verify-bugs-and-bisect-regressions.rst

.. SPDX-License-Identifier: (GPL-2.0+ OR CC-BY-4.0)
.. [see the bottom of this file for redistribution information]

=========================================
How to verify bugs and bisect regressions
=========================================

This document describes how to check if some Linux kernel problem occurs in code
currently supported by developers -- to then explain how to locate the change
causing the issue, if it is a regression (e.g. did not happen with earlier
versions).

The text aims at people running kernels from mainstream Linux distributions on
commodity hardware who want to report a kernel bug to the upstream Linux
developers. Despite this intent, the instructions work just as well for users
who are already familiar with building their own kernels: they help avoid
mistakes occasionally made even by experienced developers.

..
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..
   Find the latest rendered version of this text here:
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The essence of the process (aka 'TL;DR')
========================================

*[If you are new to building or bisecting Linux, ignore this section and head
over to the* ':ref:`step-by-step guide <introguide_bissbs>`' *below. It utilizes
the same commands as this section while describing them in brief fashion. The
steps are nevertheless easy to follow and together with accompanying entries
in a reference section mention many alternatives, pitfalls, and additional
aspects, all of which might be essential in your present case.]*

**In case you want to check if a bug is present in code currently supported by
developers**, execute just the *preparations* and *segment 1*; while doing so,
consider the newest Linux kernel you regularly use to be the 'working' kernel.
In the following example that's assumed to be 6.0, which is why its sources
will be used to prepare the .config file.

**In case you face a regression**, follow the steps at least till the end of
*segment 2*. Then you can submit a preliminary report -- or continue with
*segment 3*, which describes how to perform a bisection needed for a
full-fledged regression report. In the following example 6.0.13 is assumed to be
the 'working' kernel and 6.1.5 to be the first 'broken', which is why 6.0
will be considered the 'good' release and used to prepare the .config file.

* **Preparations**: set up everything to build your own kernels::

    # * Remove any software that depends on externally maintained kernel modules
    #   or builds any automatically during bootup.
    # * Ensure Secure Boot permits booting self-compiled Linux kernels.
    # * If you are not already running the 'working' kernel, reboot into it.
    # * Install compilers and everything else needed for building Linux.
    # * Ensure to have 15 Gigabyte free space in your home directory.
    git clone -o mainline --no-checkout \
      https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git ~/linux/
    cd ~/linux/
    git remote add -t master stable \
      https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
    git switch --detach v6.0
    # * Hint: if you used an existing clone, ensure no stale .config is around.
    make olddefconfig
    # * Ensure the former command picked the .config of the 'working' kernel.
    # * Connect external hardware (USB keys, tokens, ...), start a VM, bring up
    #   VPNs, mount network shares, and briefly try the feature that is broken.
    yes '' | make localmodconfig
    ./scripts/config --set-str CONFIG_LOCALVERSION '-local'
    ./scripts/config -e CONFIG_LOCALVERSION_AUTO
    # * Note, when short on storage space, check the guide for an alternative:
    ./scripts/config -d DEBUG_INFO_NONE -e KALLSYMS_ALL -e DEBUG_KERNEL \
      -e DEBUG_INFO -e DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT -e KALLSYMS
    # * Hint: at this point you might want to adjust the build configuration;
    #   you'll have to, if you are running Debian.
    make olddefconfig
    cp .config ~/kernel-config-working

* **Segment 1**: build a kernel from the latest mainline codebase.

  This among others checks if the problem was fixed already and which developers
  later need to be told about the problem; in case of a regression, this rules
  out a .config change as root of the problem.

  a) Checking out latest mainline code::

       cd ~/linux/
       git switch --discard-changes --detach mainline/master

  b) Build, install, and boot a kernel::

       cp ~/kernel-config-working .config
       make olddefconfig
       make -j $(nproc --all)
       # * Make sure there is enough disk space to hold another kernel:
       df -h /boot/ /lib/modules/
       # * Note: on Arch Linux, its derivatives and a few other distributions
       #   the following commands will do nothing at all or only part of the
       #   job. See the step-by-step guide for further details.
       sudo make modules_install
       command -v installkernel && sudo make install
       # * Check how much space your self-built kernel actually needs, which
       #   enables you to make better estimates later:
       du -ch /boot/*$(make -s kernelrelease)* | tail -n 1
       du -sh /lib/modules/$(make -s kernelrelease)/
       # * Hint: the output of the following command will help you pick the
       #   right kernel from the boot menu:
       make -s kernelrelease | tee -a ~/kernels-built
       reboot
       # * Once booted, ensure you are running the kernel you just built by
       #   checking if the output of the next two commands matches:
       tail -n 1 ~/kernels-built
       uname -r
       cat /proc/sys/kernel/tainted

  c) Check if the problem occurs with this kernel as well.

