Re: [RFC][PATCH] unique sigs in the msdos label

[John Gilmore]

I think writing 'unique signatures into disk labels' is a terrible idea.

Intel is working on a Legacy-BIOS-free spec.  It should provide a way
to determine exactly what device is the boot device -- without this
hocus pocus about storing different magic numbers in every partition
table, retrieving them via different routes to compare and contrast.
By getting involved in the writing of this spec, we should be able to
put a stake through the heart of this issue once and for all, in new
hardware.  Like the concept of cylinders, 'BIOS drive numbers' should
hsve been d-e-a-d two decades ago.

As far as I know, nobody cares what device numbers the BIOS assigned
to devices other than the boot disk.  The main question is, "What
disk and partition did I, this kernel/bootstrapper/etc, get read in from?"

In sane systems, the processor's boot ROMs don't even know the format
of partition tables.

> Windows NT and later store their own 32-bit unique signature here at 
> partition table creation, and the "Disk Administrator" (older NT) or 
> "Disk Management" (>=W2K) tools ensure they're unique across the system.  
> If you've ever moved a disk from another system into yours and run these 
> tools, you'll see it prompt you to write a signature to the disk, and 
> maybe reboot.

This comment makes me think that mere uniqueness isn't all that
Microsoft is doing here.  There'd be no need to rewrite the signature
of a disk that moved from a different CPU, unless there was some kind of
processor-ID in the signature.  The chance of getting two disks with
matching random 4-byte values would be nil, and Mr. Domsch would never
have seen the Microsoft code notice that it needed to rewrite one of
them.  (What does it do about read-only media that it CAN'T rewrite?
Linux, at least, needs to be able to boot from such media, and use them
freely.)

        John