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1=================================
2HOWTO interact with BPF subsystem
3=================================
4
5This document provides information for the BPF subsystem about various
6workflows related to reporting bugs, submitting patches, and queueing
7patches for stable kernels.
8
9For general information about submitting patches, please refer to
10Documentation/process/submitting-patches.rst. This document only describes
11additional specifics related to BPF.
12
13.. contents::
14 :local:
15 :depth: 2
16
17Reporting bugs
18==============
19
20Q: How do I report bugs for BPF kernel code?
21--------------------------------------------
22A: Since all BPF kernel development as well as bpftool and iproute2 BPF
23loader development happens through the bpf kernel mailing list,
24please report any found issues around BPF to the following mailing
25list:
26
27 bpf@vger.kernel.org
28
29This may also include issues related to XDP, BPF tracing, etc.
30
31Given netdev has a high volume of traffic, please also add the BPF
32maintainers to Cc (from kernel ``MAINTAINERS`` file):
33
34* Alexei Starovoitov <ast@kernel.org>
35* Daniel Borkmann <daniel@iogearbox.net>
36
37In case a buggy commit has already been identified, make sure to keep
38the actual commit authors in Cc as well for the report. They can
39typically be identified through the kernel's git tree.
40
41**Please do NOT report BPF issues to bugzilla.kernel.org since it
42is a guarantee that the reported issue will be overlooked.**
43
44Submitting patches
45==================
46
47Q: How do I run BPF CI on my changes before sending them out for review?
48------------------------------------------------------------------------
49A: BPF CI is GitHub based and hosted at https://github.com/kernel-patches/bpf.
50While GitHub also provides a CLI that can be used to accomplish the same
51results, here we focus on the UI based workflow.
52
53The following steps lay out how to start a CI run for your patches:
54
55- Create a fork of the aforementioned repository in your own account (one time
56 action)
57
58- Clone the fork locally, check out a new branch tracking either the bpf-next
59 or bpf branch, and apply your to-be-tested patches on top of it
60
61- Push the local branch to your fork and create a pull request against
62 kernel-patches/bpf's bpf-next_base or bpf_base branch, respectively
63
64Shortly after the pull request has been created, the CI workflow will run. Note
65that capacity is shared with patches submitted upstream being checked and so
66depending on utilization the run can take a while to finish.
67
68Note furthermore that both base branches (bpf-next_base and bpf_base) will be
69updated as patches are pushed to the respective upstream branches they track. As
70such, your patch set will automatically (be attempted to) be rebased as well.
71This behavior can result in a CI run being aborted and restarted with the new
72base line.
73
74Q: To which mailing list do I need to submit my BPF patches?
75------------------------------------------------------------
76A: Please submit your BPF patches to the bpf kernel mailing list:
77
78 bpf@vger.kernel.org
79
80In case your patch has changes in various different subsystems (e.g.
81networking, tracing, security, etc), make sure to Cc the related kernel mailing
82lists and maintainers from there as well, so they are able to review
83the changes and provide their Acked-by's to the patches.
84
85Q: Where can I find patches currently under discussion for BPF subsystem?
86-------------------------------------------------------------------------
87A: All patches that are Cc'ed to netdev are queued for review under netdev
88patchwork project:
89
90 https://patchwork.kernel.org/project/netdevbpf/list/
91
92Those patches which target BPF, are assigned to a 'bpf' delegate for
93further processing from BPF maintainers. The current queue with
94patches under review can be found at:
95
96 https://patchwork.kernel.org/project/netdevbpf/list/?delegate=121173
97
98Once the patches have been reviewed by the BPF community as a whole
99and approved by the BPF maintainers, their status in patchwork will be
100changed to 'Accepted' and the submitter will be notified by mail. This
101means that the patches look good from a BPF perspective and have been
102applied to one of the two BPF kernel trees.
103
104In case feedback from the community requires a respin of the patches,
105their status in patchwork will be set to 'Changes Requested', and purged
106from the current review queue. Likewise for cases where patches would
107get rejected or are not applicable to the BPF trees (but assigned to
108the 'bpf' delegate).
109
110Q: How do the changes make their way into Linux?
