Why do I need superuser privileges when I just want to write to a regular file

categories: debian, linux

I have written a number of scripts to create Debian foreign architecture (mostly armel and armhf) rootfs images for SD cards or NAND flashing. I started with putting Debian on my Openmoko gta01 and gta02 and continued with devices like the qi nanonote, a marvel kirkwood based device, the Always Innovating Touchbook (close to the Beagleboard), the Notion Ink Adam and most recently the Golden Delicious gta04. Once it has been manufactured, I will surely also get my hands dirty with the Neo900 whose creators are currently looking for potential donors/customers to increase the size of the first batch and get the price per unit further down.

Creating a Debian rootfs disk image for all these devices basically follows the same steps:

  1. create an disk image file, partition it, format the partitions and mount the / partition into a directory
  2. use debootstrap or multistrap to extract a selection of armel or armhf packages into the directory
  3. copy over /usr/bin/qemu-arm-static for qemu user mode emulation
  4. chroot into the directory to execute package maintainer scripts with dpkg --configure -a
  5. copy the disk image onto the sd card

It was not long until I started wondering why I had to run all of the above steps with superuser privileges even though everything except the final step (which I will not cover here) was in principle nothing else than writing some magic bytes to files I had write access to (the disk image file) in some more or less fancy ways.

So I tried using fakeroot+fakechroot and after some initial troubles I managed to build a foreign architecture rootfs without needing root priviliges for steps two, three and four. I wrote about my solution which still included some workarounds in another article here. These workarounds were soon not needed anymore as upstream fixed the outstanding issues. As a result I wrote the polystrap tool which combines multistrap, fakeroot, fakechroot and qemu user mode emulation. Recently I managed to integrate proot support in a separate branch of polystrap.

Last year I got the LEGO ev3 robot for christmas and since it runs Linux I also wanted to put Debian on it by following the instructions given by the ev3dev project. Even though ev3dev calls itself a "distribution" it only deviates from pure Debian by its kernel, some configuration options and its initial package selection. Otherwise it's vanilla Debian. The project also supplies some multistrap based scripts which create the rootfs and then partition and populate an SD card. All of this is of course done as the superuser.

While the creation of the file/directory structure of the foreign Debian armel rootfs can by now easily be done without superuser priviliges by running multistrap under fakeroot/fakechroot/proot, creating the SD card image still seems to be a bit more tricky. While it is no problem to write a partition table to a regular file, it turned out to be tricky to mount these partition because tools like kpartx and losetup require superuser permissions. Tools like mkfs.ext3 and fuse-ext2 which otherwise would be able to work on a regular file without superuser privileges do not seem to allow to specify the required offsets that the partitions have within the disk image. With fuseloop there exists a tool which allows to "loop-mount" parts of a file in userspace to a new file and thus allows tools like mkfs.ext3 and fuse-ext2 to work as they normally do. But fuseloop is not packaged for Debian yet and thus also not in the current Debian stable. An obvious workaround would be to create and fill each partition in a separate file and concatenate them together. But why do I have to write my data twice just because I do not want to become the superuser? Even worse: because parted refuses to write a partition table to a file which is too small to hold the specified partitions, one spends twice the disk space of the final image: the image with the partition table plus the image with the main partition's content.

So lets summarize: a bootable foreign architecture SD card disk image is nothing else than a regular file representing the contents of the SD card as a block device. This disk image is created in my home directory and given enough free disk space there is nothing stopping me from writing any possible permutation of bits to that file. Obviously I'm interested in a permutation representing a valid partition table and file systems with sensible content. Why do I need superuser privileges to generate such a sensible permutation of bits?

Gladly it seems that the (at least in my opinion) hardest part of faking chroot and executing foreign architecture package maintainer scripts is already possible without superuser privileges by using fakeroot and fakechroot or proot together with qemu user mode emulation. But then there is still the blocker of creating the disk image itself through some user mode loop mounting of a filesystem occupying a virtual "partition" in the disk image.

Why has all this only become available so very recently and still requires a number of workarounds to fully work in userspace? There exists a surprising amount of scripts which wrap debootstrap/multistrap. Most of them require superuser privileges. Does everybody just accept that they have to put a sudo in front of every invocation and hope for the best? While this might be okay for well tested code like debootstrap and multistrap the countless wrapper scripts might accidentally (be it a bug in the code or a typo in the given command line arguments) write to your primary hard disk instead of your SD card. Such behavior can easily be mitigated by not executing any such script with superuser privileges in the first place.

Operations like loop mounting affect the whole system. Why do I have to touch anything outside of my home directory (/dev/loop in this case) to populate a file in it with some meaningful bits? Virtualization is no option because every virtualization solution again requires root privileges.

One might argue that a number of solutions just require some initial setup by root to then later be used by a regular user (for example /etc/fstab configuration or the schroot approach). But then again: why do I have to write anything outside of my home directory (even if it is only once) to be able to write something meaningful to a file in it?

The latter approach also does not work if one cannot become root in the first place or is limited by a virtualized environment. Imagine you are trying to build a Debian rootfs on a machine where you just have a regular user account. Or a situation I was recently in: I had a virtual server which denied me operations like loop mounting.

Given all these downsides, why is it still so common to just assume that one is able and willing to use sudo and be done with it in most cases?

I really wonder why technologies like fakeroot and fakechroot have only been developed this late. Has this problem not been around since the earliest days of Linux/Unix?

Am I missing something and rambling around for nothing? Is this idea a lost cause or something that is worth spending time and energy on to extend and fix the required tools?

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