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disk/edisk: allow printing and readonly inspection of hybrid MBR/GPT disks (thanks aiju)

dumping hybrid MBR/GPT disks is fine, which can sometimes be found
on USB sticks. but prohibit editing.

however, always barf on disks with dos partitions and missing
protecive MBR partition entry.

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

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

@@ -713,6 +713,7 @@

 static uchar*

 readmbr(Disk *disk)

 {

+	int dosparts, protected;

 	uchar *mbr, *magic;

 	Tentry *t;

 	int i;

@@ -722,16 +723,24 @@

 	if(magic[0] != 0x55 || magic[1] != 0xAA)

 		sysfatal("did not find master boot record");

+	dosparts = protected = 0;

 	for(i=0; i<NTentry; i++){

 		t = (Tentry*)&mbr[disk->secsize - 2 - (i+1)*Tentrysiz];

 		switch(t->type){

 		case 0xEE:

+			protected = 1;

 		case 0xEF:

 		case 0x00:

 			continue;

 		}

-		sysfatal("dos partition table in use");

+		dosparts++;

 	}

+

+	if(dosparts && protected && !(printflag || rdonly))

+		sysfatal("potential hybrid MBR/GPT detected, not editing");

+

+	if(dosparts && !protected)

+		sysfatal("dos partition table in use and no protective partition found");

 	return mbr;

 }