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ReStructuredText
696 lines
29 KiB
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=================================
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HOWTO interact with BPF subsystem
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=================================
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This document provides information for the BPF subsystem about various
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workflows related to reporting bugs, submitting patches, and queueing
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patches for stable kernels.
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For general information about submitting patches, please refer to
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Documentation/process/submitting-patches.rst. This document only describes
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additional specifics related to BPF.
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.. contents::
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:local:
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:depth: 2
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Reporting bugs
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==============
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Q: How do I report bugs for BPF kernel code?
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--------------------------------------------
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A: Since all BPF kernel development as well as bpftool and iproute2 BPF
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loader development happens through the bpf kernel mailing list,
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please report any found issues around BPF to the following mailing
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list:
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bpf@vger.kernel.org
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This may also include issues related to XDP, BPF tracing, etc.
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Given netdev has a high volume of traffic, please also add the BPF
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maintainers to Cc (from kernel ``MAINTAINERS`` file):
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* Alexei Starovoitov <ast@kernel.org>
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* Daniel Borkmann <daniel@iogearbox.net>
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In case a buggy commit has already been identified, make sure to keep
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the actual commit authors in Cc as well for the report. They can
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typically be identified through the kernel's git tree.
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**Please do NOT report BPF issues to bugzilla.kernel.org since it
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is a guarantee that the reported issue will be overlooked.**
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Submitting patches
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==================
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Q: How do I run BPF CI on my changes before sending them out for review?
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------------------------------------------------------------------------
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A: BPF CI is GitHub based and hosted at https://github.com/kernel-patches/bpf.
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While GitHub also provides a CLI that can be used to accomplish the same
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results, here we focus on the UI based workflow.
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The following steps lay out how to start a CI run for your patches:
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- Create a fork of the aforementioned repository in your own account (one time
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action)
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- Clone the fork locally, check out a new branch tracking either the bpf-next
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or bpf branch, and apply your to-be-tested patches on top of it
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- Push the local branch to your fork and create a pull request against
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kernel-patches/bpf's bpf-next_base or bpf_base branch, respectively
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Shortly after the pull request has been created, the CI workflow will run. Note
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that capacity is shared with patches submitted upstream being checked and so
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depending on utilization the run can take a while to finish.
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Note furthermore that both base branches (bpf-next_base and bpf_base) will be
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updated as patches are pushed to the respective upstream branches they track. As
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such, your patch set will automatically (be attempted to) be rebased as well.
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This behavior can result in a CI run being aborted and restarted with the new
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base line.
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Q: To which mailing list do I need to submit my BPF patches?
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------------------------------------------------------------
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A: Please submit your BPF patches to the bpf kernel mailing list:
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bpf@vger.kernel.org
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In case your patch has changes in various different subsystems (e.g.
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networking, tracing, security, etc), make sure to Cc the related kernel mailing
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lists and maintainers from there as well, so they are able to review
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the changes and provide their Acked-by's to the patches.
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Q: Where can I find patches currently under discussion for BPF subsystem?
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-------------------------------------------------------------------------
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A: All patches that are Cc'ed to netdev are queued for review under netdev
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patchwork project:
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https://patchwork.kernel.org/project/netdevbpf/list/
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Those patches which target BPF, are assigned to a 'bpf' delegate for
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further processing from BPF maintainers. The current queue with
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patches under review can be found at:
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https://patchwork.kernel.org/project/netdevbpf/list/?delegate=121173
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Once the patches have been reviewed by the BPF community as a whole
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and approved by the BPF maintainers, their status in patchwork will be
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changed to 'Accepted' and the submitter will be notified by mail. This
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means that the patches look good from a BPF perspective and have been
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applied to one of the two BPF kernel trees.
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In case feedback from the community requires a respin of the patches,
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their status in patchwork will be set to 'Changes Requested', and purged
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from the current review queue. Likewise for cases where patches would
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get rejected or are not applicable to the BPF trees (but assigned to
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the 'bpf' delegate).
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Q: How do the changes make their way into Linux?