111------------------------------------------------
112A: There are two BPF kernel trees (git repositories). Once patches have
113been accepted by the BPF maintainers, they will be applied to one
114of the two BPF trees:
115
116 * https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/
117 * https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/
118
119The bpf tree itself is for fixes only, whereas bpf-next for features,
120cleanups or other kind of improvements ("next-like" content). This is
121analogous to net and net-next trees for networking. Both bpf and
122bpf-next will only have a master branch in order to simplify against
123which branch patches should get rebased to.
124
125Accumulated BPF patches in the bpf tree will regularly get pulled
126into the net kernel tree. Likewise, accumulated BPF patches accepted
127into the bpf-next tree will make their way into net-next tree. net and
128net-next are both run by David S. Miller. From there, they will go
129into the kernel mainline tree run by Linus Torvalds. To read up on the
130process of net and net-next being merged into the mainline tree, see
131the documentation on netdev subsystem at
132Documentation/process/maintainer-netdev.rst.
133
134
135
136Occasionally, to prevent merge conflicts, we might send pull requests
137to other trees (e.g. tracing) with a small subset of the patches, but
138net and net-next are always the main trees targeted for integration.
139
140The pull requests will contain a high-level summary of the accumulated
141patches and can be searched on netdev kernel mailing list through the
142following subject lines (``yyyy-mm-dd`` is the date of the pull
143request)::
144
145 pull-request: bpf yyyy-mm-dd
146 pull-request: bpf-next yyyy-mm-dd
147
148Q: How do I indicate which tree (bpf vs. bpf-next) my patch should be applied to?
149---------------------------------------------------------------------------------
150
151A: The process is the very same as described in the netdev subsystem
152documentation at Documentation/process/maintainer-netdev.rst,
153so please read up on it. The subject line must indicate whether the
154patch is a fix or rather "next-like" content in order to let the
155maintainers know whether it is targeted at bpf or bpf-next.
156
157For fixes eventually landing in bpf -> net tree, the subject must
158look like::
159
160 git format-patch --subject-prefix='PATCH bpf' start..finish
161
162For features/improvements/etc that should eventually land in
163bpf-next -> net-next, the subject must look like::
164
165 git format-patch --subject-prefix='PATCH bpf-next' start..finish
166
167If unsure whether the patch or patch series should go into bpf
168or net directly, or bpf-next or net-next directly, it is not a
169problem either if the subject line says net or net-next as target.
170It is eventually up to the maintainers to do the delegation of
171the patches.
172
173If it is clear that patches should go into bpf or bpf-next tree,
174please make sure to rebase the patches against those trees in
175order to reduce potential conflicts.
176
177In case the patch or patch series has to be reworked and sent out
178again in a second or later revision, it is also required to add a
179version number (``v2``, ``v3``, ...) into the subject prefix::
180
181 git format-patch --subject-prefix='PATCH bpf-next v2' start..finish
182
183When changes have been requested to the patch series, always send the
184whole patch series again with the feedback incorporated (never send
185individual diffs on top of the old series).
186
187Q: What does it mean when a patch gets applied to bpf or bpf-next tree?
188-----------------------------------------------------------------------
189A: It means that the patch looks good for mainline inclusion from
190a BPF point of view.
191
192Be aware that this is not a final verdict that the patch will
193automatically get accepted into net or net-next trees eventually:
194
195On the bpf kernel mailing list reviews can come in at any point
196in time. If discussions around a patch conclude that they cannot
197get included as-is, we will either apply a follow-up fix or drop
198them from the trees entirely. Therefore, we also reserve to rebase
199the trees when deemed necessary. After all, the purpose of the tree
200is to:
201
202i) accumulate and stage BPF patches for integration into trees
203 like net and net-next, and
204
205ii) run extensive BPF test suite and
206 workloads on the patches before they make their way any further.
207
208Once the BPF pull request was accepted by David S. Miller, then
209the patches end up in net or net-next tree, respectively, and
210make their way from there further into mainline. Again, see the
211documentation for netdev subsystem at
212Documentation/process/maintainer-netdev.rst for additional information
213e.g. on how often they are merged to mainline.
214
215Q: How long do I need to wait for feedback on my BPF patches?
216-------------------------------------------------------------
217A: We try to keep the latency low. The usual time to feedback will
218be around 2 or 3 business days. It may vary depending on the
219complexity of changes and current patch load.