* **Segment 2**: ensure the 'good' kernel is also a 'working' kernel.

  This among others verifies the trimmed .config file actually works well, as
  bisecting with it otherwise would be a waste of time:

  a) Start by checking out the sources of the 'good' version::

       cd ~/linux/
       git switch --discard-changes --detach v6.0

  b) Build, install, and boot a kernel as described earlier in *segment 1,
     section b* -- just feel free to skip the 'du' commands, as you have a rough
     estimate already.

  c) Ensure the feature that regressed with the 'broken' kernel actually works
     with this one.

* **Segment 3**: perform and validate the bisection.

  a) Retrieve the sources for your 'bad' version::

       git remote set-branches --add stable linux-6.1.y
       git fetch stable

  b) Initialize the bisection::

       cd ~/linux/
       git bisect start
       git bisect good v6.0
       git bisect bad v6.1.5

  c) Build, install, and boot a kernel as described earlier in *segment 1,
     section b*.

     In case building or booting the kernel fails for unrelated reasons, run
     ``git bisect skip``. In all other outcomes, check if the regressed feature
     works with the newly built kernel. If it does, tell Git by executing
     ``git bisect good``; if it does not, run ``git bisect bad`` instead.

     All three commands will make Git check out another commit; then re-execute
     this step (e.g. build, install, boot, and test a kernel to then tell Git
     the outcome). Do so again and again until Git shows which commit broke
     things. If you run short of disk space during this process, check the
     section 'Complementary tasks: cleanup during and after the process'
     below.

  d) Once your finished the bisection, put a few things away::

       cd ~/linux/
       git bisect log > ~/bisect-log
       cp .config ~/bisection-config-culprit
       git bisect reset

  e) Try to verify the bisection result::

       git switch --discard-changes --detach mainline/master
       git revert --no-edit cafec0cacaca0
       cp ~/kernel-config-working .config
       ./scripts/config --set-str CONFIG_LOCALVERSION '-local-cafec0cacaca0-reverted'

    This is optional, as some commits are impossible to revert. But if the
    second command worked flawlessly, build, install, and boot one more kernel
    kernel; just this time skip the first command copying the base .config file
    over, as that already has been taken care off.

* **Complementary tasks**: cleanup during and after the process.

  a) To avoid running out of disk space during a bisection, you might need to
     remove some kernels you built earlier. You most likely want to keep those
     you built during segment 1 and 2 around for a while, but you will most
     likely no longer need kernels tested during the actual bisection
     (Segment 3 c). You can list them in build order using::

       ls -ltr /lib/modules/*-local*

    To then for example erase a kernel that identifies itself as
    '6.0-rc1-local-gcafec0cacaca0', use this::

       sudo rm -rf /lib/modules/6.0-rc1-local-gcafec0cacaca0
       sudo kernel-install -v remove 6.0-rc1-local-gcafec0cacaca0
       # * Note, on some distributions kernel-install is missing
       #   or does only part of the job.

  b) If you performed a bisection and successfully validated the result, feel
     free to remove all kernels built during the actual bisection (Segment 3 c);
     the kernels you built earlier and later you might want to keep around for
     a week or two.

* **Optional task**: test a debug patch or a proposed fix later::

    git fetch mainline
    git switch --discard-changes --detach mainline/master
    git apply /tmp/foobars-proposed-fix-v1.patch
    cp ~/kernel-config-working .config
    ./scripts/config --set-str CONFIG_LOCALVERSION '-local-foobars-fix-v1'

  Build, install, and boot a kernel as described in *segment 1, section b* --
  but this time omit the first command copying the build configuration over,
  as that has been taken care of already.

.. _introguide_bissbs:

Step-by-step guide on how to verify bugs and bisect regressions
===============================================================

This guide describes how to set up your own Linux kernels for investigating bugs
or regressions you intend to report. How far you want to follow the instructions
depends on your issue:

Execute all steps till the end of *segment 1* to **verify if your kernel problem
is present in code supported by Linux kernel developers**. If it is, you are all
set to report the bug -- unless it did not happen with earlier kernel versions,
as then your want to at least continue with *segment 2* to **check if the issue
qualifies as regression** which receive priority treatment. Depending on the
outcome you then are ready to report a bug or submit a preliminary regression
report; instead of the latter your could also head straight on and follow
*segment 3* to **perform a bisection** for a full-fledged regression report
developers are obliged to act upon.

