diff --git a/sys/fs/devfs/devfs_devs.c b/sys/fs/devfs/devfs_devs.c
index 1d5e3c90f50a..45f8d58f5b43 100644
--- a/sys/fs/devfs/devfs_devs.c
+++ b/sys/fs/devfs/devfs_devs.c
@@ -125,7 +125,6 @@ devfs_alloc(void)
 	cdp->cdp_maxdirent = 0;
 
 	cdev = &cdp->cdp_c;
-	cdev->si_priv = cdp;
 
 	cdev->si_name = cdev->__si_namebuf;
 	LIST_INIT(&cdev->si_children);
@@ -137,7 +136,7 @@ devfs_free(struct cdev *cdev)
 {
 	struct cdev_priv *cdp;
 
-	cdp = cdev->si_priv;
+	cdp = cdev2priv(cdev);
 	if (cdev->si_cred != NULL)
 		crfree(cdev->si_cred);
 	if (cdp->cdp_inode > 0)
@@ -510,7 +509,7 @@ devfs_create(struct cdev *dev)
 	struct cdev_priv *cdp;
 
 	mtx_assert(&devmtx, MA_OWNED);
-	cdp = dev->si_priv;
+	cdp = cdev2priv(dev);
 	cdp->cdp_flags |= CDP_ACTIVE;
 	cdp->cdp_inode = alloc_unrl(devfs_inos);
 	dev_refl(dev);
@@ -524,7 +523,7 @@ devfs_destroy(struct cdev *dev)
 	struct cdev_priv *cdp;
 
 	mtx_assert(&devmtx, MA_OWNED);
-	cdp = dev->si_priv;
+	cdp = cdev2priv(dev);
 	cdp->cdp_flags &= ~CDP_ACTIVE;
 	devfs_generation++;
 }
diff --git a/sys/fs/devfs/devfs_int.h b/sys/fs/devfs/devfs_int.h
index df0ad56069d9..5a61dd4a79b0 100644
--- a/sys/fs/devfs/devfs_int.h
+++ b/sys/fs/devfs/devfs_int.h
@@ -68,6 +68,8 @@ struct cdev_priv {
 	LIST_HEAD(, cdev_privdata) cdp_fdpriv;
 };
 
+#define	cdev2priv(c)	member2struct(cdev_priv, cdp_c, c)
+
 struct cdev *devfs_alloc(void);
 void devfs_free(struct cdev *);
 void devfs_create(struct cdev *dev);
diff --git a/sys/fs/devfs/devfs_vnops.c b/sys/fs/devfs/devfs_vnops.c
index 649f492dc61e..8130642f1154 100644
--- a/sys/fs/devfs/devfs_vnops.c
+++ b/sys/fs/devfs/devfs_vnops.c
@@ -132,7 +132,7 @@ devfs_set_cdevpriv(void *priv, cdevpriv_dtr_t priv_dtr)
 	fp = curthread->td_fpop;
 	if (fp == NULL)
 		return (ENOENT);
-	cdp = ((struct cdev *)fp->f_data)->si_priv;
+	cdp = cdev2priv((struct cdev *)fp->f_data);
 	p = malloc(sizeof(struct cdev_privdata), M_CDEVPDATA, M_WAITOK);
 	p->cdpd_data = priv;
 	p->cdpd_dtr = priv_dtr;
@@ -541,7 +541,7 @@ devfs_getattr(struct vop_getattr_args *ap)
 		fix(dev->si_ctime);
 		vap->va_ctime = dev->si_ctime;
 
-		vap->va_rdev = dev->si_priv->cdp_inode;
+		vap->va_rdev = cdev2priv(dev)->cdp_inode;
 	}
 	vap->va_gen = 0;
 	vap->va_flags = 0;
@@ -742,7 +742,7 @@ devfs_lookupx(struct vop_lookup_args *ap, int *dm_unlock)
 		}
 
 		dev_lock();
-		dde = &cdev->si_priv->cdp_dirents[dmp->dm_idx];
+		dde = &cdev2priv(cdev)->cdp_dirents[dmp->dm_idx];
 		if (dde != NULL && *dde != NULL)
 			de = *dde;
 		dev_unlock();
@@ -1141,7 +1141,7 @@ devfs_revoke(struct vop_revoke_args *ap)
 	KASSERT((ap->a_flags & REVOKEALL) != 0, ("devfs_revoke !REVOKEALL"));
 
 	dev = vp->v_rdev;
-	cdp = dev->si_priv;
+	cdp = cdev2priv(dev);
  
 	dev_lock();
 	cdp->cdp_inuse++;
@@ -1419,7 +1419,7 @@ dev2udev(struct cdev *x)
 {
 	if (x == NULL)
 		return (NODEV);
-	return (x->si_priv->cdp_inode);
+	return (cdev2priv(x)->cdp_inode);
 }
 
