diff --git a/usr.sbin/installboot/bootstrap.c b/usr.sbin/installboot/bootstrap.c
index 7576b1cb5e4..9abb63823b8 100644
--- a/usr.sbin/installboot/bootstrap.c
+++ b/usr.sbin/installboot/bootstrap.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: bootstrap.c,v 1.14 2025/09/17 16:12:10 deraadt Exp $	*/
+/*	$OpenBSD: bootstrap.c,v 1.15 2025/11/19 15:05:04 deraadt Exp $	*/
 
 /*
  * Copyright (c) 2013 Joel Sing <jsing@openbsd.org>
@@ -112,12 +112,12 @@ bootstrap(int devfd, char *dev, char *bootfile)
 	 */
 	lp = (struct disklabel *)(boot + (LABELSECTOR * DEV_BSIZE) +
 	    LABELOFFSET);
-	for (i = 0, p = (char *)lp; i < (int)sizeof(*lp); i++)
+	for (i = 0, p = (char *)lp; i < dl16sz; i++)
 		if (p[i] != 0)
 			errx(1, "bootstrap has data in disklabel area");
 
 	/* Patch the disklabel into the bootstrap code. */
-	memcpy(lp, &dl, sizeof(dl));
+	memcpy(lp, &dl, dl16sz);
 
 	/* Write the bootstrap out to the disk. */
 	if (verbose)
diff --git a/usr.sbin/installboot/installboot.c b/usr.sbin/installboot/installboot.c
index dbbd3309d32..cd6d692b46d 100644
--- a/usr.sbin/installboot/installboot.c
+++ b/usr.sbin/installboot/installboot.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: installboot.c,v 1.17 2025/02/19 21:30:46 kettenis Exp $	*/
+/*	$OpenBSD: installboot.c,v 1.18 2025/11/19 15:05:04 deraadt Exp $	*/
 
 /*
  * Copyright (c) 2012, 2013 Joel Sing <jsing@openbsd.org>
@@ -16,11 +16,14 @@
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
+#include <sys/types.h>
+#include <sys/disklabel.h>
 #include <err.h>
 #include <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <stddef.h>
 #include <unistd.h>
 #include <util.h>
 
@@ -36,6 +39,14 @@ char	*root;
 char	*stage1;
 char	*stage2;
 
+/*
+ * XXX Only read the old smaller "skinny" label for now which
+ * has 16 partitions. offsetof() is used to carve struct disklabel.
+ * Later we'll add code to read and process the "fat" label with
+ * 52 partitions.
+ */
+size_t dl16sz = offsetof(struct disklabel, d_partitions[MAXPARTITIONS16]);
+
 static __dead void
 usage(void)
 {
diff --git a/usr.sbin/installboot/installboot.h b/usr.sbin/installboot/installboot.h
index 5b460a63ab3..55256287747 100644
--- a/usr.sbin/installboot/installboot.h
+++ b/usr.sbin/installboot/installboot.h
@@ -1,4 +1,4 @@
-/*	$OpenBSD: installboot.h,v 1.17 2025/02/19 21:30:46 kettenis Exp $	*/
+/*	$OpenBSD: installboot.h,v 1.18 2025/11/19 15:05:04 deraadt Exp $	*/
 /*
  * Copyright (c) 2012, 2013 Joel Sing <jsing@openbsd.org>
  *
@@ -28,6 +28,8 @@ extern char *root;
 extern char *stage1;
 extern char *stage2;
 
+extern size_t dl16sz;
+
 #ifdef BOOTSTRAP
 void	bootstrap(int, char *, char *);
 #endif
diff --git a/usr.sbin/installboot/landisk_installboot.c b/usr.sbin/installboot/landisk_installboot.c
index b8ed8ddbbae..e0440e6b196 100644
--- a/usr.sbin/installboot/landisk_installboot.c
+++ b/usr.sbin/installboot/landisk_installboot.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: landisk_installboot.c,v 1.13 2025/09/17 16:12:10 deraadt Exp $	*/
+/*	$OpenBSD: landisk_installboot.c,v 1.14 2025/11/19 15:05:04 deraadt Exp $	*/
 
 /*
  * Copyright (c) 2013 Joel Sing <jsing@openbsd.org>
@@ -208,12 +208,12 @@ md_bootstrap(int devfd, char *dev, char *bootfile)
 	 */
 	lp = (struct disklabel *)(boot + (LABELSECTOR * DEV_BSIZE) +
 	    LABELOFFSET);
-	for (i = 0, p = (char *)lp; i < (int)sizeof(*lp); i++)
+	for (i = 0, p = (char *)lp; i < dl16sz; i++)
 		if (p[i] != 0)
 			errx(1, "bootstrap has data in disklabel area");
 
 	/* Patch the disklabel into the bootstrap code. */
-	memcpy(lp, &dl, sizeof(dl));
+	memcpy(lp, &dl, dl16sz);
 
 	/* Write the bootstrap out to the disk. */
 	bootpos *= dl.d_secsize;