 :ref:`Preparations: set up everything to build your own kernels <introprep_bissbs>`.

 :ref:`Segment 1: try to reproduce the problem with the latest codebase <introlatestcheck_bissbs>`.

 :ref:`Segment 2: check if the kernels you build work fine <introworkingcheck_bissbs>`.

 :ref:`Segment 3: perform a bisection and validate the result <introbisect_bissbs>`.

 :ref:`Complementary tasks: cleanup during and after following this guide <introclosure_bissbs>`.

 :ref:`Optional tasks: test reverts, patches, or later versions <introoptional_bissbs>`.

The steps in each segment illustrate the important aspects of the process, while
a comprehensive reference section holds additional details for almost all of the
steps. The reference section sometimes also outlines alternative approaches,
pitfalls, as well as problems that might occur at the particular step -- and how
to get things rolling again.

For further details on how to report Linux kernel issues or regressions check
out Documentation/admin-guide/reporting-issues.rst, which works in conjunction
with this document. It among others explains why you need to verify bugs with
the latest 'mainline' kernel (e.g. versions like 6.0, 6.1-rc1, or 6.1-rc6),
even if you face a problem with a kernel from a 'stable/longterm' series
(say 6.0.13).

For users facing a regression that document also explains why sending a
preliminary report after segment 2 might be wise, as the regression and its
culprit might be known already. For further details on what actually qualifies
as a regression check out Documentation/admin-guide/reporting-regressions.rst.

If you run into any problems while following this guide or have ideas how to
improve it, :ref:`please let the kernel developers know <submit_improvements>`.

.. _introprep_bissbs:

Preparations: set up everything to build your own kernels
---------------------------------------------------------

The following steps lay the groundwork for all further tasks.

Note: the instructions assume you are building and testing on the same
machine; if you want to compile the kernel on another system, check
:ref:`Build kernels on a different machine <buildhost_bis>` below.

.. _backup_bissbs:

* Create a fresh backup and put system repair and restore tools at hand, just
  to be prepared for the unlikely case of something going sideways.

  [:ref:`details <backup_bisref>`]

.. _vanilla_bissbs:

* Remove all software that depends on externally developed kernel drivers or
  builds them automatically. That includes but is not limited to DKMS, openZFS,
  VirtualBox, and Nvidia's graphics drivers (including the GPLed kernel module).

  [:ref:`details <vanilla_bisref>`]

.. _secureboot_bissbs:

* On platforms with 'Secure Boot' or similar solutions, prepare everything to
  ensure the system will permit your self-compiled kernel to boot. The
  quickest and easiest way to achieve this on commodity x86 systems is to
  disable such techniques in the BIOS setup utility; alternatively, remove
  their restrictions through a process initiated by
  ``mokutil --disable-validation``.

  [:ref:`details <secureboot_bisref>`]

.. _rangecheck_bissbs:

* Determine the kernel versions considered 'good' and 'bad' throughout this
  guide.

  Do you follow this guide to verify if a bug is present in the code developers
  care for? Then consider the mainline release your 'working' kernel (the newest
  one you regularly use) is based on to be the 'good' version; if your 'working'
  kernel for example is 6.0.13, then your 'good' kernel is 6.0.

  In case you face a regression, it depends on the version range where the
  regression was introduced:

  * Something which used to work in Linux 6.0 broke when switching to Linux
    6.1-rc1? Then henceforth regard 6.0 as the last known 'good' version
    and 6.1-rc1 as the first 'bad' one.

  * Some function stopped working when updating from 6.0.13 to 6.1.5? Then for
    the time being consider 6.0 as the last 'good' version and 6.1.5 as
    the 'bad' one. Note, at this point it is merely assumed that 6.0 is fine;
    this assumption will be checked in segment 2.

  * A feature you used in 6.0.13 does not work at all or worse in 6.1.15? In
    that case you want to bisect within a stable/longterm series: consider
    6.0.13 as the last known 'good' version and 6.0.15 as the first 'bad'
    one. Note, in this case you still want to compile and test a mainline kernel
    as explained in segment 1: the outcome will determine if you need to report
    your issue to the regular developers or the stable team.