220
221Q: How often do you send pull requests to major kernel trees like net or net-next?
222----------------------------------------------------------------------------------
223
224A: Pull requests will be sent out rather often in order to not
225accumulate too many patches in bpf or bpf-next.
226
227As a rule of thumb, expect pull requests for each tree regularly
228at the end of the week. In some cases pull requests could additionally
229come also in the middle of the week depending on the current patch
230load or urgency.
231
232Q: Are patches applied to bpf-next when the merge window is open?
233-----------------------------------------------------------------
234A: For the time when the merge window is open, bpf-next will not be
235processed. This is roughly analogous to net-next patch processing,
236so feel free to read up on the netdev docs at
237Documentation/process/maintainer-netdev.rst about further details.
238
239During those two weeks of merge window, we might ask you to resend
240your patch series once bpf-next is open again. Once Linus released
241a ``v*-rc1`` after the merge window, we continue processing of bpf-next.
242
243For non-subscribers to kernel mailing lists, there is also a status
244page run by David S. Miller on net-next that provides guidance:
245
246 http://vger.kernel.org/~davem/net-next.html
247
248Q: Verifier changes and test cases
249----------------------------------
250Q: I made a BPF verifier change, do I need to add test cases for
251BPF kernel selftests_?
252
253A: If the patch has changes to the behavior of the verifier, then yes,
254it is absolutely necessary to add test cases to the BPF kernel
255selftests_ suite. If they are not present and we think they are
256needed, then we might ask for them before accepting any changes.
257
258In particular, test_verifier.c is tracking a high number of BPF test
259cases, including a lot of corner cases that LLVM BPF back end may
260generate out of the restricted C code. Thus, adding test cases is
261absolutely crucial to make sure future changes do not accidentally
262affect prior use-cases. Thus, treat those test cases as: verifier
263behavior that is not tracked in test_verifier.c could potentially
264be subject to change.
265
266Q: samples/bpf preference vs selftests?
267---------------------------------------
268Q: When should I add code to ``samples/bpf/`` and when to BPF kernel
269selftests_?
270
271A: In general, we prefer additions to BPF kernel selftests_ rather than
272``samples/bpf/``. The rationale is very simple: kernel selftests are
273regularly run by various bots to test for kernel regressions.
274
275The more test cases we add to BPF selftests, the better the coverage
276and the less likely it is that those could accidentally break. It is
277not that BPF kernel selftests cannot demo how a specific feature can
278be used.
279
280That said, ``samples/bpf/`` may be a good place for people to get started,
281so it might be advisable that simple demos of features could go into
282``samples/bpf/``, but advanced functional and corner-case testing rather
283into kernel selftests.
284
285If your sample looks like a test case, then go for BPF kernel selftests
286instead!
287
288Q: When should I add code to the bpftool?
289-----------------------------------------
290A: The main purpose of bpftool (under tools/bpf/bpftool/) is to provide
291a central user space tool for debugging and introspection of BPF programs
292and maps that are active in the kernel. If UAPI changes related to BPF
293enable for dumping additional information of programs or maps, then
294bpftool should be extended as well to support dumping them.
295
296Q: When should I add code to iproute2's BPF loader?
297---------------------------------------------------
298A: For UAPI changes related to the XDP or tc layer (e.g. ``cls_bpf``),
299the convention is that those control-path related changes are added to
300iproute2's BPF loader as well from user space side. This is not only
301useful to have UAPI changes properly designed to be usable, but also
302to make those changes available to a wider user base of major
303downstream distributions.
304
305Q: Do you accept patches as well for iproute2's BPF loader?
306-----------------------------------------------------------
307A: Patches for the iproute2's BPF loader have to be sent to:
308
309 netdev@vger.kernel.org
310
311While those patches are not processed by the BPF kernel maintainers,
312please keep them in Cc as well, so they can be reviewed.
