Re: Bug#1098638: fdisk: creates partitions one sector too large

On Sun, Feb 23, 2025 at 02:22:21PM GMT, Chris Hofstaedtler wrote:
> Control: tags -1 + upstream
> 
> Looping in upstream.
> 
> On Sat, Feb 22, 2025 at 12:57:38AM +0100, Thorsten Glaser wrote:
> > Package: fdisk
> > Version: 2.38.1-5+deb12u1
> > 
> > When I create a new partition with +sectors, fdisk creates one
> > that is one sector too large:
> > 
> > 
> > Command (m for help): n
> > Partition number (1-128, default 1):
> > First sector (2048-7814037134, default 2048):
> > Last sector, +/-sectors or +/-size{K,M,G,T,P} (2048-7814037134, default 7814035455): +2097152
> > 
> > Created a new partition 1 of type 'Linux filesystem' and of size 1 GiB.
> > 
> > Command (m for help): p
> > Disk /dev/sdb: 3.64 TiB, 4000787030016 bytes, 7814037168 sectors
> > Disk model: ST4000NM0245-1Z2
> > Units: sectors of 1 * 512 = 512 bytes
> > Sector size (logical/physical): 512 bytes / 4096 bytes
> > I/O size (minimum/optimal): 4096 bytes / 4096 bytes
> > Disklabel type: gpt
> > Disk identifier: DDA88AFF-1E6A-7448-AB5A-85470938DBC7
> > 
> > Device       Start        End    Sectors  Size Type
> > /dev/sdb1     2048    2099200    2097153    1G Linux filesystem

This is how fdisk has functioned since its inception. I noticed this
in the v2.13 code and was able to replicate it with v2.14 (year 2008).

The convention of +<sectors> is strictly evaluated as <first> + <size>,
without subtracting 1. This is different from +<size>{M,G,T,..} where the final
size is reduced by 1 and aligned to the disk I/O limits.

I have doubts about changing this after 20 years due to backward
compatibility. Additionally, it should be noted that people use 'echo
"something" | fdisk' in scripts ...

    Karel

-- 
 Karel Zak  <[email protected]>
 http://karelzak.blogspot.com