  *Note, do not confuse 'good' version with 'working' kernel; the latter term
  throughout this guide will refer to the last kernel that has been working
  fine.*

  [:ref:`details <rangecheck_bisref>`]

.. _bootworking_bissbs:

* Boot into the 'working' kernel and briefly use the apparently broken feature.

  [:ref:`details <bootworking_bisref>`]

.. _diskspace_bissbs:

* Ensure to have enough free space for building Linux. 15 Gigabyte in your home
  directory should typically suffice. If you have less available, be sure to pay
  attention to later steps about retrieving the Linux sources and handling of
  debug symbols: both explain approaches reducing the amount of space, which
  should allow you to master these tasks with about 4 Gigabytes free space.

  [:ref:`details <diskspace_bisref>`]

.. _buildrequires_bissbs:

* Install all software required to build a Linux kernel. Often you will need:
  'bc', 'binutils' ('ld' et al.), 'bison', 'flex', 'gcc', 'git', 'openssl',
  'pahole', 'perl', and the development headers for 'libelf' and 'openssl'. The
  reference section shows how to quickly install those on various popular Linux
  distributions.

  [:ref:`details <buildrequires_bisref>`]

.. _sources_bissbs:

* Retrieve the mainline Linux sources; then change into the directory holding
  them, as all further commands in this guide are meant to be executed from
  there.

  *Note, the following describe how to retrieve the sources using a full
  mainline clone, which downloads about 2,75 GByte as of early 2024. The*
  :ref:`reference section describes two alternatives <sources_bisref>` *:
  one downloads less than 500 MByte, the other works better with unreliable
  internet connections.*

  Execute the following command to retrieve a fresh mainline codebase while
  preparing things to add branches for stable/longterm series later::

    git clone -o mainline --no-checkout \
      https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git ~/linux/
    cd ~/linux/
    git remote add -t master stable \
      https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git

  [:ref:`details <sources_bisref>`]

.. _stablesources_bissbs:

* Retrieve the sources for any stable or longterm series you might need.

  Is the version you earlier established as 'bad' a stable or longterm release?
  Then download the code for the series it belongs to ('linux-6.1.y' in this
  example)::

    git remote set-branches --add stable linux-6.1.y
    git fetch stable

  If the version earlier established as 'good' is from a different stable or
  longterm series (say 6.0.13), repeat the previous step, but this time for the
  branch holding the series the 'good' version belongs to (e.g. linux-6.0.y).

.. _oldconfig_bissbs:

* Start preparing a kernel build configuration (the '.config' file).

  Before doing so, ensure you are still running the 'working' kernel an earlier
  step told you to boot; if you are unsure, check the current kernelrelease
  identifier using ``uname -r``.

  Afterwards check out the source code for the version earlier established as
  'good'. In the following example command this is assumed to be 6.0; note that
  the version number in this and all later Git commands needs to be prefixed
  with a 'v'::

    git switch --discard-changes --detach v6.0

  Now create a build configuration file::

    make olddefconfig

  The kernel build scripts then will try to locate the build configuration file
  for the running kernel and then adjust it for the needs of the kernel sources
  you checked out. While doing so, it will print a few lines you need to check.

  Look out for a line starting with '# using defaults found in'. It should be
  followed by a path to a file in '/boot/' that contains the release identifier
  of your currently working kernel. If the line instead continues with something
  like 'arch/x86/configs/x86_64_defconfig', then the build infra failed to find
  the .config file for your running kernel -- in which case you have to put one
  there manually, as explained in the reference section.

  In case you can not find such a line, look for one containing '# configuration
  written to .config'. If that's the case you have a stale build configuration
  lying around. Unless you intend to use it, delete it; afterwards run
  'make olddefconfig' again and check if it now picked up the right config file
  as base.

  [:ref:`details <oldconfig_bisref>`]

.. _localmodconfig_bissbs:

* Disable any kernel modules apparently superfluous for your setup. This is
  optional, but especially wise for bisections, as it speeds up the build
  process enormously -- at least unless the .config file picked up in the
  previous step was already tailored to your and your hardware needs, in which
  case you should skip this step.

  To prepare the trimming, connect external hardware you occasionally use (USB
  keys, tokens, ...), quickly start a VM, and bring up VPNs. And if you rebooted
  since you started that guide, ensure that you tried using the feature causing
  trouble since you started the system. Only then trim your .config::

     yes '' | make localmodconfig

  There is a catch to this, as the 'apparently' in initial sentence of this step
  and the preparation instructions already hinted at:

  The 'localmodconfig' target easily disables kernel modules for features only
  used occasionally -- like modules for external peripherals not yet connected
  since booting, virtualization software not yet utilized, VPN tunnels, and a
  few other things. That's because some tasks rely on kernel modules Linux only
  loads when you execute tasks like the aforementioned ones for the first time.