313
314The official git repository for iproute2 is run by Stephen Hemminger
315and can be found at:
316
317 https://git.kernel.org/pub/scm/linux/kernel/git/shemminger/iproute2.git/
318
319The patches need to have a subject prefix of '``[PATCH iproute2
320master]``' or '``[PATCH iproute2 net-next]``'. '``master``' or
321'``net-next``' describes the target branch where the patch should be
322applied to. Meaning, if kernel changes went into the net-next kernel
323tree, then the related iproute2 changes need to go into the iproute2
324net-next branch, otherwise they can be targeted at master branch. The
325iproute2 net-next branch will get merged into the master branch after
326the current iproute2 version from master has been released.
327
328Like BPF, the patches end up in patchwork under the netdev project and
329are delegated to 'shemminger' for further processing:
330
331 http://patchwork.ozlabs.org/project/netdev/list/?delegate=389
332
333Q: What is the minimum requirement before I submit my BPF patches?
334------------------------------------------------------------------
335A: When submitting patches, always take the time and properly test your
336patches *prior* to submission. Never rush them! If maintainers find
337that your patches have not been properly tested, it is a good way to
338get them grumpy. Testing patch submissions is a hard requirement!
339
340Note, fixes that go to bpf tree *must* have a ``Fixes:`` tag included.
341The same applies to fixes that target bpf-next, where the affected
342commit is in net-next (or in some cases bpf-next). The ``Fixes:`` tag is
343crucial in order to identify follow-up commits and tremendously helps
344for people having to do backporting, so it is a must have!
345
346We also don't accept patches with an empty commit message. Take your
347time and properly write up a high quality commit message, it is
348essential!
349
350Think about it this way: other developers looking at your code a month
351from now need to understand *why* a certain change has been done that
352way, and whether there have been flaws in the analysis or assumptions
353that the original author did. Thus providing a proper rationale and
354describing the use-case for the changes is a must.
355
356Patch submissions with >1 patch must have a cover letter which includes
357a high level description of the series. This high level summary will
358then be placed into the merge commit by the BPF maintainers such that
359it is also accessible from the git log for future reference.
360
361Q: Features changing BPF JIT and/or LLVM
362----------------------------------------
363Q: What do I need to consider when adding a new instruction or feature
364that would require BPF JIT and/or LLVM integration as well?
365
366A: We try hard to keep all BPF JITs up to date such that the same user
367experience can be guaranteed when running BPF programs on different
368architectures without having the program punt to the less efficient
369interpreter in case the in-kernel BPF JIT is enabled.
370
371If you are unable to implement or test the required JIT changes for
372certain architectures, please work together with the related BPF JIT
373developers in order to get the feature implemented in a timely manner.
374Please refer to the git log (``arch/*/net/``) to locate the necessary
375people for helping out.
376
377Also always make sure to add BPF test cases (e.g. test_bpf.c and
378test_verifier.c) for new instructions, so that they can receive
379broad test coverage and help run-time testing the various BPF JITs.
380
381In case of new BPF instructions, once the changes have been accepted
382into the Linux kernel, please implement support into LLVM's BPF back
383end. See LLVM_ section below for further information.
384
385Stable submission
386=================
387
388Q: I need a specific BPF commit in stable kernels. What should I do?
389--------------------------------------------------------------------
390A: In case you need a specific fix in stable kernels, first check whether
391the commit has already been applied in the related ``linux-*.y`` branches:
392
393 https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/
394
395If not the case, then drop an email to the BPF maintainers with the
396netdev kernel mailing list in Cc and ask for the fix to be queued up:
397
398 netdev@vger.kernel.org
399
400The process in general is the same as on netdev itself, see also the
401the documentation on networking subsystem at
402Documentation/process/maintainer-netdev.rst.
403
404Q: Do you also backport to kernels not currently maintained as stable?
405----------------------------------------------------------------------
406A: No. If you need a specific BPF commit in kernels that are currently not
407maintained by the stable maintainers, then you are on your own.
408
409The current stable and longterm stable kernels are all listed here:
410
411 https://www.kernel.org/
412
413Q: The BPF patch I am about to submit needs to go to stable as well
414-------------------------------------------------------------------
415What should I do?
416
417A: The same rules apply as with netdev patch submissions in general, see
418the netdev docs at Documentation/process/maintainer-netdev.rst.
419
420Never add "``Cc: stable@vger.kernel.org``" to the patch description, but
421ask the BPF maintainers to queue the patches instead. This can be done
422with a note, for example, under the ``---`` part of the patch which does
423not go into the git log. Alternatively, this can be done as a simple
424request by mail instead.
