Re: [Bug-xorriso] fdisk complains: GPT PMBR size mismatch

[Thomas Schmitt]

Hi,

> when passing an iso to `fdisk -l` it complains that there's a "GPT
> PMBR size mismatch".
> GPT PMBR size mismatch (583191 != 583807) will be corrected by w(rite).

Isn't gdisk in charge for GPT ?

"PMBR" probably means "Protective MBR". See
  http://en.wikipedia.org/wiki/GUID_Partition_Table#Legacy_MBR_.28LBA_0.29

583807 - 583191 = 616 blocks = 308 KiB.

We have a traditional 0-byte padding of 300 KiB, in case the image
ends up on a CD written with write type TAO. (Linux CD readahead bug.)
The padding is covered by the MBR partitions, because they shall
cover the whole ISO image.
But the padding cannot be covered by GPT, because a GPT disk ends
by the GPT backup header block. That block is valuable and thus
needs to be stored before the padding.

Now from where do we get the surplus 8 KiB ?

Obviously fdisk displays the GPT entries and not the MBR partition table:

> tests-grub.iso1    64          487   212K Microsoft basic data
> tests-grub.iso2   488          879   196K EFI System
> tests-grub.iso3   880       583127 284.3M Microsoft basic data

I'd like to inspect the first 34 KiB of that image

  dd if=tests-grub.iso bs=1K count=34 of=for_bug_xorriso.bin

This will include MBR, GPT, and the Primary Volume Descriptor of
the ISO image. No file names, and no file data, except what you
submitted by -boot_image.


> Though, the iso boots as expected. Can this warning (?) be ignored?

It should be quite harmless for booting. (EFI booting tolerates
wrong checksums in the GPT. So it will probably not care for the
MBR at all.)
But it might indicate upcomming trouble with partition editors.

Try xorriso command
   -padding 0
and look whether the warning disappears.
If not, then send me the first 34 KiB of that ISO , too.

What xorriso commands exactly do you use ?

What do other partition editors like gdisk report ?

Have a nice day :)

Thomas