  This drawback of localmodconfig is nothing you should lose sleep over, but
  something to keep in mind: if something is misbehaving with the kernels built
  during this guide, this is most likely the reason. You can reduce or nearly
  eliminate the risk with tricks outlined in the reference section; but when
  building a kernel just for quick testing purposes this is usually not worth
  spending much effort on, as long as it boots and allows to properly test the
  feature that causes trouble.

  [:ref:`details <localmodconfig_bisref>`]

.. _tagging_bissbs:

* Ensure all the kernels you will build are clearly identifiable using a special
  tag and a unique version number::

    ./scripts/config --set-str CONFIG_LOCALVERSION '-local'
    ./scripts/config -e CONFIG_LOCALVERSION_AUTO

  [:ref:`details <tagging_bisref>`]

.. _debugsymbols_bissbs:

* Decide how to handle debug symbols.

  In the context of this document it is often wise to enable them, as there is a
  decent chance you will need to decode a stack trace from a 'panic', 'Oops',
  'warning', or 'BUG'::

    ./scripts/config -d DEBUG_INFO_NONE -e KALLSYMS_ALL -e DEBUG_KERNEL \
      -e DEBUG_INFO -e DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT -e KALLSYMS

  But if you are extremely short on storage space, you might want to disable
  debug symbols instead::

    ./scripts/config -d DEBUG_INFO -d DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT \
      -d DEBUG_INFO_DWARF4 -d DEBUG_INFO_DWARF5 -e CONFIG_DEBUG_INFO_NONE

  [:ref:`details <debugsymbols_bisref>`]

.. _configmods_bissbs:

* Check if you may want or need to adjust some other kernel configuration
  options:

  * Are you running Debian? Then you want to avoid known problems by performing
    additional adjustments explained in the reference section.

    [:ref:`details <configmods_distros_bisref>`].

  * If you want to influence other aspects of the configuration, do so now using
    your preferred tool. Note, to use make targets like 'menuconfig' or
    'nconfig', you will need to install the development files of ncurses; for
    'xconfig' you likewise need the Qt5 or Qt6 headers.

    [:ref:`details <configmods_individual_bisref>`].

.. _saveconfig_bissbs:

* Reprocess the .config after the latest adjustments and store it in a safe
  place::

     make olddefconfig
     cp .config ~/kernel-config-working

  [:ref:`details <saveconfig_bisref>`]

.. _introlatestcheck_bissbs:

Segment 1: try to reproduce the problem with the latest codebase
----------------------------------------------------------------

The following steps verify if the problem occurs with the code currently
supported by developers. In case you face a regression, it also checks that the
problem is not caused by some .config change, as reporting the issue then would
be a waste of time. [:ref:`details <introlatestcheck_bisref>`]

.. _checkoutmaster_bissbs:

* Check out the latest Linux codebase::

    cd ~/linux/
    git switch --discard-changes --detach mainline/master

  [:ref:`details <checkoutmaster_bisref>`]

.. _build_bissbs:

* Build the image and the modules of your first kernel using the config file you
  prepared::

    cp ~/kernel-config-working .config
    make olddefconfig
    make -j $(nproc --all)

  If you want your kernel packaged up as deb, rpm, or tar file, see the
  reference section for alternatives, which obviously will require other
  steps to install as well.

  [:ref:`details <build_bisref>`]

.. _install_bissbs:

* Install your newly built kernel.

  Before doing so, consider checking if there is still enough space for it::

    df -h /boot/ /lib/modules/

  For now assume 150 MByte in /boot/ and 200 in /lib/modules/ will suffice; how
  much your kernels actually require will be determined later during this guide.

  Now install the kernel's modules and its image, which will be stored in
  parallel to the your Linux distribution's kernels::

    sudo make modules_install
    command -v installkernel && sudo make install

  The second command ideally will take care of three steps required at this
  point: copying the kernel's image to /boot/, generating an initramfs, and
  adding an entry for both to the boot loader's configuration.

  Sadly some distributions (among them Arch Linux, its derivatives, and many
  immutable Linux distributions) will perform none or only some of those tasks.
  You therefore want to check if all of them were taken care of and manually
  perform those that were not. The reference section provides further details on
  that; your distribution's documentation might help, too.

