From 3d40667ede5978c665cc4fbff0aaf3b92c16a5da Mon Sep 17 00:00:00 2001
From: krw <krw@openbsd.org>
Date: Tue, 26 Jul 2022 14:30:37 +0000
Subject: [PATCH] Allow editing of an MBR of all zeros.

Fallout from regress failure spotted by anton@.
---
 sbin/fdisk/mbr.c | 13 +++++++++++--
 sbin/fdisk/mbr.h |  3 ++-
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/sbin/fdisk/mbr.c b/sbin/fdisk/mbr.c
index 2ef6783110f..6e6b6c58caf 100644
--- a/sbin/fdisk/mbr.c
+++ b/sbin/fdisk/mbr.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: mbr.c,v 1.120 2022/07/25 17:45:16 krw Exp $	*/
+/*	$OpenBSD: mbr.c,v 1.121 2022/07/26 14:30:37 krw Exp $	*/
 
 /*
  * Copyright (c) 1997 Tobias Weingartner
@@ -105,7 +105,13 @@ dos_mbr_to_mbr(const struct dos_mbr *dmbr, const uint64_t lba_self,
     const uint64_t lba_firstembr, struct mbr *mbr)
 {
 	struct dos_partition	dos_parts[NDOSPART];
-	int			i;
+	uint8_t			*p;
+	unsigned int		 i;
+
+	p = (uint8_t *)dmbr;
+	mbr->mbr_dmbrzeros = 0;
+	for (i = 0; i < sizeof(struct dos_mbr) && *p == 0; i++, p++)
+		mbr->mbr_dmbrzeros++;
 
 	memcpy(mbr->mbr_code, dmbr->dmbr_boot, sizeof(mbr->mbr_code));
 	mbr->mbr_lba_self = lba_self;
@@ -188,6 +194,9 @@ MBR_valid_prt(const struct mbr *mbr)
 	unsigned int		i, nprt;
 	unsigned char		id;
 
+	if (mbr->mbr_dmbrzeros == sizeof(struct dos_mbr))
+		return 1;	/* All zeros struct dos_mbr is editable. */
+
 	nprt = 0;
 	for (i = 0; i < NDOSPART; i++) {
 		bs = mbr->mbr_prt[i].prt_bs;
diff --git a/sbin/fdisk/mbr.h b/sbin/fdisk/mbr.h
index 0c56a3db4e0..3aee1dabdb5 100644
--- a/sbin/fdisk/mbr.h
+++ b/sbin/fdisk/mbr.h
@@ -1,4 +1,4 @@
-/*	$OpenBSD: mbr.h,v 1.43 2022/07/25 17:45:16 krw Exp $	*/
+/*	$OpenBSD: mbr.h,v 1.44 2022/07/26 14:30:37 krw Exp $	*/
 
 /*
  * Copyright (c) 1997 Tobias Weingartner
@@ -22,6 +22,7 @@ struct mbr {
 	unsigned char	mbr_code[DOSPARTOFF];
 	struct prt	mbr_prt[NDOSPART];
 	uint16_t	mbr_signature;
+	unsigned int	mbr_dmbrzeros;
 };
 
 extern struct dos_mbr	default_dmbr;
-- 
2.20.1