
+sbreserve_locked_limit(struct socket *so, sb_which which, u_long cc,
+    u_long buf_max, struct thread *td)
 {
 	struct sockbuf *sb = sobuf(so, which);
 	rlim_t sbsize_limit;
@@ -635,7 +636,7 @@
 	 * appropriate thread resource limits are available.  In that case,
 	 * we don't apply a process limit.
 	 */
-	if (cc > sb_max_adj)
+	if (cc > BUF_MAX_ADJ(buf_max))
 		return (false);
 	if (td != NULL) {
 		sbsize_limit = lim_cur(td, RLIMIT_SBSIZE);
@@ -644,12 +645,19 @@
 	if (!chgsbsize(so->so_cred->cr_uidinfo, &sb->sb_hiwat, cc,
 	    sbsize_limit))
 		return (false);
-	sb->sb_mbmax = min(cc * sb_efficiency, sb_max);
+	sb->sb_mbmax = min(cc * sb_efficiency, buf_max);
 	if (sb->sb_lowat > sb->sb_hiwat)
 		sb->sb_lowat = sb->sb_hiwat;
 	return (true);
 }
 
+bool
+sbreserve_locked(struct socket *so, sb_which which, u_long cc,
+    struct thread *td)
+{
+	return (sbreserve_locked_limit(so, which, cc, sb_max, td));
+}
+
 int
 sbsetopt(struct socket *so, struct sockopt *sopt)
 {
diff --git a/sys/sys/sockbuf.h b/sys/sys/sockbuf.h
--- a/sys/sys/sockbuf.h
+++ b/sys/sys/sockbuf.h
@@ -231,6 +231,8 @@
 int	sbsetopt(struct socket *so, struct sockopt *);
 bool	sbreserve_locked(struct socket *so, sb_which which, u_long cc,
 	    struct thread *td);
+bool	sbreserve_locked_limit(struct socket *so, sb_which which, u_long cc,
+	    u_long buf_max, struct thread *td);
 void	sbsndptr_adv(struct sockbuf *sb, struct mbuf *mb, u_int len);
 struct mbuf *
 	sbsndptr_noadv(struct sockbuf *sb, u_int off, u_int *moff);