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------------------------------------------------
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A: There are two BPF kernel trees (git repositories). Once patches have
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been accepted by the BPF maintainers, they will be applied to one
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of the two BPF trees:
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* https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/
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* https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/
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The bpf tree itself is for fixes only, whereas bpf-next for features,
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cleanups or other kind of improvements ("next-like" content). This is
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analogous to net and net-next trees for networking. Both bpf and
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bpf-next will only have a master branch in order to simplify against
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which branch patches should get rebased to.
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Accumulated BPF patches in the bpf tree will regularly get pulled
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into the net kernel tree. Likewise, accumulated BPF patches accepted
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into the bpf-next tree will make their way into net-next tree. net and
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net-next are both run by David S. Miller. From there, they will go
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into the kernel mainline tree run by Linus Torvalds. To read up on the
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process of net and net-next being merged into the mainline tree, see
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the documentation on netdev subsystem at
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Documentation/process/maintainer-netdev.rst.
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Occasionally, to prevent merge conflicts, we might send pull requests
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to other trees (e.g. tracing) with a small subset of the patches, but
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net and net-next are always the main trees targeted for integration.
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The pull requests will contain a high-level summary of the accumulated
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patches and can be searched on netdev kernel mailing list through the
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following subject lines (``yyyy-mm-dd`` is the date of the pull
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request)::
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pull-request: bpf yyyy-mm-dd
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pull-request: bpf-next yyyy-mm-dd
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Q: How do I indicate which tree (bpf vs. bpf-next) my patch should be applied to?
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---------------------------------------------------------------------------------
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A: The process is the very same as described in the netdev subsystem
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documentation at Documentation/process/maintainer-netdev.rst,
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so please read up on it. The subject line must indicate whether the
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patch is a fix or rather "next-like" content in order to let the
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maintainers know whether it is targeted at bpf or bpf-next.
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For fixes eventually landing in bpf -> net tree, the subject must
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look like::
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git format-patch --subject-prefix='PATCH bpf' start..finish
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For features/improvements/etc that should eventually land in
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bpf-next -> net-next, the subject must look like::
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git format-patch --subject-prefix='PATCH bpf-next' start..finish
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If unsure whether the patch or patch series should go into bpf
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or net directly, or bpf-next or net-next directly, it is not a
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problem either if the subject line says net or net-next as target.
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It is eventually up to the maintainers to do the delegation of
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the patches.
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If it is clear that patches should go into bpf or bpf-next tree,
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please make sure to rebase the patches against those trees in
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order to reduce potential conflicts.
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In case the patch or patch series has to be reworked and sent out
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again in a second or later revision, it is also required to add a
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version number (``v2``, ``v3``, ...) into the subject prefix::
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git format-patch --subject-prefix='PATCH bpf-next v2' start..finish
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When changes have been requested to the patch series, always send the
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whole patch series again with the feedback incorporated (never send
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individual diffs on top of the old series).
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Q: What does it mean when a patch gets applied to bpf or bpf-next tree?
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-----------------------------------------------------------------------
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A: It means that the patch looks good for mainline inclusion from
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a BPF point of view.
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Be aware that this is not a final verdict that the patch will
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automatically get accepted into net or net-next trees eventually:
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On the bpf kernel mailing list reviews can come in at any point
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in time. If discussions around a patch conclude that they cannot
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get included as-is, we will either apply a follow-up fix or drop
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them from the trees entirely. Therefore, we also reserve to rebase
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the trees when deemed necessary. After all, the purpose of the tree
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is to:
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i) accumulate and stage BPF patches for integration into trees
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like net and net-next, and
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ii) run extensive BPF test suite and
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workloads on the patches before they make their way any further.
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Once the BPF pull request was accepted by David S. Miller, then
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the patches end up in net or net-next tree, respectively, and
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make their way from there further into mainline. Again, see the
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documentation for netdev subsystem at
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Documentation/process/maintainer-netdev.rst for additional information
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e.g. on how often they are merged to mainline.
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Q: How long do I need to wait for feedback on my BPF patches?