  Once you figured out the steps needed at this point, consider writing them
  down: if you will build more kernels as described in segment 2 and 3, you will
  have to perform those again after executing ``command -v installkernel [...]``.

  [:ref:`details <install_bisref>`]

.. _storagespace_bissbs:

* In case you plan to follow this guide further, check how much storage space
  the kernel, its modules, and other related files like the initramfs consume::

    du -ch /boot/*$(make -s kernelrelease)* | tail -n 1
    du -sh /lib/modules/$(make -s kernelrelease)/

  Write down or remember those two values for later: they enable you to prevent
  running out of disk space accidentally during a bisection.

  [:ref:`details <storagespace_bisref>`]

.. _kernelrelease_bissbs:

* Show and store the kernelrelease identifier of the kernel you just built::

    make -s kernelrelease | tee -a ~/kernels-built

  Remember the identifier momentarily, as it will help you pick the right kernel
  from the boot menu upon restarting.

* Reboot into your newly built kernel. To ensure your actually started the one
  you just built, you might want to verify if the output of these commands
  matches::

    tail -n 1 ~/kernels-built
    uname -r

.. _tainted_bissbs:

* Check if the kernel marked itself as 'tainted'::

    cat /proc/sys/kernel/tainted

  If that command does not return '0', check the reference section, as the cause
  for this might interfere with your testing.

  [:ref:`details <tainted_bisref>`]

.. _recheckbroken_bissbs:

* Verify if your bug occurs with the newly built kernel. If it does not, check
  out the instructions in the reference section to ensure nothing went sideways
  during your tests.

  [:ref:`details <recheckbroken_bisref>`]

.. _recheckstablebroken_bissbs:

* Are you facing a problem within a stable/longterm series, but failed to
  reproduce it with the mainline kernel you just built? One that according to
  the `front page of kernel.org <https://kernel.org/>`_ is still supported? Then
  check if the latest codebase for the particular series might already fix the
  problem. To do so, check out that series latest version (again, this here is
  assumed to be 6.0)::

    cd ~/linux/
    git switch --discard-changes --detach linux-6.0.y

  Now use the checked out code to build and install another kernel using the
  commands the earlier steps already described in more detail::

    cp ~/kernel-config-working .config
    make olddefconfig
    make -j $(nproc --all)
    # * Check if the free space suffices holding another kernel:
    df -h /boot/ /lib/modules/
    sudo make modules_install
    command -v installkernel && sudo make install
    make -s kernelrelease | tee -a ~/kernels-built
    reboot

  Confirm you booted the kernel you intended to start and check its tainted
  status::

    tail -n 1 ~/kernels-built
    uname -r
    cat /proc/sys/kernel/tainted

  Now verify if this kernel is showing the problem.

  [:ref:`details <recheckstablebroken_bisref>`]

Do you follow this guide to verify if a problem is present in the code
currently supported by Linux kernel developers? Then you are done at this
point. If you later want to remove the kernel you just built, check out
:ref:`Complementary tasks: cleanup during and after following this guide <introclosure_bissbs>`.

In case you face a regression, move on and execute at least the next segment
as well.

.. _introworkingcheck_bissbs:

Segment 2: check if the kernels you build work fine
---------------------------------------------------

In case of a regression, you now want to ensure the trimmed configuration file
you created earlier works as expected; a bisection with the .config file
otherwise would be a waste of time. [:ref:`details <introworkingcheck_bisref>`]

.. _recheckworking_bissbs:

* Build your own variant of the 'working' kernel and check if the feature that
  regressed works as expected with it.

  Start by checking out the sources for the version earlier established as
  'good' (once again assumed to be 6.0 here)::

    cd ~/linux/
    git switch --discard-changes --detach v6.0

  Now use the checked out code to configure, build, and install another kernel
  using the commands the previous subsection explained in more detail::

    cp ~/kernel-config-working .config
    make olddefconfig
    make -j $(nproc --all)
    # * Check if the free space suffices holding another kernel:
    df -h /boot/ /lib/modules/
    sudo make modules_install
    command -v installkernel && sudo make install
    make -s kernelrelease | tee -a ~/kernels-built
    reboot

  When the system booted, you may want to verify once again that the
  kernel you started is the one you just built::

    tail -n 1 ~/kernels-built
    uname -r

  Now check if this kernel works as expected; if not, consult the reference
  section for further instructions.

