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Kernel Maintenance

CIP announced two relevant decisions during the second half of 2016:

  • At LinuxCon North America 2016 we announced Ben Hutchings as the CIP kernel maintainer. Ben is currently the Debian Long Term Support kernel maintainer and a reputed kernel hacker, specialised in kernel maintainership.
  • At Embedded Linux Conference Europe 2016 we announced that our first Super Long Term Support kernel branch will be based on Linux 4.4.

Maintenance policies

CIP kernel - SLTS kernel

When does CIP define/label a kernel as SLTS (Super Long Term Support)?

New major versions of commercial Linux distributions are released at 3-4 year intervals, so that typically only 4 versions need to be supported at one time. Given that CIP's support period is meant to be even longer, it won’t be sustainable to extend every LTS branch, but only to take on a new branch every 2-4 years.

The longer the intervals between new SLTS branches, the greater need there will be for CIP or individual members to backport new hardware support (which carries its own risks). This trade-off is perhaps the most difficult issue to decide.

CIP will measure the effort involved in maintaining the kernel branch we already labelled as SLTS (4.4) and study the technical implications of this work during 2017 in order to define when the next SLTS branch should be started.

How long will 4.4 SLTS branch be maintained?

The use cases CIP project is targeting have a life cycle of between 25 and 50 years. In theory, this is the time in which products shipped with the CIP kernel will be under maintenance. However, identifying and backporting relevant fixes becomes increasingly difficult as upstream kernel development diverges further from a stable branch. Any given SLTS branch is unlikely to be maintainable for more than 10-20 years.

The Linux kernel 4.4 was released on January 10th 2016. It was declared LTS (Long Term Support) by the stable team which means Greg Kroah-Hartman will maintain it for two years (Feb 2018) following the kernel LTS process. Another stable maintainer could extend LTS maintenance for some time beyond that. After that period, CIP will maintain it.

Suitable changes

The policy for which changes to accept is based on stable-kernel-rules.rst. For reference, these are the rules with numbering added:

  1. It must be obviously correct and tested.
  2. It cannot be bigger than 100 lines, with context.
  3. It must fix only one thing.
  4. It must fix a real bug that bothers people (not a, “This could be a problem…” type thing).
  5. It must fix a problem that causes a build error (but not for things marked CONFIG_BROKEN), an oops, a hang, data corruption, a real security issue, or some “oh, that's not good” issue. In short, something critical.
  6. Serious issues as reported by a user of a distribution kernel may also be considered if they fix a notable performance or interactivity issue. As these fixes are not as obvious and have a higher risk of a subtle regression they should only be submitted by a distribution kernel maintainer and include an addendum linking to a bugzilla entry if it exists and additional information on the user-visible impact.
  7. New device IDs and quirks are also accepted.
  8. No “theoretical race condition” issues, unless an explanation of how the race can be exploited is also provided.
  9. It cannot contain any “trivial” fixes in it (spelling changes, whitespace cleanups, etc).
  10. It must follow the Documentation/SubmittingPatches rules.
  11. It or an equivalent fix must already exist in Linus' tree (upstream).

In practice, exceptions are sometimes made to rules 2, 3, 8 and 9, and we will likewise treat those as should rather than must rules.

After the end of LTS at, the fixes made to a SLTS branch will be more restricted:

  • Architecture and hardware support will be limited to the needs of CIP members; fixes for other hardware will not be applied except where that makes it easier to apply the fixes we do need. For the 4.4 branch, the architectures we've committed to so far are arm (32-bit) and x86_64.
  • Some features, particularly those labelled as experimental, may also be excluded from support. We have not done so yet.

Unlike an LTS branch, an SLTS branch may include larger changes to support new hardware, to bridge the gap between SLTS branches. However, this should only be done on the newest SLTS branch.


CIP will release sources, not binaries, except for the CIP platforms once we start to build a more complete platform. The SLTS kernel maintainer will push a signed tag to the kernel git repository for each release.

Once we have automated testing in place, a release candidate must be tested on all the platforms that are covered, with no regressions, before it is tagged as a release.

As we will be backporting fixes from mainline, which may change in-kernel APIs, we will not guarantee API or ABI stability within an SLTS branch. It is the responsibility of members to update any out-of-tree components they use in case they are affected by such a change. However, incompatible changes to user-space APIs or ABIs will not be acceptable.



CIP maintainers will support and respond to bug reports from CIP members and their developers, but not system administrators or end users.

The embedded systems that CIP will be used in will also often require out-of-tree drivers and will sometimes include other changes of unknown quality to their kernel. These modifications are in general unsupported. If a bug is found in such a modified kernel, Members will first demonstrate that it exists in the CIP source release in order for the CIP maintainers to act on it.

Security fixes

CIP works towards reducing the window of vulnerability to zero. To achieve this goal, CIP will collaborate with the wider kernel community in initiatives like the Kernel Self Protection Project.

Although this goal may be achievable for CIP, its members may take much longer to release and deploy binary updates, maybe due to valid concerns about the risk of regression or limited opportunities to deploy updates. In the worst case, they may use CIP as an advertising/compliance point - please don't do that!

We plan to track CVE status in each branch and provide this information either to members or publicly.

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civilinfrastructureplatform/cipkernelmaintenance.txt · Last modified: 2017/03/30 11:04 by Agustin Benito Bethencourt