425
426Q: Queue stable patches
427-----------------------
428Q: Where do I find currently queued BPF patches that will be submitted
429to stable?
430
431A: Once patches that fix critical bugs got applied into the bpf tree, they
432are queued up for stable submission under:
433
434 http://patchwork.ozlabs.org/bundle/bpf/stable/?state=*
435
436They will be on hold there at minimum until the related commit made its
437way into the mainline kernel tree.
438
439After having been under broader exposure, the queued patches will be
440submitted by the BPF maintainers to the stable maintainers.
441
442Testing patches
443===============
444
445Q: How to run BPF selftests
446---------------------------
447A: After you have booted into the newly compiled kernel, navigate to
448the BPF selftests_ suite in order to test BPF functionality (current
449working directory points to the root of the cloned git tree)::
450
451 $ cd tools/testing/selftests/bpf/
452 $ make
453
454To run the verifier tests::
455
456 $ sudo ./test_verifier
457
458The verifier tests print out all the current checks being
459performed. The summary at the end of running all tests will dump
460information of test successes and failures::
461
462 Summary: 418 PASSED, 0 FAILED
463
464In order to run through all BPF selftests, the following command is
465needed::
466
467 $ sudo make run_tests
468
469See :doc:`kernel selftest documentation </dev-tools/kselftest>`
470for details.
471
472To maximize the number of tests passing, the .config of the kernel
473under test should match the config file fragment in
474tools/testing/selftests/bpf as closely as possible.
475
476Finally to ensure support for latest BPF Type Format features -
477discussed in Documentation/bpf/btf.rst - pahole version 1.16
478is required for kernels built with CONFIG_DEBUG_INFO_BTF=y.
479pahole is delivered in the dwarves package or can be built
480from source at
481
482https://github.com/acmel/dwarves
483
484pahole starts to use libbpf definitions and APIs since v1.13 after the
485commit 21507cd3e97b ("pahole: add libbpf as submodule under lib/bpf").
486It works well with the git repository because the libbpf submodule will
487use "git submodule update --init --recursive" to update.
488
489Unfortunately, the default github release source code does not contain
490libbpf submodule source code and this will cause build issues, the tarball
491from https://git.kernel.org/pub/scm/devel/pahole/pahole.git/ is same with
492github, you can get the source tarball with corresponding libbpf submodule
493codes from
494
495https://fedorapeople.org/~acme/dwarves
496
497Some distros have pahole version 1.16 packaged already, e.g.
498Fedora, Gentoo.
499
500Q: Which BPF kernel selftests version should I run my kernel against?
501---------------------------------------------------------------------
502A: If you run a kernel ``xyz``, then always run the BPF kernel selftests
503from that kernel ``xyz`` as well. Do not expect that the BPF selftest
504from the latest mainline tree will pass all the time.
505
506In particular, test_bpf.c and test_verifier.c have a large number of
507test cases and are constantly updated with new BPF test sequences, or
508existing ones are adapted to verifier changes e.g. due to verifier
509becoming smarter and being able to better track certain things.
510
511LLVM
512====
513
514Q: Where do I find LLVM with BPF support?
515-----------------------------------------
516A: The BPF back end for LLVM is upstream in LLVM since version 3.7.1.
517
518All major distributions these days ship LLVM with BPF back end enabled,
519so for the majority of use-cases it is not required to compile LLVM by
520hand anymore, just install the distribution provided package.
521
522LLVM's static compiler lists the supported targets through
523``llc --version``, make sure BPF targets are listed. Example::
524
525 $ llc --version
526 LLVM (http://llvm.org/):
527 LLVM version 10.0.0
528 Optimized build.
529 Default target: x86_64-unknown-linux-gnu
530 Host CPU: skylake
531
532 Registered Targets:
533 aarch64 - AArch64 (little endian)
534 bpf - BPF (host endian)
535 bpfeb - BPF (big endian)
536 bpfel - BPF (little endian)
537 x86 - 32-bit X86: Pentium-Pro and above
538 x86-64 - 64-bit X86: EM64T and AMD64
539
540For developers in order to utilize the latest features added to LLVM's
541BPF back end, it is advisable to run the latest LLVM releases. Support
542for new BPF kernel features such as additions to the BPF instruction
543set are often developed together.
