The Design and Implementation of the Anykernel and Rump Kernels

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4.2.2 makefs
For a source tree to be fully cross-buildable with build.sh [77], the build process
cannot rely on any non-standard kernel functionality because the functionality might
not exist on a non-NetBSD build host. The build.sh mandate also demands that a
build can be performed fully unprivileged, i.e. a root account is not required.
Before build.sh, the canonical approach to building a file system image for boot
media was to be to create a regular file, mount it using the loopback driver, copy the
files to the file system and unmount the image. This approach was not compatible
with the goals of build.sh.
When build.sh was introduced to NetBSD, it came with a tool called makefs which
creates a file system image from a given directory tree using only application level
code. In other words, the makefs application contains the file system driver implemented
for a userspace environment. This approach does not require privileges to
mount a file system or support of the target file system in the kernel. The original
utility had support for Berkeley FFS and was implemented by modifying and
reimplementing the FFS kernel code to be able to run in userspace. This copy-andmodify
approach was the only good approach available at the time.
The process of makefs consists of four phases:
1. scan the source directory
2. calculate target image size based on scan data
3. create the target image
4. copy source directory files to the target image