 static struct fileops devfs_ops_f = {
diff --git a/sys/kern/kern_conf.c b/sys/kern/kern_conf.c
index c9eac628796a..e328ca97b38e 100644
--- a/sys/kern/kern_conf.c
+++ b/sys/kern/kern_conf.c
@@ -115,7 +115,7 @@ dev_free_devlocked(struct cdev *cdev)
 	struct cdev_priv *cdp;
 
 	mtx_assert(&devmtx, MA_OWNED);
-	cdp = cdev->si_priv;
+	cdp = cdev2priv(cdev);
 	TAILQ_INSERT_HEAD(&cdevp_free_list, cdp, cdp_list);
 }
 
@@ -187,7 +187,7 @@ dev_refthread(struct cdev *dev)
 	dev_lock();
 	csw = dev->si_devsw;
 	if (csw != NULL) {
-		cdp = dev->si_priv;
+		cdp = cdev2priv(dev);
 		if ((cdp->cdp_flags & CDP_SCHED_DTR) == 0)
 			dev->si_threadcount++;
 		else
@@ -208,7 +208,7 @@ devvn_refthread(struct vnode *vp, struct cdev **devp)
 	dev_lock();
 	*devp = vp->v_rdev;
 	if (*devp != NULL) {
-		cdp = (*devp)->si_priv;
+		cdp = cdev2priv(*devp);
 		if ((cdp->cdp_flags & CDP_SCHED_DTR) == 0) {
 			csw = (*devp)->si_devsw;
 			if (csw != NULL)
@@ -851,7 +851,7 @@ destroy_devl(struct cdev *dev)
 	dev_unlock();
 	notify_destroy(dev);
 	mtx_lock(&cdevpriv_mtx);
-	LIST_FOREACH_SAFE(p, &dev->si_priv->cdp_fdpriv, cdpd_list, p1) {
+	LIST_FOREACH_SAFE(p, &cdev2priv(dev)->cdp_fdpriv, cdpd_list, p1) {
 		devfs_destroy_cdevpriv(p);
 		mtx_lock(&cdevpriv_mtx);
 	}
@@ -1071,7 +1071,7 @@ clone_cleanup(struct clonedevs **cdp)
 		KASSERT(dev->si_flags & SI_CLONELIST,
 		    ("Dev %p(%s) should be on clonelist", dev, dev->si_name));
 		dev->si_flags &= ~SI_CLONELIST;
-		cp = dev->si_priv;
+		cp = cdev2priv(dev);
 		if (!(cp->cdp_flags & CDP_SCHED_DTR)) {
 			cp->cdp_flags |= CDP_SCHED_DTR;
 			KASSERT(dev->si_flags & SI_NAMED,
@@ -1125,7 +1125,7 @@ destroy_dev_sched_cbl(struct cdev *dev, void (*cb)(void *), void *arg)
 	struct cdev_priv *cp;
 
 	mtx_assert(&devmtx, MA_OWNED);
-	cp = dev->si_priv;
+	cp = cdev2priv(dev);
 	if (cp->cdp_flags & CDP_SCHED_DTR) {
 		dev_unlock();
 		return (0);
diff --git a/sys/sys/conf.h b/sys/sys/conf.h
index cd2bb8d1dcfa..739c3fe9cebd 100644
--- a/sys/sys/conf.h
+++ b/sys/sys/conf.h
@@ -53,7 +53,7 @@ struct cdevsw;
 struct file;
 
 struct cdev {
-	struct cdev_priv	*si_priv;
+	void		*__si_reserved;
 	u_int		si_flags;
 #define SI_ALIAS	0x0002	/* carrier of alias name */
 #define SI_NAMED	0x0004	/* make_dev{_alias} has been called */
diff --git a/usr.bin/fstat/fstat.c b/usr.bin/fstat/fstat.c
index 90ffba8207a6..886c3bb4746c 100644
--- a/usr.bin/fstat/fstat.c
+++ b/usr.bin/fstat/fstat.c
@@ -97,6 +97,7 @@ __FBSDID("$FreeBSD$");
 #include <pwd.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <stddef.h>
 #include <string.h>
 #include <unistd.h>
 #include <netdb.h>
@@ -895,10 +896,8 @@ dev_t
 dev2udev(struct cdev *dev)
 {
 	struct cdev_priv priv;
-	struct cdev si;
 
-	if (KVM_READ(dev, &si, sizeof si) &&
-	    KVM_READ(si.si_priv, &priv, sizeof priv)) {
+	if (KVM_READ(cdev2priv(dev), &priv, sizeof priv)) {
 		return ((dev_t)priv.cdp_inode);
 	} else {
 		dprintf(stderr, "can't convert cdev *%p to a dev_t\n", dev);