544
545All LLVM releases can be found at: http://releases.llvm.org/
546
547Q: Got it, so how do I build LLVM manually anyway?
548--------------------------------------------------
549A: We recommend that developers who want the fastest incremental builds
550use the Ninja build system, you can find it in your system's package
551manager, usually the package is ninja or ninja-build.
552
553You need ninja, cmake and gcc-c++ as build requisites for LLVM. Once you
554have that set up, proceed with building the latest LLVM and clang version
555from the git repositories::
556
557 $ git clone https://github.com/llvm/llvm-project.git
558 $ mkdir -p llvm-project/llvm/build
559 $ cd llvm-project/llvm/build
560 $ cmake .. -G "Ninja" -DLLVM_TARGETS_TO_BUILD="BPF;X86" \
561 -DLLVM_ENABLE_PROJECTS="clang" \
562 -DCMAKE_BUILD_TYPE=Release \
563 -DLLVM_BUILD_RUNTIME=OFF
564 $ ninja
565
566The built binaries can then be found in the build/bin/ directory, where
567you can point the PATH variable to.
568
569Set ``-DLLVM_TARGETS_TO_BUILD`` equal to the target you wish to build, you
570will find a full list of targets within the llvm-project/llvm/lib/Target
571directory.
572
573Q: Reporting LLVM BPF issues
574----------------------------
575Q: Should I notify BPF kernel maintainers about issues in LLVM's BPF code
576generation back end or about LLVM generated code that the verifier
577refuses to accept?
578
579A: Yes, please do!
580
581LLVM's BPF back end is a key piece of the whole BPF
582infrastructure and it ties deeply into verification of programs from the
583kernel side. Therefore, any issues on either side need to be investigated
584and fixed whenever necessary.
585
586Therefore, please make sure to bring them up at netdev kernel mailing
587list and Cc BPF maintainers for LLVM and kernel bits:
588
589* Yonghong Song <yhs@fb.com>
590* Alexei Starovoitov <ast@kernel.org>
591* Daniel Borkmann <daniel@iogearbox.net>
592
593LLVM also has an issue tracker where BPF related bugs can be found:
594
595 https://bugs.llvm.org/buglist.cgi?quicksearch=bpf
596
597However, it is better to reach out through mailing lists with having
598maintainers in Cc.
599
600Q: New BPF instruction for kernel and LLVM
601------------------------------------------
602Q: I have added a new BPF instruction to the kernel, how can I integrate
603it into LLVM?
604
605A: LLVM has a ``-mcpu`` selector for the BPF back end in order to allow
606the selection of BPF instruction set extensions. By default the
607``generic`` processor target is used, which is the base instruction set
608(v1) of BPF.
609
610LLVM has an option to select ``-mcpu=probe`` where it will probe the host
611kernel for supported BPF instruction set extensions and selects the
612optimal set automatically.
613
614For cross-compilation, a specific version can be select manually as well ::
615
616 $ llc -march bpf -mcpu=help
617 Available CPUs for this target:
618
619 generic - Select the generic processor.
620 probe - Select the probe processor.
621 v1 - Select the v1 processor.
622 v2 - Select the v2 processor.
623 [...]
624
625Newly added BPF instructions to the Linux kernel need to follow the same
626scheme, bump the instruction set version and implement probing for the
627extensions such that ``-mcpu=probe`` users can benefit from the
628optimization transparently when upgrading their kernels.
629
630If you are unable to implement support for the newly added BPF instruction
631please reach out to BPF developers for help.
632
633By the way, the BPF kernel selftests run with ``-mcpu=probe`` for better
634test coverage.
635
636Q: clang flag for target bpf?
637-----------------------------
638Q: In some cases clang flag ``--target=bpf`` is used but in other cases the
639default clang target, which matches the underlying architecture, is used.
640What is the difference and when I should use which?
641
642A: Although LLVM IR generation and optimization try to stay architecture
643independent, ``--target=<arch>`` still has some impact on generated code:
644
645- BPF program may recursively include header file(s) with file scope
646 inline assembly codes. The default target can handle this well,
647 while ``bpf`` target may fail if bpf backend assembler does not
648 understand these assembly codes, which is true in most cases.
