Sublevel version mismatch when trying to compile current kernel

Question

I am trying to recompile the sound/usb module on Ubuntu 18.10 against the running kernel. I get no compilation errors but when I try to load the module I get this error:

Invalid module format

I am running the 4.18.0-21-lowlatency kernel.

I obtain the kernel source this way

apt source linux

This downloads and extracts the source of the 4.18.0 kernel into the linux-4.18.0 folder.

I copy the .config and Module.symvers files from /lib/modules/4.18.0-21-lowlatency/build to the root of my kernel source directory.

I run make EXTRAVERSION=-21-lowlatency modules_prepare, and then make EXTRAVERSION=-21-lowlatency M=sound/usb

Running insmod results in the following error written into syslog

snd_usb_audio: version magic '4.18.20-21-lowlatency SMP preempt mod_unload ' should be '4.18.0-21-lowlatency SMP preempt mod_unload '

Running modinfo /lib/modules/uname -r/kernel/sound/usb/snd-usb-audio.ko | grep vermagic returns this

vermagic:       4.18.0-21-lowlatency SMP preempt mod_unload

Running modinfo on my newly compiled module returns this

vermagic:       4.18.20-21-lowlatency SMP preempt mod_unload

I traced back the issue to the first few lines of the Makefile

VERSION = 4
PATCHLEVEL = 18
SUBLEVEL = 20

If I change the SUBLEVEL to 0 and then compile, I can successfully load the module.

So even though I am running the 4.18.0 kernel, and the apt source linux seems to download the 4.18.0 kernel, the downloaded files are versioned 4.18.20.

Is this normal or I am missing something?

Answer 1

I had similar problem. The trouble lies with the method how you obtain the kernel source and how you "build" the kernel. There is a link to what the official way of doing it. I followed it from https://help.ubuntu.com/community/Kernel/Compile. By the look of it, it appears to be out of date and long reading.

Based on the situation I want to use the official released Ubuntu kernel (not from anywhere else) apt-get source xxxxx. It is a folder with Debian folder and a tar ball. Then I follow partial instructions from Trying to build kernel on 18.04. No editconfigs option

1. Download kernel sources (deb-src should be uncommented in /etc/apt/sources.list)

   $ apt-get install linux-source kernel-package
   

2. Go to folder with kernel sources and untar

   $ cd /usr/src/linux-source-x.x.x
   $ tar jxvf linux-source-x.x.x.tar.bz2
   

3. Move content to current folder

   $ mv linux-source-x.x.x/* .
   $ rm -rf linux-source-x.x.x/
   

4. Get necessary packages

   $ apt-get build-dep linux-source
   $ mkdir debian/stamps
   

Then I can run fakeroot debian/rules clean and fakeroot debian/rules binary-headers binary-generic binary-perarch. In the middle I have to manually copy one amd gpu header file to the missing location, otherwise the compilation failed. This way the fakeroot debian/rules is using your running kernel configuration (uname -r).

If I use the make menuconfigure and .config, I end up the same problem as you. So you can use my sample to use official released Ubuntu kernel and fakeroot debian/rules to compile. The version of the module could match.

My version is Ubuntu 18.04, and my uname -r is 5.3.0-51. I use apt-get to get source around 2020-4-15.

I believe there are other ways to use make instead of fakeroot debian/rules, and it could help to build in-tree modules.