libfdisk: don't use FAT as MBR

The current libfdisk MBR detection is too weak, the result is that it
reuses MBR from FAT. The correct behavior is to create a new MBR, wipe
first sector (on write) and warn about obsolete FAT superblock.

Reported-by: Mike Fleetwood <mike.fleetwood@googlemail.com>
Signed-off-by: Karel Zak <kzak@redhat.com>

2 files changed, 15 insertions, 5 deletions

diff --git a/libfdisk/src/context.c b/libfdisk/src/context.c
index 770aa4d52..bc284f34e 100644
--- a/libfdisk/src/context.c
+++ b/libfdisk/src/context.c
@@ -619,14 +619,20 @@ static int fdisk_assign_fd(struct fdisk_context *cxt, int fd,
 	if (fdisk_read_firstsector(cxt) < 0)
 		goto fail;
 
+	/* warn about obsolete stuff on the device if we aren't in list-only */
+	if (!fdisk_is_listonly(cxt) && fdisk_check_collisions(cxt) < 0)
+		goto fail;
+
 	fdisk_probe_labels(cxt);
 	fdisk_apply_label_device_properties(cxt);
 
-	/* warn about obsolete stuff on the device if we aren't in
-	 * list-only mode and there is not PT yet */
-	if (!fdisk_is_listonly(cxt) && !fdisk_has_label(cxt)
-	    && fdisk_check_collisions(cxt) < 0)
-		goto fail;
+	/* Don't report collision if there is already a valid partition table.
+	 * The bootbits are wiped when we create a *new* partition table only. */
+	if (fdisk_is_ptcollision(cxt) && fdisk_has_label(cxt)) {
+		cxt->pt_collision = 0;
+		free(cxt->collision);
+		cxt->collision = NULL;
+	}
 
 	DBG(CXT, ul_debugobj(cxt, "initialized for %s [%s]",
 			      fname, readonly ? "READ-ONLY" : "READ-WRITE"));
diff --git a/libfdisk/src/dos.c b/libfdisk/src/dos.c
index 6dc276e8a..bd964db15 100644
--- a/libfdisk/src/dos.c
+++ b/libfdisk/src/dos.c
@@ -814,6 +814,10 @@ static int dos_probe_label(struct fdisk_context *cxt)
 	if (!mbr_is_valid_magic(cxt->firstsector))
 		return 0;
 
+	/* ignore disks with FAT */
+	if (cxt->collision && strcmp(cxt->collision, "vfat") == 0)
+		return 0;
+
 	dos_init(cxt);
 
 	get_partition_table_geometry(cxt, &h, &s);