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CategoryGit CategoryGit CategoryDebugging

On this page is described an easy way to debug upstream Linux kernel issues, by using "git bisect". The idea of this tool is to track down a particular issue (or regression) by selecting the faulty revision.

/!\ Before starting to bisect, it would be a good idea to use Debian's wayback machine to narrow down the range of Linux kernel versions where the faulty revision may occur so that the amount of compiling is reduced somewhat.

If the issue does not appear in the upstream Linux kernel, but does appear in the Debian Linux kernel images, please refer to the Debian Linux Kernel Handbook for information about how to debug and report the issue.

Use cases

  • Hibernation works under 2.6.25-rc6 but not anymore under 2.6.25-rc7 and I would like to know which revision caused the regression.

See Also

Needed tools

For these regression tests, you will need the following packages (to install with your favorite package manager):

$ sudo apt install git gitk fakeroot

Alternatively, just install all the Linux kernel build-dependencies and git:

$ sudo apt install git
$ sudo apt build-dep linux

At times the Linux kernel and the toolchain versions might be out of sync, resulting in lots of strange build failures. In that case, it would be a good idea to use a Debian stable chroot to do the building.

It is recommended to have your shell prompt display git repository status information. The git package includes prompt support for bash and zsh.

Getting the Linux git source

In this step, we will get a local copy of the whole Linux kernel source code by using Linux's distributed version control system: git.

Move to your source directory

This is where you will be building the code.

$ cd ~/src

Clone Linux git repository

This will download several gigabytes data and thus may need a long time.

You can either clone Linus Torvald's main Linux repository, or the repository containing the Linux kernel stable release series. The stable repository contains more commits and is useful for bisecting bugs introduced by stable updates.

$ git clone https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/
$ git clone https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/

To get the best of both worlds, you can set up a single local repository which contains references to both of the upstream repositories.

$ git clone --origin torvalds https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/
$ cd linux
$ git remote add stable https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/
$ git fetch stable

Separate the wheat from the chaff

In this step, we will mark the versions good or bad.

Start the git-bisect process

$ git bisect start

Mark the ''good'' version

For example:

$ git bisect good v2.6.25-rc6 

Mark the ''bad'' version

For example:

$ git bisect bad v2.6.25-rc7 

At this point, this command should answer you something like that :

Bisecting: 182 revisions left to test after this
[2c7871982cf27caaddbaeb7e2121ce1374b520ff] Merge git://git.kernel.org/pub/scm/linux/kernel/git/lethal/sh-2.6.25

The interesting part is the number (in our case, 182) which is the number of Linux kernel revisions that lay between your good and your bad.

Build your Linux kernel

In this step, we will configure the future Linux kernel according to the actual configuration and then compile the new Linux kernel.

Copy a valid configuration

For example, the configuration of the running Linux kernel:

$ cp /boot/config-$(uname -r) .config

Put git commit IDs into versions

This ensures that you can easily map the installed Linux kernel images to a git commit ID. If you do not have much disk space on /boot you should either skip this step or be prepared during testing to remove some Linux kernel images that have already been tested.

$ scripts/config --enable LOCALVERSION_AUTO

Disabling debug information

If your bisect won't need to debug the kernel, it is a good idea to disable debug info to reduce the build time, especially for the linux-image-*-dbg package, which will be very large and can take a long time to compress.

$ scripts/config --enable DEBUG_INFO_NONE # kernels since v5.18-rc1
$ scripts/config --disable DEBUG_INFO     # kernels before v5.18-rc1

Disable Secure Boot

If you are using a config from a Linux kernel from Debian or other distro kernel that has SecureBoot enabled using the distro key, then your builds will fail because you don't have a copy of the private distro key. First disable Secure Boot in your UEFI boot setup and then disable it in your Linux kernel config:

$ scripts/config --disable SYSTEM_TRUSTED_KEYRING
$ scripts/config --set-str SYSTEM_TRUSTED_KEYS ''

Disable Stack Protector

If you are using a newer GCC while bisecting older kernel versions, you may need to disable the stack protector during the bisect.

$ scripts/config --disable STACKPROTECTOR_STRONG

Configure the Linux kernel

If you do not know the answer to each question, you can just accept the defaults:

$ make olddefconfig

If you want to customise the configuration, you can run this instead:

$ make oldconfig

If you want to reduce the build time to just what you need you can use this:

$ make localmodconfig

If you want are doing this for a remote host, copy the remote lsmod output locally and use this instead:

$ make LSMOD=$(pwd)/lsmod localmodconfig

Build the Linux kernel

This will take a lot of time and computation power too. Ensure your system is adequately cooled so it will not overheat.

# Detect the amount of CPU cores you have
$ jobs=$(nproc --all)
# Detect if the Linux kernel version needs fakeroot or not
$ fakeroot=$(grep -q 'Rules-Requires-Root: no' scripts/package/mkdebian || grep -q fakeroot scripts/Makefile.package scripts/package/Makefile || echo fakeroot)
# Build Debian binary packages of the Linux kernel version
# Make sure you include the $ sign before fakeroot on the next line
$ $fakeroot make -j$jobs bindeb-pkg

Transfer the Linux kernel image package

If you are building on a different system to the one you are testing on, at this point you should transfer the Linux kernel image package to the system you are testing on.

It is recommended to do the testing in a spare system with the same hardware, or virtual machine (for bisects of code that is not hardware-specific) to avoid any old fixed Linux kernel bugs affecting your main system, especially old fixed issues that may corrupt your data storage.

Install the newly created Linux kernel

According to the name printed by the last command.

$ sudo apt install ../linux-2.6.25-rc6_2.6.25-rc6-2_amd64.deb

Reboot under newly built Linux kernel

$ sudo reboot

Test your issue under this Linux kernel

If the feature you are testing works in the newly booted Linux kernel, mark the commit as good:

$ cd ~/src/linux
$ git bisect good

If it does not work, mark the commit as bad:

$ cd ~/src/linux
$ git bisect bad

These commands will then choose a new commit to be configured, built and tested. Repeat the procedure until git bisect decides one particular commit is at fault. Once the commit has been found you can inspect it to see why it caused the regression and or file a bug about it against the Debian Linux kernel package.

CategoryGit CategoryDebugging