Don't ignore a GPT that claims the last usable LBA is located

past the address where alternate partition entries would be
written.

Just adjust the GPT header value (gh_lba_end) to the highest safe
value and carry on.

Issue encountered in the wild by mlarkin@ while accessing some
disk images.

ok deraadt@

diff --git a/sbin/fdisk/gpt.c b/sbin/fdisk/gpt.c
index 1d9a552..fd0e7ed 100644 (file)
--- a/sbin/fdisk/gpt.c
+++ b/sbin/fdisk/gpt.c
@@ -1,4 +1,4 @@
-/*     $OpenBSD: gpt.c,v 1.27 2021/06/13 13:24:45 krw Exp $    */
+/*     $OpenBSD: gpt.c,v 1.28 2021/06/13 13:48:00 krw Exp $    */
 /*
  * Copyright (c) 2015 Markus Muller <mmu@grummel.net>
  * Copyright (c) 2015 Kenneth R Westerback <krw@openbsd.org>
@@ -55,7 +55,7 @@ int
 get_header(off_t where)
 {
        char *secbuf;
-       uint64_t partlastlba;
+       uint64_t partlastlba, partslen, lba_end;
        int partspersec;
        uint32_t orig_gh_csum, new_gh_csum;
 
@@ -112,10 +112,14 @@ get_header(off_t where)
                return (1);
        }
 
-       if (letoh64(gh.gh_lba_end) >= DL_GETDSIZE(&dl)) {
-               DPRINTF("gpt last usable LBA: expected < %lld, got %llu\n",
-                   DL_GETDSIZE(&dl), letoh64(gh.gh_lba_end));
-               return (1);
+       /* XXX Assume part_num * part_size is multiple of secsize. */
+       partslen = letoh32(gh.gh_part_num) * letoh32(gh.gh_part_size) /
+           dl.d_secsize;
+       lba_end = DL_GETDSIZE(&dl) - partslen - 2;
+       if (letoh64(gh.gh_lba_end) > lba_end) {
+               DPRINTF("gpt last usable LBA: reduced from %llu to %llu\n",
+                   letoh64(gh.gh_lba_end), lba_end);
+               gh.gh_lba_end = htole64(lba_end);
        }
 
        if (letoh64(gh.gh_lba_start) >= letoh64(gh.gh_lba_end)) {