Workaround for ubuntu mainline kernel cross-compile issues with module building files compiled in wrong architecture

before-dkms-ubuntu-mainline

#!/bin/bash

# This should go in /etc/kernel/header_postinst.d


kernelver=$1

files=("scripts/recordmcount" "scripts/mod/modpost" \
        "scripts/basic/fixdep")
rebuild=0
ARCH=`uname -m`
for i in "${files[@]}"
do
ARCHCHECK=`file /usr/src/linux-headers-$kernelver/$i |grep -c  $ARCH`
if [ "$ARCHCHECK" = "1" ] ; then
echo "correct arch for $i"
else
echo "wrong arch for $i, rebuilding scripts"
rm /usr/src/linux-headers-$kernelver/$i
rebuild=1
fi
done
if [ "$rebuild" = "0" ] ;
then exit 0
fi

echo "Rebuilding Ubuntu Mainline module infrastructure for non-AMD64 arch"
echo "Note also that you will need to install the following"
echo "packages first using:"
echo "sudo apt install bison dkms flex gcc libssl-dev -y"
echo "Otherwise this process will fail."

majorversion=`grep VERSION /usr/src/linux-headers-$kernelver/Makefile | head -1 | awk -F ' = ' '{print $2}'`
patchlevel=`grep PATCHLEVEL /usr/src/linux-headers-$kernelver/Makefile | head -1 | awk -F ' = ' '{print $2}'`
sublevel=`grep SUBLEVEL /usr/src/linux-headers-$kernelver/Makefile | head -1 | awk -F ' = ' '{print $2}'`
extraversion=`grep EXTRAVERSION /usr/src/linux-headers-$kernelver/Makefile | head -1 | awk -F ' = ' '{print $2}'`
if [ ! -z "$extraversion" ]; then
        export kernid=$majorversion.$patchlevel$extraversion
else
        export kernid=$majorversion.$patchlevel.$sublevel
fi

echo "Now downloading missing headers needed for rebuild."
files=("security/selinux/include/classmap.h" \
"security/selinux/include/initial_sid_to_string.h" \
"tools/include/tools/be_byteshift.h" \
"tools/include/tools/le_byteshift.h")
for i in "${files[@]}"
do
if [ ! -f /usr/src/linux-headers-$kernelver/$i ]; then
    echo "Downloading $kernelver/$i"
    # curl --create-dirs -o /usr/src/linux-headers-$kernelver/$i https://git.launchpad.net/~ubuntu-kernel-test/ubuntu/+source/linux/+git/mainline-crack/plain/$i?id=v$kernid
    curl --create-dirs -o /usr/src/linux-headers-$kernelver/$i \
    https://raw.githubusercontent.com/torvalds/linux/v$kernid/$i
fi
done

cd /usr/src/linux-headers-$kernelver
# This next step is known to fail, but gets most of what is needed done.
make modules_prepare 2>/dev/null
#make scripts_basic
#make scripts/recordmcount
#make scripts/mod/modpost
exit 0