Documentation/bpf/bpf_devel_QA.txt

This document provides information for the BPF subsystem about various
workflows related to reporting bugs, submitting patches, and queueing
patches for stable kernels.

For general information about submitting patches, please refer to
Documentation/process/. This document only describes additional specifics
related to BPF.

Reporting bugs:
---------------

Q: How do I report bugs for BPF kernel code?

A: Since all BPF kernel development as well as bpftool and iproute2 BPF
   loader development happens through the netdev kernel mailing list,
   please report any found issues around BPF to the following mailing
   list:

     netdev@vger.kernel.org

   This may also include issues related to XDP, BPF tracing, etc.

   Given netdev has a high volume of traffic, please also add the BPF
   maintainers to Cc (from kernel MAINTAINERS file):

     Alexei Starovoitov <ast@kernel.org>
     Daniel Borkmann <daniel@iogearbox.net>

   In case a buggy commit has already been identified, make sure to keep
   the actual commit authors in Cc as well for the report. They can
   typically be identified through the kernel's git tree.

   Please do *not* report BPF issues to bugzilla.kernel.org since it
   is a guarantee that the reported issue will be overlooked.

Submitting patches:
-------------------

Q: To which mailing list do I need to submit my BPF patches?

A: Please submit your BPF patches to the netdev kernel mailing list:

     netdev@vger.kernel.org

   Historically, BPF came out of networking and has always been maintained
   by the kernel networking community. Although these days BPF touches
   many other subsystems as well, the patches are still routed mainly
   through the networking community.

   In case your patch has changes in various different subsystems (e.g.
   tracing, security, etc), make sure to Cc the related kernel mailing
   lists and maintainers from there as well, so they are able to review
   the changes and provide their Acked-by's to the patches.

Q: Where can I find patches currently under discussion for BPF subsystem?

A: All patches that are Cc'ed to netdev are queued for review under netdev
   patchwork project:

     http://patchwork.ozlabs.org/project/netdev/list/

   Those patches which target BPF, are assigned to a 'bpf' delegate for
   further processing from BPF maintainers. The current queue with
   patches under review can be found at:

     https://patchwork.ozlabs.org/project/netdev/list/?delegate=77147

   Once the patches have been reviewed by the BPF community as a whole
   and approved by the BPF maintainers, their status in patchwork will be
   changed to 'Accepted' and the submitter will be notified by mail. This
   means that the patches look good from a BPF perspective and have been
   applied to one of the two BPF kernel trees.

   In case feedback from the community requires a respin of the patches,
   their status in patchwork will be set to 'Changes Requested', and purged
   from the current review queue. Likewise for cases where patches would
   get rejected or are not applicable to the BPF trees (but assigned to
   the 'bpf' delegate).

Q: How do the changes make their way into Linux?

A: There are two BPF kernel trees (git repositories). Once patches have
   been accepted by the BPF maintainers, they will be applied to one
   of the two BPF trees:

     https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/
     https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/

   The bpf tree itself is for fixes only, whereas bpf-next for features,
   cleanups or other kind of improvements ("next-like" content). This is
   analogous to net and net-next trees for networking. Both bpf and
   bpf-next will only have a master branch in order to simplify against
   which branch patches should get rebased to.

   Accumulated BPF patches in the bpf tree will regularly get pulled
   into the net kernel tree. Likewise, accumulated BPF patches accepted
   into the bpf-next tree will make their way into net-next tree. net and
   net-next are both run by David S. Miller. From there, they will go
   into the kernel mainline tree run by Linus Torvalds. To read up on the
   process of net and net-next being merged into the mainline tree, see
   the netdev FAQ under:

     Documentation/networking/netdev-FAQ.txt

   Occasionally, to prevent merge conflicts, we might send pull requests
   to other trees (e.g. tracing) with a small subset of the patches, but
   net and net-next are always the main trees targeted for integration.

   The pull requests will contain a high-level summary of the accumulated
   patches and can be searched on netdev kernel mailing list through the
   following subject lines (yyyy-mm-dd is the date of the pull request):

     pull-request: bpf yyyy-mm-dd
     pull-request: bpf-next yyyy-mm-dd

Q: How do I indicate which tree (bpf vs. bpf-next) my patch should be
   applied to?

A: The process is the very same as described in the netdev FAQ, so
   please read up on it. The subject line must indicate whether the
   patch is a fix or rather "next-like" content in order to let the
   maintainers know whether it is targeted at bpf or bpf-next.

   For fixes eventually landing in bpf -> net tree, the subject must
   look like:

     git format-patch --subject-prefix='PATCH bpf' start..finish

   For features/improvements/etc that should eventually land in
   bpf-next -> net-next, the subject must look like:

     git format-patch --subject-prefix='PATCH bpf-next' start..finish

   If unsure whether the patch or patch series should go into bpf
   or net directly, or bpf-next or net-next directly, it is not a
   problem either if the subject line says net or net-next as target.
   It is eventually up to the maintainers to do the delegation of
   the patches.

   If it is clear that patches should go into bpf or bpf-next tree,
   please make sure to rebase the patches against those trees in
   order to reduce potential conflicts.