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-------------------------------------------------------------
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A: We try to keep the latency low. The usual time to feedback will
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be around 2 or 3 business days. It may vary depending on the
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complexity of changes and current patch load.
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Q: How often do you send pull requests to major kernel trees like net or net-next?
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----------------------------------------------------------------------------------
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A: Pull requests will be sent out rather often in order to not
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accumulate too many patches in bpf or bpf-next.
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As a rule of thumb, expect pull requests for each tree regularly
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at the end of the week. In some cases pull requests could additionally
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come also in the middle of the week depending on the current patch
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load or urgency.
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Q: Are patches applied to bpf-next when the merge window is open?
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-----------------------------------------------------------------
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A: For the time when the merge window is open, bpf-next will not be
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processed. This is roughly analogous to net-next patch processing,
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so feel free to read up on the netdev docs at
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Documentation/process/maintainer-netdev.rst about further details.
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During those two weeks of merge window, we might ask you to resend
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your patch series once bpf-next is open again. Once Linus released
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a ``v*-rc1`` after the merge window, we continue processing of bpf-next.
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For non-subscribers to kernel mailing lists, there is also a status
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page run by David S. Miller on net-next that provides guidance:
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http://vger.kernel.org/~davem/net-next.html
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Q: Verifier changes and test cases
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----------------------------------
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Q: I made a BPF verifier change, do I need to add test cases for
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BPF kernel selftests_?
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A: If the patch has changes to the behavior of the verifier, then yes,
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it is absolutely necessary to add test cases to the BPF kernel
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selftests_ suite. If they are not present and we think they are
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needed, then we might ask for them before accepting any changes.
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In particular, test_verifier.c is tracking a high number of BPF test
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cases, including a lot of corner cases that LLVM BPF back end may
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generate out of the restricted C code. Thus, adding test cases is
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absolutely crucial to make sure future changes do not accidentally
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affect prior use-cases. Thus, treat those test cases as: verifier
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behavior that is not tracked in test_verifier.c could potentially
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be subject to change.
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Q: samples/bpf preference vs selftests?
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---------------------------------------
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Q: When should I add code to ``samples/bpf/`` and when to BPF kernel
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selftests_?
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A: In general, we prefer additions to BPF kernel selftests_ rather than
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``samples/bpf/``. The rationale is very simple: kernel selftests are
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regularly run by various bots to test for kernel regressions.
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The more test cases we add to BPF selftests, the better the coverage
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and the less likely it is that those could accidentally break. It is
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not that BPF kernel selftests cannot demo how a specific feature can
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be used.
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That said, ``samples/bpf/`` may be a good place for people to get started,
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so it might be advisable that simple demos of features could go into
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``samples/bpf/``, but advanced functional and corner-case testing rather
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into kernel selftests.
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If your sample looks like a test case, then go for BPF kernel selftests
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instead!
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Q: When should I add code to the bpftool?
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-----------------------------------------
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A: The main purpose of bpftool (under tools/bpf/bpftool/) is to provide
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a central user space tool for debugging and introspection of BPF programs
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and maps that are active in the kernel. If UAPI changes related to BPF
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enable for dumping additional information of programs or maps, then
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bpftool should be extended as well to support dumping them.
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Q: When should I add code to iproute2's BPF loader?
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---------------------------------------------------
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A: For UAPI changes related to the XDP or tc layer (e.g. ``cls_bpf``),
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the convention is that those control-path related changes are added to
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iproute2's BPF loader as well from user space side. This is not only
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useful to have UAPI changes properly designed to be usable, but also
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to make those changes available to a wider user base of major
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downstream distributions.
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Q: Do you accept patches as well for iproute2's BPF loader?
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-----------------------------------------------------------
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A: Patches for the iproute2's BPF loader have to be sent to:
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netdev@vger.kernel.org
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While those patches are not processed by the BPF kernel maintainers,
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please keep them in Cc as well, so they can be reviewed.
