Re: Cache coherency bug: stale reads on /dev/sda1

[Joshua Hudson]

On Sun, Aug 27, 2023 at 9:30 AM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> On Sun, Aug 27, 2023 at 07:39:03AM -0700, Joshua Hudson wrote:
> > Offset 0x1C into the FAT filesystem is defined as "Count of hidden
> > sectors preceding
> > the partition that contains this FAT volume." It's been there since
> > DOS 3.0. The Linux
> > Kernel does not care what's in this field, but I have tools that have
> > a hard time of it
> > not being there. One example of a tool is BootDuet.
> >
> > The rationale is to reduce the number of tools that have to walk the
> > partition table, the
> > most obvious one being the boot sector itself which doesn't have room.
>
> ???
>
> That makes no sense whatsoever; "boot sector" here is the first sector
> of _partition_, not that of the entire disk (that would be MBR).
>
> To quote the same wikipedia,
> ----
> A volume boot record (VBR) (also known as a volume boot sector,
> a partition boot record or a partition boot sector) is a type of boot
> sector introduced by the IBM Personal Computer. It may be found on a
> partitioned data storage device, such as a hard disk, or an unpartitioned
> device, such as a floppy disk, and contains machine code for bootstrapping
> programs (usually, but not necessarily, operating systems) stored in other
> parts of the device. On non-partitioned storage devices, it is the first
> sector of the device. On partitioned devices, it is the first sector of
> an individual partition on the device, with the first sector of the entire
> device being a Master Boot Record (MBR) containing the partition table.
> ----
>
> So your rationale doesn't work - you need to know where the partition is
> just to read the sector that contains that field.  Or have access to
> something that can be asked to read from partition, as opposed to the
> entire disk (kernel, for example), but that something can usually be
> asked where the hell does partition start.

The MBR doesn't look at the BPB. It looks at the first four bytes of the
partition table entry, loads that sector (which is the VBR), and jumps to
it. In the late 90s it became a thing to pass DL all the way through to
enable booting from ZIP disks, but nothing else is passed from the MBR
to the VBR. Thus, the BPB is in the VBR and describes the filesystem
and knows its own offset (otherwise the disk would not boot).

https://github.com/FDOS/kernel/blob/132a0a9f94d23f13c90319bb0e0232a5f33164d8/boot/boot.asm#L203

Almost everybody depended on this exact behavior for dual booting. Each
OS was defined in the boot menu as a path to a 512 byte file, which was
that OS's boot sector. They don't access disk relative to their own position,
but rather what their position should be.

So what I did: unmounted /boot/efi, started a virtual machine with /dev/sda
as the disk, booted a different OS than the host OS, shut it down, and got
very confused when changes to /boot/efi via the guest OS were not reflected
in the host OS.

>
> On anything with UEFI the first sector of the entire disk is likely to be
> the "protective MBR" in the beginning of GPT.
>

# dd if=/dev/sda bs=c skip=446 count=66 | hexdump -C
00000000  00 00 02 00 ee ff ff ff  01 00 00 00 af 88 e0 e8  |....îÿÿÿ....¯.àè|
00000010  80 20 21 00 06 df 13 0c  00 08 00 00 00 20 03 00  |. !..ß....... ..|
00000020  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000030  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000040  55 aa                                             |Uª|
00000042

I wouldn't be too sure of that. And yes, the second line corresponds
exactly to the EFI partition's entry in GPT.