   In case the patch or patch series has to be reworked and sent out
   again in a second or later revision, it is also required to add a
   version number (v2, v3, ...) into the subject prefix:

     git format-patch --subject-prefix='PATCH net-next v2' start..finish

   When changes have been requested to the patch series, always send the
   whole patch series again with the feedback incorporated (never send
   individual diffs on top of the old series).

Q: What does it mean when a patch gets applied to bpf or bpf-next tree?

A: It means that the patch looks good for mainline inclusion from
   a BPF point of view.

   Be aware that this is not a final verdict that the patch will
   automatically get accepted into net or net-next trees eventually:

   On the netdev kernel mailing list reviews can come in at any point
   in time. If discussions around a patch conclude that they cannot
   get included as-is, we will either apply a follow-up fix or drop
   them from the trees entirely. Therefore, we also reserve to rebase
   the trees when deemed necessary. After all, the purpose of the tree
   is to i) accumulate and stage BPF patches for integration into trees
   like net and net-next, and ii) run extensive BPF test suite and
   workloads on the patches before they make their way any further.

   Once the BPF pull request was accepted by David S. Miller, then
   the patches end up in net or net-next tree, respectively, and
   make their way from there further into mainline. Again, see the
   netdev FAQ for additional information e.g. on how often they are
   merged to mainline.

Q: How long do I need to wait for feedback on my BPF patches?

A: We try to keep the latency low. The usual time to feedback will
   be around 2 or 3 business days. It may vary depending on the
   complexity of changes and current patch load.

Q: How often do you send pull requests to major kernel trees like
   net or net-next?

A: Pull requests will be sent out rather often in order to not
   accumulate too many patches in bpf or bpf-next.

   As a rule of thumb, expect pull requests for each tree regularly
   at the end of the week. In some cases pull requests could additionally
   come also in the middle of the week depending on the current patch
   load or urgency.

Q: Are patches applied to bpf-next when the merge window is open?

A: For the time when the merge window is open, bpf-next will not be
   processed. This is roughly analogous to net-next patch processing,
   so feel free to read up on the netdev FAQ about further details.

   During those two weeks of merge window, we might ask you to resend
   your patch series once bpf-next is open again. Once Linus released
   a v*-rc1 after the merge window, we continue processing of bpf-next.

   For non-subscribers to kernel mailing lists, there is also a status
   page run by David S. Miller on net-next that provides guidance:

     http://vger.kernel.org/~davem/net-next.html

Q: I made a BPF verifier change, do I need to add test cases for
   BPF kernel selftests?

A: If the patch has changes to the behavior of the verifier, then yes,
   it is absolutely necessary to add test cases to the BPF kernel
   selftests suite. If they are not present and we think they are
   needed, then we might ask for them before accepting any changes.

   In particular, test_verifier.c is tracking a high number of BPF test
   cases, including a lot of corner cases that LLVM BPF back end may
   generate out of the restricted C code. Thus, adding test cases is
   absolutely crucial to make sure future changes do not accidentally
   affect prior use-cases. Thus, treat those test cases as: verifier
   behavior that is not tracked in test_verifier.c could potentially
   be subject to change.

Q: When should I add code to samples/bpf/ and when to BPF kernel
   selftests?

A: In general, we prefer additions to BPF kernel selftests rather than
   samples/bpf/. The rationale is very simple: kernel selftests are
   regularly run by various bots to test for kernel regressions.

   The more test cases we add to BPF selftests, the better the coverage
   and the less likely it is that those could accidentally break. It is
   not that BPF kernel selftests cannot demo how a specific feature can
   be used.

   That said, samples/bpf/ may be a good place for people to get started,
   so it might be advisable that simple demos of features could go into
   samples/bpf/, but advanced functional and corner-case testing rather
   into kernel selftests.

   If your sample looks like a test case, then go for BPF kernel selftests
   instead!

Q: When should I add code to the bpftool?

A: The main purpose of bpftool (under tools/bpf/bpftool/) is to provide
   a central user space tool for debugging and introspection of BPF programs
   and maps that are active in the kernel. If UAPI changes related to BPF
   enable for dumping additional information of programs or maps, then
   bpftool should be extended as well to support dumping them.

Q: When should I add code to iproute2's BPF loader?

A: For UAPI changes related to the XDP or tc layer (e.g. cls_bpf), the
   convention is that those control-path related changes are added to
   iproute2's BPF loader as well from user space side. This is not only
   useful to have UAPI changes properly designed to be usable, but also
   to make those changes available to a wider user base of major
   downstream distributions.

Q: Do you accept patches as well for iproute2's BPF loader?

A: Patches for the iproute2's BPF loader have to be sent to:

     netdev@vger.kernel.org

   While those patches are not processed by the BPF kernel maintainers,
   please keep them in Cc as well, so they can be reviewed.

