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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?

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