  [:ref:`details <recheckworking_bisref>`]

.. _introbisect_bissbs:

Segment 3: perform the bisection and validate the result
--------------------------------------------------------

With all the preparations and precaution builds taken care of, you are now ready
to begin the bisection. This will make you build quite a few kernels -- usually
about 15 in case you encountered a regression when updating to a newer series
(say from 6.0.13 to 6.1.5). But do not worry, due to the trimmed build
configuration created earlier this works a lot faster than many people assume:
overall on average it will often just take about 10 to 15 minutes to compile
each kernel on commodity x86 machines.

.. _bisectstart_bissbs:

* Start the bisection and tell Git about the versions earlier established as
  'good' (6.0 in the following example command) and 'bad' (6.1.5)::

    cd ~/linux/
    git bisect start
    git bisect good v6.0
    git bisect bad v6.1.5

  [:ref:`details <bisectstart_bisref>`]

.. _bisectbuild_bissbs:

* Now use the code Git checked out to build, install, and boot a kernel using
  the commands introduced earlier::

    cp ~/kernel-config-working .config
    make olddefconfig
    make -j $(nproc --all)
    # * Check if the free space suffices holding another kernel:
    df -h /boot/ /lib/modules/
    sudo make modules_install
    command -v installkernel && sudo make install
    make -s kernelrelease | tee -a ~/kernels-built
    reboot

  If compilation fails for some reason, run ``git bisect skip`` and restart
  executing the stack of commands from the beginning.

  In case you skipped the 'test latest codebase' step in the guide, check its
  description as for why the 'df [...]' and 'make -s kernelrelease [...]'
  commands are here.

  Important note: the latter command from this point on will print release
  identifiers that might look odd or wrong to you -- which they are not, as it's
  totally normal to see release identifiers like '6.0-rc1-local-gcafec0cacaca0'
  if you bisect between versions 6.1 and 6.2 for example.

  [:ref:`details <bisectbuild_bisref>`]

.. _bisecttest_bissbs:

* Now check if the feature that regressed works in the kernel you just built.

  You again might want to start by making sure the kernel you booted is the one
  you just built::

    cd ~/linux/
    tail -n 1 ~/kernels-built
    uname -r

  Now verify if the feature that regressed works at this kernel bisection point.
  If it does, run this::

    git bisect good

  If it does not, run this::

    git bisect bad

  Be sure about what you tell Git, as getting this wrong just once will send the
  rest of the bisection totally off course.

  While the bisection is ongoing, Git will use the information you provided to
  find and check out another bisection point for you to test. While doing so, it
  will print something like 'Bisecting: 675 revisions left to test after this
  (roughly 10 steps)' to indicate how many further changes it expects to be
  tested. Now build and install another kernel using the instructions from the
  previous step; afterwards follow the instructions in this step again.

  Repeat this again and again until you finish the bisection -- that's the case
  when Git after tagging a change as 'good' or 'bad' prints something like
  'cafecaca0c0dacafecaca0c0dacafecaca0c0da is the first bad commit'; right
  afterwards it will show some details about the culprit including the patch
  description of the change. The latter might fill your terminal screen, so you
  might need to scroll up to see the message mentioning the culprit;
  alternatively, run ``git bisect log > ~/bisection-log``.

  [:ref:`details <bisecttest_bisref>`]

.. _bisectlog_bissbs:

* Store Git's bisection log and the current .config file in a safe place before
  telling Git to reset the sources to the state before the bisection::

    cd ~/linux/
    git bisect log > ~/bisection-log
    cp .config ~/bisection-config-culprit
    git bisect reset

  [:ref:`details <bisectlog_bisref>`]

.. _revert_bissbs:

* Try reverting the culprit on top of latest mainline to see if this fixes your
  regression.

  This is optional, as it might be impossible or hard to realize. The former is
  the case, if the bisection determined a merge commit as the culprit; the
  latter happens if other changes depend on the culprit. But if the revert
  succeeds, it is worth building another kernel, as it validates the result of
  a bisection, which can easily deroute; it furthermore will let kernel
  developers know, if they can resolve the regression with a quick revert.

