Challenge ·Easy

Run a Glibc-incompatible Binary Using chroot

Başar Subaşı
by  Başar Subaşı · on
Linux
Export an Alpine container rootfs using crane, set up a chroot environment, and execute a musl-linked binary that fails to run on the host system.
#chroot#musl#glibc#crane#alpine

You are provided with a binary located at /home/laborant/app/processor.

Try executing it directly on this Ubuntu machine:

/home/laborant/app/processor

You will notice that execution fails immediately with cannot execute: required file not found. This happens when a dynamically linked binary depends on a dynamic linker/interpreter that is not present on the host's root filesystem.

To solve this dependency issue without altering the host OS, you will isolate and run the application inside an Alpine Linux root filesystem at /tmp/newroot using chroot.

Linux Filesystem Isolation with chroot: Host filesystem vs. /tmp/newroot

Objectives

  1. Find out the linking information of the binary
  2. Get yourself a rootfs that contains the required dynamic libraries
  3. Run the app inside the correct rootfs
Hint 1: Inspecting the binary with file

Run file /home/laborant/app/processor directly in your host terminal to find the program interpreter required by the binary.

Hint 2: Exporting an image rootfs with crane

crane export <image> - streams the container filesystem tarball to standard output. You can pipe it directly to tar:

mkdir -p /tmp/newroot
crane export alpine:latest - | tar -xf - -C /tmp/newroot
Hint 3: Running a command in chroot

Once the rootfs is extracted and the app is placed inside /tmp/newroot/app/, execute:

sudo chroot /tmp/newroot /app/processor
Hint 4: Need a Refresher on chroot Basics?

Check out the tutorial Change Your Root: Chroot Basics to learn how the chroot system call manages process root pointers, shared library resolution, and root filesystem isolation.

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