Lines of Code/Binary Size/Features in the Linux kernel

[dimakuv]

For comparison, I wanted to have a more or less reasonable metric of Lines of Code (LoC) in used-in-production Linux kernels.

Method for counting LoC

We use the inotifywait utility that will log which files are opened during make vmlinux build.

NOTE: both terminals must be in the Linux sources dir for pwd tricks to work correctly.

• In first terminal, prepare the Linux build:

# this is to get rid of `git status` invocation that pollutes the list of opened files,
# see https://github.com/torvalds/linux/blob/f1a09972a45a/scripts/setlocalversion#L123
mv .git .git.hide

cp ../generic-kernel-v5.19.0-ubuntu22.04-config ./.config
make clean
make olddefconfig

• In second terminal, start the inotifywait tool:

rm -rf kernel_build.log
inotifywait -m -r -e open --format '%w%f' -o kernel_build.log $PWD

• In first terminal again, start the Linux build:

make vmlinux -j32

Prepare kernel-build.log for cloc

cat kernel_build.log | cut -c `pwd | wc -c | awk '{print $1 + 1}'`- | sort -u \
    | grep -E "\.c$|\.h$|\.H$|\.s$|\.S$" \
    | grep -v -E "^\.|^certs|^samples|^scripts|^tools|^usr" > kernel_build.log.prepared

Time to cloc!

cloc --list-file=kernel_build.log.prepared

Result for generic Ubuntu 22.04 v5.19

Language                     files          blank        comment           code
-------------------------------------------------------------------------------
C                             2920         335002         417673        1576590
C/C++ Header                  3175          96563         193066         466800
Assembly                        64           1326           3974           5373
-------------------------------------------------------------------------------
SUM:                          6159         432891         614713        2048763

We get the config from the linux-buildinfo package:

wget http://security.ubuntu.com/ubuntu/pool/main/l/linux-hwe-5.19/linux-buildinfo-5.19.0-50-generic_5.19.0-50.50_amd64.deb
ar vx linux-buildinfo-5.19.0-50-generic_5.19.0-50.50_amd64.deb
tar --use-compress-program=unzstd -xvf data.tar.zst

cd ../
cp ubuntu22.04-linux-buildinfo-5.19/usr/lib/linux/5.19.0-50-generic/config ./generic-kernel-v5.19.0-ubuntu22.04-config

Note on .config file: To remove the need for Debian certs and keys, follow this advice:
https://stackoverflow.com/questions/67670169/compiling-kernel-gives-error-no-rule-to-make-target-debian-certs-debian-uefi-ce.
Basically, we need to remove debian/ from everywhere in the .config file. The attached config files are already like this.

Result for Amazon Firecracker v6.1

Language                     files          blank        comment           code
-------------------------------------------------------------------------------
C                             1247         157534         212474         754827
C/C++ Header                  2196          66274         126476         309104
Assembly                        42            922           2944           3511
-------------------------------------------------------------------------------
SUM:                          3485         224730         341894        1067442

Config: https://github.com/firecracker-microvm/firecracker/tree/main/resources/guest_configs

[dimakuv]

Binary sizes

Method

We use the explanations from here: https://elinux.org/Kernel_Size_Tuning_Guide

We use the size tool on the ELF (uncompressed binaries).

To extract vmlinuz into uncompressed ELF vmlinux, we use this: https://blog.packagecloud.io/how-to-extract-and-disassmble-a-linux-kernel-image-vmlinuz/. I.e., we download and use the extract-vmlinux script.

Results

$ size --format=GNU ubuntu-5.19-vmlinux
      text       data        bss      total filename
  19496120   22748264   17760256   60004640 ubuntu-5.19-vmlinux

$ size --format=GNU amazon-6.1-vmlinux
      text       data        bss      total filename
   8619247   10337888    3362816   22319951 amazon-6.1-vmlinux

ls results

$ ls -lah
68M ubuntu-5.19-vmlinux
27M amazon-6.1-vmlinux

[dimakuv]

Features of Linux .config

# normal Linux kernel
$ grep -rI "=y" generic-kernel-v5.19.0-ubuntu22.04-config | wc --lines
2877

$ grep -rI "=m" generic-kernel-v5.19.0-ubuntu22.04-config | wc --lines
5988

$ grep -rI "=y" amazon-firecracker-microvm-kernel-v6.1-config | wc --lines
789

$ grep -rI "=m" amazon-firecracker-microvm-kernel-v6.1-config | wc --lines
0