  Begin by checking out the latest codebase depending on the range you bisected:

  * Did you face a regression within a stable/longterm series (say between
    6.0.13 and 6.0.15) that does not happen in mainline? Then check out the
    latest codebase for the affected series like this::

      git fetch stable
      git switch --discard-changes --detach linux-6.0.y

  * In all other cases check out latest mainline::

      git fetch mainline
      git switch --discard-changes --detach mainline/master

    If you bisected a regression within a stable/longterm series that also
    happens in mainline, there is one more thing to do: look up the mainline
    commit-id. To do so, use a command like ``git show abcdcafecabcd`` to
    view the patch description of the culprit. There will be a line near
    the top which looks like 'commit cafec0cacaca0 upstream.' or
    'Upstream commit cafec0cacaca0'; use that commit-id in the next command
    and not the one the bisection blamed.

  Now try reverting the culprit by specifying its commit id::

    git revert --no-edit cafec0cacaca0

  If that fails, give up trying and move on to the next step; if it works,
  adjust the tag to facilitate the identification and prevent accidentally
  overwriting another kernel::

    cp ~/kernel-config-working .config
    ./scripts/config --set-str CONFIG_LOCALVERSION '-local-cafec0cacaca0-reverted'

  Build a kernel using the familiar command sequence, just without copying the
  the base .config over::

    make olddefconfig &&
    make -j $(nproc --all)
    # * Check if the free space suffices holding another kernel:
    df -h /boot/ /lib/modules/
    sudo make modules_install
    command -v installkernel && sudo make install
    make -s kernelrelease | tee -a ~/kernels-built
    reboot

  Now check one last time if the feature that made you perform a bisection works
  with that kernel: if everything went well, it should not show the regression.

  [:ref:`details <revert_bisref>`]

.. _introclosure_bissbs:

Complementary tasks: cleanup during and after the bisection
-----------------------------------------------------------

During and after following this guide you might want or need to remove some of
the kernels you installed: the boot menu otherwise will become confusing or
space might run out.

.. _makeroom_bissbs:

* To remove one of the kernels you installed, look up its 'kernelrelease'
  identifier. This guide stores them in '~/kernels-built', but the following
  command will print them as well::

    ls -ltr /lib/modules/*-local*

  You in most situations want to remove the oldest kernels built during the
  actual bisection (e.g. segment 3 of this guide). The two ones you created
  beforehand (e.g. to test the latest codebase and the version considered
  'good') might become handy to verify something later -- thus better keep them
  around, unless you are really short on storage space.

  To remove the modules of a kernel with the kernelrelease identifier
  '*6.0-rc1-local-gcafec0cacaca0*', start by removing the directory holding its
  modules::

    sudo rm -rf /lib/modules/6.0-rc1-local-gcafec0cacaca0

  Afterwards try the following command::

    sudo kernel-install -v remove 6.0-rc1-local-gcafec0cacaca0

  On quite a few distributions this will delete all other kernel files installed
  while also removing the kernel's entry from the boot menu. But on some
  distributions kernel-install does not exist or leaves boot-loader entries or
  kernel image and related files behind; in that case remove them as described
  in the reference section.

  [:ref:`details <makeroom_bisref>`]

.. _finishingtouch_bissbs:

* Once you have finished the bisection, do not immediately remove anything you
  set up, as you might need a few things again. What is safe to remove depends
  on the outcome of the bisection:

  * Could you initially reproduce the regression with the latest codebase and
    after the bisection were able to fix the problem by reverting the culprit on
    top of the latest codebase? Then you want to keep those two kernels around
    for a while, but safely remove all others with a '-local' in the release
    identifier.

  * Did the bisection end on a merge-commit or seems questionable for other
    reasons? Then you want to keep as many kernels as possible around for a few
    days: it's pretty likely that you will be asked to recheck something.

  * In other cases it likely is a good idea to keep the following kernels around
    for some time: the one built from the latest codebase, the one created from
    the version considered 'good', and the last three or four you compiled
    during the actual bisection process.

  [:ref:`details <finishingtouch_bisref>`]

.. _introoptional_bissbs:

Optional: test reverts, patches, or later versions
--------------------------------------------------

While or after reporting a bug, you might want or potentially will be asked to
test reverts, debug patches, proposed fixes, or other versions. In that case
follow these instructions.

* Update your Git clone and check out the latest code.

  * In case you want to test mainline, fetch its latest changes before checking
    its code out::

      git fetch mainline
      git switch --discard-changes --detach mainline/master

  * In case you want to test a stable or longterm kernel, first add the branch
    holding the series you are interested in (6.2 in the example), unless you
    already did so earlier::

      git remote set-branches --add stable linux-6.2.y

    Then fetch the latest changes and check out the latest version from the
    series::

      git fetch stable
      git switch --discard-changes --detach stable/linux-6.2.y

* Copy your kernel build configuration over::