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The official git repository for iproute2 is run by Stephen Hemminger
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and can be found at:
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https://git.kernel.org/pub/scm/linux/kernel/git/shemminger/iproute2.git/
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The patches need to have a subject prefix of '``[PATCH iproute2
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master]``' or '``[PATCH iproute2 net-next]``'. '``master``' or
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'``net-next``' describes the target branch where the patch should be
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applied to. Meaning, if kernel changes went into the net-next kernel
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tree, then the related iproute2 changes need to go into the iproute2
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net-next branch, otherwise they can be targeted at master branch. The
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iproute2 net-next branch will get merged into the master branch after
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the current iproute2 version from master has been released.
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Like BPF, the patches end up in patchwork under the netdev project and
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are delegated to 'shemminger' for further processing:
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http://patchwork.ozlabs.org/project/netdev/list/?delegate=389
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Q: What is the minimum requirement before I submit my BPF patches?
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------------------------------------------------------------------
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A: When submitting patches, always take the time and properly test your
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patches *prior* to submission. Never rush them! If maintainers find
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that your patches have not been properly tested, it is a good way to
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get them grumpy. Testing patch submissions is a hard requirement!
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Note, fixes that go to bpf tree *must* have a ``Fixes:`` tag included.
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The same applies to fixes that target bpf-next, where the affected
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commit is in net-next (or in some cases bpf-next). The ``Fixes:`` tag is
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crucial in order to identify follow-up commits and tremendously helps
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for people having to do backporting, so it is a must have!
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We also don't accept patches with an empty commit message. Take your
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time and properly write up a high quality commit message, it is
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essential!
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Think about it this way: other developers looking at your code a month
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from now need to understand *why* a certain change has been done that
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way, and whether there have been flaws in the analysis or assumptions
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that the original author did. Thus providing a proper rationale and
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describing the use-case for the changes is a must.
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Patch submissions with >1 patch must have a cover letter which includes
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a high level description of the series. This high level summary will
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then be placed into the merge commit by the BPF maintainers such that
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it is also accessible from the git log for future reference.
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Q: Features changing BPF JIT and/or LLVM
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----------------------------------------
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Q: What do I need to consider when adding a new instruction or feature
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that would require BPF JIT and/or LLVM integration as well?
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A: We try hard to keep all BPF JITs up to date such that the same user
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experience can be guaranteed when running BPF programs on different
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architectures without having the program punt to the less efficient
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interpreter in case the in-kernel BPF JIT is enabled.
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If you are unable to implement or test the required JIT changes for
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certain architectures, please work together with the related BPF JIT
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developers in order to get the feature implemented in a timely manner.
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Please refer to the git log (``arch/*/net/``) to locate the necessary
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people for helping out.
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Also always make sure to add BPF test cases (e.g. test_bpf.c and
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test_verifier.c) for new instructions, so that they can receive
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broad test coverage and help run-time testing the various BPF JITs.
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In case of new BPF instructions, once the changes have been accepted
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into the Linux kernel, please implement support into LLVM's BPF back
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end. See LLVM_ section below for further information.
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Stable submission
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=================
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Q: I need a specific BPF commit in stable kernels. What should I do?
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--------------------------------------------------------------------
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A: In case you need a specific fix in stable kernels, first check whether
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the commit has already been applied in the related ``linux-*.y`` branches:
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https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/
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If not the case, then drop an email to the BPF maintainers with the
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netdev kernel mailing list in Cc and ask for the fix to be queued up:
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netdev@vger.kernel.org
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The process in general is the same as on netdev itself, see also the
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the documentation on networking subsystem at
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Documentation/process/maintainer-netdev.rst.
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Q: Do you also backport to kernels not currently maintained as stable?
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----------------------------------------------------------------------
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A: No. If you need a specific BPF commit in kernels that are currently not
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maintained by the stable maintainers, then you are on your own.
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The current stable and longterm stable kernels are all listed here:
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https://www.kernel.org/
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Q: The BPF patch I am about to submit needs to go to stable as well
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-------------------------------------------------------------------
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What should I do?
