fdisk/sfdisk: creates scripts that don't work

[smurfix]

Cf. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=931087

I have created a GPT-partitioned image where the first partition
needs to start at block 64. The problem is that fdisk/sfdisk is unable to
replicate this partition layout using an sfdisk script, because the
generated script doesn't work when it's read back in.

This seems to be a problem with interpreting the "table-length" value.
It appears to be multiplied by 4 instead of divided by 4 when checking
sector limits.

# ls -l /tmp/bmb_root
-rw-r--r-- 1 smurf smurf 1170228224 Jun 25 22:16 /tmp/bmb_root
# bc
bc 1.07.1
Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006, 2008, 2012-2017 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'. 
1170228224/512-34
2285568

# cat /tmp/lb  ### minimum/maximum working LBA values
label: gpt
label-id: 3C0E3DAC-3069-354E-BA0E-CB74029F5A5C
device: /tmp/bmb_root
unit: sectors
table-length: 8
first-lba: 34
last-lba: 2285568

# sfdisk /tmp/bmb_root </tmp/lb
Checking that no-one is using this disk right now ... OK

Disk /tmp/bmb_root: 1,1 GiB, 1170228224 bytes, 2285602 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 3C0E3DAC-3069-354E-BA0E-CB74029F5A5C

Created a new GPT disklabel (GUID: 3C0E3DAC-3069-354E-BA0E-CB74029F5A5C).
Partition table length changed from 128 to 8.

New situation:
Disklabel type: gpt
Disk identifier: 3C0E3DAC-3069-354E-BA0E-CB74029F5A5C

# sfdisk -d /tmp/bmb_root 
label: gpt
label-id: 3C0E3DAC-3069-354E-BA0E-CB74029F5A5C
device: /tmp/bmb_root
unit: sectors
first-lba: 4
last-lba: 2285598
table-length: 8
# 

Note the shifting "last-lba" value.

[smurfix]

Umm, nobody has looked into this? Owch.

[karelzak]

Sorry for the delay. It works for me, tested with v2.37.2 and the current upstream too:

# dd if=/dev/zero of=~/img bs=512 count=2285602
1170228224 bytes (1.2 GB, 1.1 GiB) copied, 3.08157 s, 380 MB/s

# cat ~/gpt
label: gpt
label-id: 3C0E3DAC-3069-354E-BA0E-CB74029F5A5C
device: /tmp/bmb_root
unit: sectors
table-length: 8
first-lba: 34
last-lba: 2285568

# sfdisk ~/img < ~/gpt
Checking that no-one is using this disk right now ... OK

Disk /root/img: 1.09 GiB, 1170228224 bytes, 2285602 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

>>> Script header accepted.
>>> Script header accepted.
>>> Script header accepted.
>>> Script header accepted.
>>> Script header accepted.
>>> Script header accepted.
>>> Script header accepted.
>>> Done.
Created a new GPT disklabel (GUID: 3C0E3DAC-3069-354E-BA0E-CB74029F5A5C).
The maximal number of partitions is 8 (default is 128).

New situation:
Disklabel type: gpt
Disk identifier: 3C0E3DAC-3069-354E-BA0E-CB74029F5A5C

The partition table has been altered.
Syncing disks.

# sfdisk -d ~/img
label: gpt
label-id: 3C0E3DAC-3069-354E-BA0E-CB74029F5A5C
device: /root/img
unit: sectors
first-lba: 34
last-lba: 2285568
table-length: 8
sector-size: 512

I guess it's fixed since commit fb64d93.

Thanks for your report and patience :-)