Allow editing of an MBR of all zeros.

Fallout from regress failure spotted by anton@.

diff --git a/sbin/fdisk/mbr.c b/sbin/fdisk/mbr.c
index 2ef6783..6e6b6c5 100644 (file)
--- 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 0c56a3d..3aee1da 100644 (file)
--- 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;