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A: The same rules apply as with netdev patch submissions in general, see
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the netdev docs at Documentation/process/maintainer-netdev.rst.
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Never add "``Cc: stable@vger.kernel.org``" to the patch description, but
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ask the BPF maintainers to queue the patches instead. This can be done
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with a note, for example, under the ``---`` part of the patch which does
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not go into the git log. Alternatively, this can be done as a simple
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request by mail instead.
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Q: Queue stable patches
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-----------------------
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Q: Where do I find currently queued BPF patches that will be submitted
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to stable?
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A: Once patches that fix critical bugs got applied into the bpf tree, they
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are queued up for stable submission under:
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http://patchwork.ozlabs.org/bundle/bpf/stable/?state=*
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They will be on hold there at minimum until the related commit made its
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way into the mainline kernel tree.
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After having been under broader exposure, the queued patches will be
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submitted by the BPF maintainers to the stable maintainers.
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Testing patches
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===============
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Q: How to run BPF selftests
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---------------------------
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A: After you have booted into the newly compiled kernel, navigate to
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the BPF selftests_ suite in order to test BPF functionality (current
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working directory points to the root of the cloned git tree)::
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$ cd tools/testing/selftests/bpf/
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$ make
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To run the verifier tests::
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$ sudo ./test_verifier
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The verifier tests print out all the current checks being
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performed. The summary at the end of running all tests will dump
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information of test successes and failures::
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Summary: 418 PASSED, 0 FAILED
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In order to run through all BPF selftests, the following command is
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needed::
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$ sudo make run_tests
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See :doc:`kernel selftest documentation </dev-tools/kselftest>`
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for details.
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To maximize the number of tests passing, the .config of the kernel
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under test should match the config file fragment in
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tools/testing/selftests/bpf as closely as possible.
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Finally to ensure support for latest BPF Type Format features -
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discussed in Documentation/bpf/btf.rst - pahole version 1.16
|
|
is required for kernels built with CONFIG_DEBUG_INFO_BTF=y.
|
|
pahole is delivered in the dwarves package or can be built
|
|
from source at
|
|
|
|
https://github.com/acmel/dwarves
|
|
|
|
pahole starts to use libbpf definitions and APIs since v1.13 after the
|
|
commit 21507cd3e97b ("pahole: add libbpf as submodule under lib/bpf").
|
|
It works well with the git repository because the libbpf submodule will
|
|
use "git submodule update --init --recursive" to update.
|
|
|
|
Unfortunately, the default github release source code does not contain
|
|
libbpf submodule source code and this will cause build issues, the tarball
|
|
from https://git.kernel.org/pub/scm/devel/pahole/pahole.git/ is same with
|
|
github, you can get the source tarball with corresponding libbpf submodule
|
|
codes from
|
|
|
|
https://fedorapeople.org/~acme/dwarves
|
|
|
|
Some distros have pahole version 1.16 packaged already, e.g.
|
|
Fedora, Gentoo.
|
|
|
|
Q: Which BPF kernel selftests version should I run my kernel against?
|
|
---------------------------------------------------------------------
|
|
A: If you run a kernel ``xyz``, then always run the BPF kernel selftests
|
|
from that kernel ``xyz`` as well. Do not expect that the BPF selftest
|
|
from the latest mainline tree will pass all the time.
|
|
|
|
In particular, test_bpf.c and test_verifier.c have a large number of
|
|
test cases and are constantly updated with new BPF test sequences, or
|
|
existing ones are adapted to verifier changes e.g. due to verifier
|
|
becoming smarter and being able to better track certain things.
|
|
|
|
LLVM
|
|
====
|
|
|
|
Q: Where do I find LLVM with BPF support?
|
|
-----------------------------------------
|
|
A: The BPF back end for LLVM is upstream in LLVM since version 3.7.1.
|
|
|
|
All major distributions these days ship LLVM with BPF back end enabled,
|
|
so for the majority of use-cases it is not required to compile LLVM by
|
|
hand anymore, just install the distribution provided package.
