Generating a .img from tarball: The /dev/loop devices

disk-imageloop-device

I'm trying to make a .img from a tarball using the following script I found:

#!/bin/bash

# Packages required
# dosfstools parted 
# Can be run on any Linux system
# loop.max_part=15 must be in kernel cmdline (cmdline.txt for rpi)
# then reboot

echo "creating image to fit on 2Gb card"
dd if=/dev/zero of=arch-rpi.img bs=1M count=1850

echo "Partitioning"
fdisk arch-rpi.img <<EOF
o
n
p
1

+100M

t
c
n
p
2


w
EOF

sleep 5

losetup -f arch-rpi.img
sleep 5

echo "Formatting vfat"
mkfs.vfat /dev/loop0p1
sleep 5
mkdir boot
echo "Mounting boot"
mount /dev/loop0p1 boot
echo "Installing"

echo "Formatting ext4"
mkfs.ext4 /dev/loop0p2
sleep 5
mkdir root
echo "Mounting root"
mount /dev/loop0p2 root

wget http://archlinuxarm.org/os/ArchLinuxARM-rpi-latest.tar.gz
echo "Installing"
bsdtar -xpf ArchLinuxARM-rpi-latest.tar.gz -C root
sync
mv root/boot/* boot
sync
umount boot root
losetup -d /dev/loop0p1
losetup -d /dev/loop0p1
losetup -d /dev/loop0
echo "All complete, image arch-rpi.img created, compressing...."
zip -9 arch-rpi.img.zip arch-rpi.img

I'm doing this on my Raspberry Pi – Raspbian Wheezy. And when it gets to the line mkfs.vfat /dev/loop0p1 It says there's no such file / directory. I've read enough on Linux to know I'm trying to mount the .img as a loop device and then use mkfs to prepare the image for the tarball, but I don't quite know why loop0p# files are not there, they are listed by fdisk -l. What do I need to to in order to get this script I found working?

Best Answer

You need to use losetup -P for creating a -Partitioned loop device, or else you need to partition the original loop device then partx -update the kernel's partition table afterward. The /dev/loop0p devices will appear after the kernel recognizes that the partition has actually been partitioned. Probably this is partly what the intent behind the sleep 5 is - but that should almost definitely be a sync - or both - instead.

Anyway, to demonstrate:

sudo sh -s <<\IN
    losetup -D                          
    fallocate "-l$((1024*1024*1024))" loop        
    printf %s\\n n '' '' '' ''  w y | gdisk loop
    sync; losetup -f loop
    lsblk /dev/loop*
IN

So the above sequence first -Detaches all current loop devices (if any) fallocates a 1GB tmp file, writes a GPT partition table to it and does a single partition on it, then syncs the filesystem and assigns it to the -first available loop device before trying to list with lsblk all available loop devices. It prints:

NAME  MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
loop0   7:0    0   1G  0 loop

But if I alter the losetup line to read:

losetup -fP loop

...it instead prints:

NAME      MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
loop0       7:0    0    1G  0 loop 
├─loop0p1 259:0    0 1023M  0 loop 
└─loop0p2 259:1    0 1007K  0 loop

...because losetup first scans the backing file for a partition table. As you can see, it's not very good at it - rather than noting the extra 1M (which is almost definitely the last iteration's partition table, actually) as unallocated space it interprets it as a separate partition, but that's probably because I'm writing to a tmpfs (twice in a row) at the top of a 1GB file hole - working w/ actual data will be much more precise (and zeroing the backing file as your script does w/ dd would handle that anyway). In any case - I can freely mkfs.whatever on the real partition there and afterwards mount it. partx -u on the /dev/loop0 would achieve the same results.

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