Creating a custom package for building the kernel

[medvednic]

Hello,

I need to create an ebuild for building and installing the kernel (4.1.15-r1) based on a custom configuration file, the following dependencies should be included:

[fpemud]

medvednic,

Welcome to Gentoo.


I have been already doing this from 2013:
Project: https://github.com/fpemud/fpemud-kernelmanager
Ebuild: https://github.com/fpemud/fpemud-overlay/tree/master/pkg-fpemud/fpemud-kernelmanager

It manages from linux-3.16 to my current kernel linux-4.4.6.
I suggest you create your own medvednic-kernelmanger by forking this project.
I'd like to offer any help you need.


Compiling linux kernel in ebuild can give perfect experience.
But I failed to make such a ebuild because I found the management procedure for linux kernel is different from applications.
I guess it is the very reason why all the ebuilds install linux source only.

fpemud-kernelmanager does the downloading, configuring, compiling, installing on its own.
And it also does many other related works such as initramfs making, grub installing, resuce kernel managing, history kernel managing.
It is hard to make a generic tool for these job.


If you don't want to bother, you can simply use sys-kernel/genkernel-next which I think you already know about.
But I don't think it can give a perfect experience.

[Genone]

Why would you _need_ such an ebuild?
While it's possible to do kernel compilation/installation in an ebuild it would require a number of hacks to achieve it and is therefore not recommended.

[medvednic]

Well I need such an ebuild because I have to do it as part of an assignment at my college, I came to this last bastion of hope to seek some help and guidance, hoping that some one can throw me a bone which I can use.

[Genone]

Thing is: you seem to be quite new to ebuild writing in general, and the kernel is a very special component, so actually answering your question in a way that helps you is a bit much for a forum post if you need an ebuild to build and install a kernel properly.

I could of course just tell you that you'll need the functions src_compile, src_install and pkg_postinst, but I don't think that helps you much. And sorry to be so direct, but that you're asking where to get the kernel source while having a DEPEND on gentoo-sources shows that you don't really understand what you're doing so far. Either you want to rely on gentoo-sources to handle the kernel sources (then your question is obsolete) or you write your own kernel ebuild in which case you'd better use kernel-2.eclass (which is a beast on its own).
And this isn't even talking about stuff like installing modules, setting up bootloaders, rebuilding external modules and some other things I likely forgot.

[steveL]

[medvednic]

Thanks for the replies, I will try to ask more specific questions after reading some documentation and trying to write something that actually performs.

What I want to achieve:
the ebuild will download the kernel source, configure the kernel according to a custom .config file and install the kernel in a directory that would be specified as an argument to make -C.

What I achieved so far:
after executing emerge on my .ebuild file: emerge gentoo-kernel
which now consists of:

[khayyam]

medvednic ... something you need to understand is that a package (such as yours) doesn't act on some part of the filesystem (ie, /usr/src/linux), what happens is that sources are unpacked in /var/tmp/portage/<category>/<package>/work and all the patching, configuring, building happens there. Once built the resulting binaries, docs, etc, etc, are installed to /var/tmp/portage/<category>/<package>/image, and it is these files that get installed ('merged') into the root of the filesystem. From the above description I can see you don't understand this fact, and this makes your task of writing such an ebuild problematic, because its these concepts, routines, etc, that constitute the ebuild. So, really, you need to understand the basics first, which I would assume is the purpose of this assignment.

An ebuild is in essence a script, what goes into that is script is simply those things required to configure, build, and install the target 'application', or 'package'. So, for that reason, forget about sys-kernel/gentoo-sources, because its not a dependency, or the sources of your package, as its a package in its own right, a package installed already into the root of the filesystem.

As Genone stated above, that is what makes the linux kernel sources different, because these are generally installed into the filesystem, configured, built, and installed, outside the package manager (the package manager is only 'managing' the sources). You could effectively do this via an ebuild but its not a simple matter, particularly if you don't have a basic understanding of what the process involves. That said, I'm trying to understand the nature of the assignment, are you expected to do prior research into ebuild development before attempting such a task, what exactly was the scope of the assignment and its background? It seems that you need to do a lot more research before fielding questions of the type above, because all you really have currently is 'skel.ebuild' with a number of mistaken assumptions about the way ebuilds work. It might be better if you ask questions of a more general nature, or for resources on the subject, because I don't think you'll make much headway without getting such a basic understanding.

best ... khay

[Genone]

And just to clarify a bit more: While building and installing the kernel manually is pretty straightforward, doing the same inside an ebuild (or any other more or less formal automated system) is a whole different story. Especially if you want to do it properly.

[C5ace]

I use a "Cut & Paste" script to install gentoo on desktops, VirtualBox and laptops, including building the kernel with genkernel. This script can easily be used as basis to write a "C" command line program to do the same.

This is the script I used last time to install gentoo on VirtualBox:

[khayyam]

[C5ace]

[khayyam]

[szatox]

[khayyam]

[szatox]

[C5ace]

[khayyam]

[C5ace]

[khayyam]

[C5ace]

[khayyam]

[Genone]

The big issue with depending on *-sources is versioning. If you have a unversioned dependency your ebuild has either no idea which path to use (you can't rely on the /usr/src/linux symlink to exist), or if you use $PV and $PVR you cannot rely on that specific version to be installed. So you have to specify versioned dependencies which means that you have to keep your ebuild up to date with the *-sources package all the time.

Btw, if you go the DEPEND="*-sources" approach you don't actually have to copy the sources as the kernel supports out-of-tree compilation natively.

Short version: Using an ebuild to build+install the kernel is like using a screwdriver to put in a nail - possible, but very inefficient.

[medvednic]

If one didn't copy the installed sources from /usr/src to emerge's build directory (as mentioned by szatox), can that be the cause of errors like:

[khayyam]

medvednic ...

please show us the ebuild and the build.log ...