|
|
|
|
LLVM's static compiler lists the supported targets through
|
|
``llc --version``, make sure BPF targets are listed. Example::
|
|
|
|
$ llc --version
|
|
LLVM (http://llvm.org/):
|
|
LLVM version 10.0.0
|
|
Optimized build.
|
|
Default target: x86_64-unknown-linux-gnu
|
|
Host CPU: skylake
|
|
|
|
Registered Targets:
|
|
aarch64 - AArch64 (little endian)
|
|
bpf - BPF (host endian)
|
|
bpfeb - BPF (big endian)
|
|
bpfel - BPF (little endian)
|
|
x86 - 32-bit X86: Pentium-Pro and above
|
|
x86-64 - 64-bit X86: EM64T and AMD64
|
|
|
|
For developers in order to utilize the latest features added to LLVM's
|
|
BPF back end, it is advisable to run the latest LLVM releases. Support
|
|
for new BPF kernel features such as additions to the BPF instruction
|
|
set are often developed together.
|
|
|
|
All LLVM releases can be found at: http://releases.llvm.org/
|
|
|
|
Q: Got it, so how do I build LLVM manually anyway?
|
|
--------------------------------------------------
|
|
A: We recommend that developers who want the fastest incremental builds
|
|
use the Ninja build system, you can find it in your system's package
|
|
manager, usually the package is ninja or ninja-build.
|
|
|
|
You need ninja, cmake and gcc-c++ as build requisites for LLVM. Once you
|
|
have that set up, proceed with building the latest LLVM and clang version
|
|
from the git repositories::
|
|
|
|
$ git clone https://github.com/llvm/llvm-project.git
|
|
$ mkdir -p llvm-project/llvm/build
|
|
$ cd llvm-project/llvm/build
|
|
$ cmake .. -G "Ninja" -DLLVM_TARGETS_TO_BUILD="BPF;X86" \
|
|
-DLLVM_ENABLE_PROJECTS="clang" \
|
|
-DCMAKE_BUILD_TYPE=Release \
|
|
-DLLVM_BUILD_RUNTIME=OFF
|
|
$ ninja
|
|
|
|
The built binaries can then be found in the build/bin/ directory, where
|
|
you can point the PATH variable to.
|
|
|
|
Set ``-DLLVM_TARGETS_TO_BUILD`` equal to the target you wish to build, you
|
|
will find a full list of targets within the llvm-project/llvm/lib/Target
|
|
directory.
|
|
|
|
Q: Reporting LLVM BPF issues
|
|
----------------------------
|
|
Q: Should I notify BPF kernel maintainers about issues in LLVM's BPF code
|
|
generation back end or about LLVM generated code that the verifier
|
|
refuses to accept?
|
|
|
|
A: Yes, please do!
|
|
|
|
LLVM's BPF back end is a key piece of the whole BPF
|
|
infrastructure and it ties deeply into verification of programs from the
|
|
kernel side. Therefore, any issues on either side need to be investigated
|
|
and fixed whenever necessary.
|
|
|
|
Therefore, please make sure to bring them up at netdev kernel mailing
|
|
list and Cc BPF maintainers for LLVM and kernel bits:
|
|
|
|
* Yonghong Song <yhs@fb.com>
|
|
* Alexei Starovoitov <ast@kernel.org>
|
|
* Daniel Borkmann <daniel@iogearbox.net>
|
|
|
|
LLVM also has an issue tracker where BPF related bugs can be found:
|
|
|
|
https://bugs.llvm.org/buglist.cgi?quicksearch=bpf
|
|
|
|
However, it is better to reach out through mailing lists with having
|
|
maintainers in Cc.
|
|
|
|
Q: New BPF instruction for kernel and LLVM
|
|
------------------------------------------
|
|
Q: I have added a new BPF instruction to the kernel, how can I integrate
|
|
it into LLVM?
|
|
|
|
A: LLVM has a ``-mcpu`` selector for the BPF back end in order to allow
|
|
the selection of BPF instruction set extensions. By default the
|
|
``generic`` processor target is used, which is the base instruction set
|
|
(v1) of BPF.
