[bug] incorrect GPT entry validation?

[Dave Reisner <d@falconindy.com>]

Hi,

A user reported problems with libblkid incorrectly reporting (or not
reporting) tags for some partitions on a GPT formatted disk. I was able
to reproduce it:

- created a 10G disk on a VM
- initialized it with GPT (using gdisk), created 5 entries
- formatted each of them with ext4

blkid correctly reads the first 2 entries on the disk, but then starts
reporting corrupted GPT entries (seen via LIBBLKID_DEBUG=0xffff):

https://dev.archlinux.org/~dreisner/blkid_bad.txt

It's interesting to note that blkid will _always_ read 2 entries before
reporting corruption, i.e.

# blkid -c /dev/null /dev/vdd{4,5,2}

This shows full info for /dev/vdd4 and /dev/vdd5, but nothing for
/dev/vdd2.

Commenting out the final crc validation in libblkid/src/partitions/gpt.c
and rebuilding lets blkid continue on and it happily prints everything:

https://dev.archlinux.org/~dreisner/blkid_no_validate.txt

Of course this is entirely wrong, but I'm not familiar at all with the
internals of libblkid or GPT to understand what's misbehaving.


As an aside, I was sadly unable to create the same layout using the
shiny new fdisk:

Command (m for help): g

Command (m for help): n
Partition number (1-128): 1
First sector (34-20971486, default 34):
Using default value 34
Last sector (34-20971486, default 20971486): 2G
Value out of range.
Last sector (34-20971486, default 20971486): +2G
Created partition 1

Command (m for help): n
Partition number (1-128): 2
First sector (4194305-20971486, default 4194305): +2G
Value out of range.
First sector (4194305-20971486, default 4194305): +1G
Value out of range.
First sector (4194305-20971486, default 4194305): +1M
Value out of range.

Cheers,
Dave