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disk/edisk: fix blank mode

there where two problems with blank (-b flag):

we did not update the backup header when there was already a valid
backup header in place. we always want to initialize a new backup header
in blank mode!

we now also check the backup header matches the primary (or the other
way arround depending on which header could be read), reporting any
mismatches and restoring the backup from the data of the primary.

the protective mbr needs to start at sector 1 not 0 (apparently, this
matters for ovmf).

--- a/sys/src/cmd/disk/prep/edisk.c

+++ b/sys/src/cmd/disk/prep/edisk.c

@@ -610,6 +610,33 @@

 	return 0;

 }

+static char*

+checkhdr(Header *a, Header *b)

+{

+	if(memcmp(a->sig, b->sig, sizeof(a->sig)) != 0)

+		return "signature";

+	if(memcmp(a->rev, b->rev, sizeof(a->rev)) != 0)

+		return "revision";

+	if(memcmp(a->hdrsiz, b->hdrsiz, sizeof(a->hdrsiz)) != 0)

+		return "header size";

+	if(memcmp(a->selflba, b->backlba, sizeof(a->selflba)) != 0

+	|| memcmp(a->backlba, b->selflba, sizeof(a->backlba)) != 0)

+		return "backup lba/self lba";

+	if(memcmp(a->firstlba, b->firstlba, sizeof(a->firstlba)) != 0)

+		return "first lba";

+	if(memcmp(a->lastlba, b->lastlba, sizeof(a->lastlba)) != 0)

+		return "last lba";

+	if(memcmp(a->devid, b->devid, sizeof(a->devid)) != 0)

+		return "device guid";

+	if(memcmp(a->entrycount, b->entrycount, sizeof(a->entrycount)) != 0)

+		return "entry count";

+	if(memcmp(a->entrysize, b->entrysize, sizeof(a->entrysize)) != 0)

+		return "entry size";

+	if(memcmp(a->tabcrc, b->tabcrc, sizeof(a->tabcrc)) != 0)

+		return "table checksum";

+	return nil;

+}

+

 static Header*

 getbakhdr(Edit *edit, Header *bhdr)

 {

@@ -619,10 +646,18 @@

 	siz = getle32(bhdr->hdrsiz);

 	lba = getle64(bhdr->backlba);

-	hdr = readhdr(edit->disk, lba);

-	if(hdr != nil)

-		return hdr;

+	if(!blank){

+		char *mismatch;

+

+		mismatch = "data";

+		hdr = readhdr(edit->disk, lba);

+		if(hdr != nil && (mismatch = checkhdr(bhdr, hdr)) == nil)

+			return hdr;

+		fprint(2, "backup header at lba %lld has mismatching %s, restoring.\n",

+			lba, mismatch);

+	}

+

 	hdr = getblock(edit->disk, lba);

 	memmove(hdr, bhdr, siz);

 	putle64(hdr->selflba, lba);

@@ -756,13 +791,13 @@

 	t->type = 0xEE;

 	t->active = 0;

-	size = disk->secs > 0xFFFFFFFF ? 0xFFFFFFFF : disk->secs;

-	putle32(t->lba, 0);

+	size = (disk->secs - 1) > 0xFFFFFFFF ? 0xFFFFFFFF : (disk->secs - 1);

+	putle32(t->lba, 1);

 	putle32(t->size, size);

 	t->starth = 0;

 	t->startc = 0;

-	t->starts = 0;

+	t->starts = 1;

 	t->endh = disk->h-1;

 	t->ends = (disk->s & 0x3F) | (((disk->c-1)>>2) & 0xC0);

 	t->endc = disk->c-1;