|
|
|
|
LLVM has an option to select ``-mcpu=probe`` where it will probe the host
|
|
kernel for supported BPF instruction set extensions and selects the
|
|
optimal set automatically.
|
|
|
|
For cross-compilation, a specific version can be select manually as well ::
|
|
|
|
$ llc -march bpf -mcpu=help
|
|
Available CPUs for this target:
|
|
|
|
generic - Select the generic processor.
|
|
probe - Select the probe processor.
|
|
v1 - Select the v1 processor.
|
|
v2 - Select the v2 processor.
|
|
[...]
|
|
|
|
Newly added BPF instructions to the Linux kernel need to follow the same
|
|
scheme, bump the instruction set version and implement probing for the
|
|
extensions such that ``-mcpu=probe`` users can benefit from the
|
|
optimization transparently when upgrading their kernels.
|
|
|
|
If you are unable to implement support for the newly added BPF instruction
|
|
please reach out to BPF developers for help.
|
|
|
|
By the way, the BPF kernel selftests run with ``-mcpu=probe`` for better
|
|
test coverage.
|
|
|
|
Q: clang flag for target bpf?
|
|
-----------------------------
|
|
Q: In some cases clang flag ``--target=bpf`` is used but in other cases the
|
|
default clang target, which matches the underlying architecture, is used.
|
|
What is the difference and when I should use which?
|
|
|
|
A: Although LLVM IR generation and optimization try to stay architecture
|
|
independent, ``--target=<arch>`` still has some impact on generated code:
|
|
|
|
- BPF program may recursively include header file(s) with file scope
|
|
inline assembly codes. The default target can handle this well,
|
|
while ``bpf`` target may fail if bpf backend assembler does not
|
|
understand these assembly codes, which is true in most cases.
|
|
|
|
- When compiled without ``-g``, additional elf sections, e.g.,
|
|
.eh_frame and .rela.eh_frame, may be present in the object file
|
|
with default target, but not with ``bpf`` target.
|
|
|
|
- The default target may turn a C switch statement into a switch table
|
|
lookup and jump operation. Since the switch table is placed
|
|
in the global readonly section, the bpf program will fail to load.
|
|
The bpf target does not support switch table optimization.
|
|
The clang option ``-fno-jump-tables`` can be used to disable
|
|
switch table generation.
|
|
|
|
- For clang ``--target=bpf``, it is guaranteed that pointer or long /
|
|
unsigned long types will always have a width of 64 bit, no matter
|
|
whether underlying clang binary or default target (or kernel) is
|
|
32 bit. However, when native clang target is used, then it will
|
|
compile these types based on the underlying architecture's conventions,
|
|
meaning in case of 32 bit architecture, pointer or long / unsigned
|
|
long types e.g. in BPF context structure will have width of 32 bit
|
|
while the BPF LLVM back end still operates in 64 bit. The native
|
|
target is mostly needed in tracing for the case of walking ``pt_regs``
|
|
or other kernel structures where CPU's register width matters.
|
|
Otherwise, ``clang --target=bpf`` is generally recommended.
|
|
|
|
You should use default target when:
|
|
|
|
- Your program includes a header file, e.g., ptrace.h, which eventually
|
|
pulls in some header files containing file scope host assembly codes.
|
|
|
|
- You can add ``-fno-jump-tables`` to work around the switch table issue.
|
|
|
|
Otherwise, you can use ``bpf`` target. Additionally, you *must* use bpf target
|
|
when:
|
|
|
|
- Your program uses data structures with pointer or long / unsigned long
|
|
types that interface with BPF helpers or context data structures. Access
|
|
into these structures is verified by the BPF verifier and may result
|
|
in verification failures if the native architecture is not aligned with
|
|
the BPF architecture, e.g. 64-bit. An example of this is
|
|
BPF_PROG_TYPE_SK_MSG require ``--target=bpf``
|
|
|
|
|
|
.. Links
|
|
.. _selftests:
|
|
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/bpf/
|
|
|
|
Happy BPF hacking!
|