   The official git repository for iproute2 is run by Stephen Hemminger
   and can be found at:

     https://git.kernel.org/pub/scm/linux/kernel/git/shemminger/iproute2.git/

   The patches need to have a subject prefix of '[PATCH iproute2 master]'
   or '[PATCH iproute2 net-next]'. 'master' or 'net-next' describes the
   target branch where the patch should be applied to. Meaning, if kernel
   changes went into the net-next kernel tree, then the related iproute2
   changes need to go into the iproute2 net-next branch, otherwise they
   can be targeted at master branch. The iproute2 net-next branch will get
   merged into the master branch after the current iproute2 version from
   master has been released.

   Like BPF, the patches end up in patchwork under the netdev project and
   are delegated to 'shemminger' for further processing:

     http://patchwork.ozlabs.org/project/netdev/list/?delegate=389

Q: What is the minimum requirement before I submit my BPF patches?

A: When submitting patches, always take the time and properly test your
   patches *prior* to submission. Never rush them! If maintainers find
   that your patches have not been properly tested, it is a good way to
   get them grumpy. Testing patch submissions is a hard requirement!

   Note, fixes that go to bpf tree *must* have a Fixes: tag included. The
   same applies to fixes that target bpf-next, where the affected commit
   is in net-next (or in some cases bpf-next). The Fixes: tag is crucial
   in order to identify follow-up commits and tremendously helps for people
   having to do backporting, so it is a must have!

   We also don't accept patches with an empty commit message. Take your
   time and properly write up a high quality commit message, it is
   essential!

   Think about it this way: other developers looking at your code a month
   from now need to understand *why* a certain change has been done that
   way, and whether there have been flaws in the analysis or assumptions
   that the original author did. Thus providing a proper rationale and
   describing the use-case for the changes is a must.

   Patch submissions with >1 patch must have a cover letter which includes
   a high level description of the series. This high level summary will
   then be placed into the merge commit by the BPF maintainers such that
   it is also accessible from the git log for future reference.

Q: What do I need to consider when adding a new instruction or feature
   that would require BPF JIT and/or LLVM integration as well?

A: We try hard to keep all BPF JITs up to date such that the same user
   experience can be guaranteed when running BPF programs on different
   architectures without having the program punt to the less efficient
   interpreter in case the in-kernel BPF JIT is enabled.

   If you are unable to implement or test the required JIT changes for
   certain architectures, please work together with the related BPF JIT
   developers in order to get the feature implemented in a timely manner.
   Please refer to the git log (arch/*/net/) to locate the necessary
   people for helping out.

   Also always make sure to add BPF test cases (e.g. test_bpf.c and
   test_verifier.c) for new instructions, so that they can receive
   broad test coverage and help run-time testing the various BPF JITs.

   In case of new BPF instructions, once the changes have been accepted
   into the Linux kernel, please implement support into LLVM's BPF back
   end. See LLVM section below for further information.

Stable submission:
------------------

Q: I need a specific BPF commit in stable kernels. What should I do?

A: In case you need a specific fix in stable kernels, first check whether
   the commit has already been applied in the related linux-*.y branches:

     https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/

   If not the case, then drop an email to the BPF maintainers with the
   netdev kernel mailing list in Cc and ask for the fix to be queued up:

     netdev@vger.kernel.org

   The process in general is the same as on netdev itself, see also the
   netdev FAQ document.

Q: Do you also backport to kernels not currently maintained as stable?

A: No. If you need a specific BPF commit in kernels that are currently not
   maintained by the stable maintainers, then you are on your own.

   The current stable and longterm stable kernels are all listed here:

     https://www.kernel.org/

Q: The BPF patch I am about to submit needs to go to stable as well. What
   should I do?

A: The same rules apply as with netdev patch submissions in general, see
   netdev FAQ under:

     Documentation/networking/netdev-FAQ.txt

   Never add "Cc: stable@vger.kernel.org" to the patch description, but
   ask the BPF maintainers to queue the patches instead. This can be done
   with a note, for example, under the "---" part of the patch which does
   not go into the git log. Alternatively, this can be done as a simple
   request by mail instead.

Q: Where do I find currently queued BPF patches that will be submitted
   to stable?

A: Once patches that fix critical bugs got applied into the bpf tree, they
   are queued up for stable submission under:

     http://patchwork.ozlabs.org/bundle/bpf/stable/?state=*

   They will be on hold there at minimum until the related commit made its
   way into the mainline kernel tree.

   After having been under broader exposure, the queued patches will be
   submitted by the BPF maintainers to the stable maintainers.

Testing patches:
----------------

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 6.0.0svn
       Optimized build.
       Default target: x86_64-unknown-linux-gnu
       Host CPU: skylake

       Registered Targets:
         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: You need 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 http://llvm.org/git/llvm.git
     $ cd llvm/tools
     $ git clone --depth 1 http://llvm.org/git/clang.git
     $ cd ..; mkdir build; cd build
     $ cmake .. -DLLVM_TARGETS_TO_BUILD="BPF;X86" \
                -DBUILD_SHARED_LIBS=OFF           \
                -DCMAKE_BUILD_TYPE=Release        \
                -DLLVM_BUILD_RUNTIME=OFF
     $ make -j $(getconf _NPROCESSORS_ONLN)

   The built binaries can then be found in the build/bin/ directory, where
   you can point the PATH variable to.

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: 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: 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'

Happy BPF hacking!