649
650- When compiled without ``-g``, additional elf sections, e.g.,
651 .eh_frame and .rela.eh_frame, may be present in the object file
652 with default target, but not with ``bpf`` target.
653
654- The default target may turn a C switch statement into a switch table
655 lookup and jump operation. Since the switch table is placed
656 in the global readonly section, the bpf program will fail to load.
657 The bpf target does not support switch table optimization.
658 The clang option ``-fno-jump-tables`` can be used to disable
659 switch table generation.
660
661- For clang ``--target=bpf``, it is guaranteed that pointer or long /
662 unsigned long types will always have a width of 64 bit, no matter
663 whether underlying clang binary or default target (or kernel) is
664 32 bit. However, when native clang target is used, then it will
665 compile these types based on the underlying architecture's conventions,
666 meaning in case of 32 bit architecture, pointer or long / unsigned
667 long types e.g. in BPF context structure will have width of 32 bit
668 while the BPF LLVM back end still operates in 64 bit. The native
669 target is mostly needed in tracing for the case of walking ``pt_regs``
670 or other kernel structures where CPU's register width matters.
671 Otherwise, ``clang --target=bpf`` is generally recommended.
672
673You should use default target when:
674
675- Your program includes a header file, e.g., ptrace.h, which eventually
676 pulls in some header files containing file scope host assembly codes.
677
678- You can add ``-fno-jump-tables`` to work around the switch table issue.
679
680Otherwise, you can use ``bpf`` target. Additionally, you *must* use bpf target
681when:
682
683- Your program uses data structures with pointer or long / unsigned long
684 types that interface with BPF helpers or context data structures. Access
685 into these structures is verified by the BPF verifier and may result
686 in verification failures if the native architecture is not aligned with
687 the BPF architecture, e.g. 64-bit. An example of this is
688 BPF_PROG_TYPE_SK_MSG require ``--target=bpf``
689
690
691.. Links
692.. _selftests:
693 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/bpf/
694
695Happy BPF hacking!
1=================================
2HOWTO interact with BPF subsystem
3=================================
4
5This document provides information for the BPF subsystem about various
6workflows related to reporting bugs, submitting patches, and queueing
7patches for stable kernels.
8
9For general information about submitting patches, please refer to
10`Documentation/process/`_. This document only describes additional specifics
11related to BPF.
12
13.. contents::
14 :local:
15 :depth: 2
16
17Reporting bugs
18==============
19
20Q: How do I report bugs for BPF kernel code?
21--------------------------------------------
22A: Since all BPF kernel development as well as bpftool and iproute2 BPF
23loader development happens through the netdev kernel mailing list,
24please report any found issues around BPF to the following mailing
25list:
26
27 netdev@vger.kernel.org
28
29This may also include issues related to XDP, BPF tracing, etc.
30
31Given netdev has a high volume of traffic, please also add the BPF
32maintainers to Cc (from kernel MAINTAINERS_ file):
33
34* Alexei Starovoitov <ast@kernel.org>
35* Daniel Borkmann <daniel@iogearbox.net>
36
37In case a buggy commit has already been identified, make sure to keep
38the actual commit authors in Cc as well for the report. They can
39typically be identified through the kernel's git tree.
40
41**Please do NOT report BPF issues to bugzilla.kernel.org since it
42is a guarantee that the reported issue will be overlooked.**
43
44Submitting patches
45==================
46
47Q: To which mailing list do I need to submit my BPF patches?
48------------------------------------------------------------
49A: Please submit your BPF patches to the netdev kernel mailing list:
50
51 netdev@vger.kernel.org
52
53Historically, BPF came out of networking and has always been maintained
54by the kernel networking community. Although these days BPF touches
55many other subsystems as well, the patches are still routed mainly
56through the networking community.
57
58In case your patch has changes in various different subsystems (e.g.
59tracing, security, etc), make sure to Cc the related kernel mailing
60lists and maintainers from there as well, so they are able to review
61the changes and provide their Acked-by's to the patches.
62
63Q: Where can I find patches currently under discussion for BPF subsystem?
64-------------------------------------------------------------------------
65A: All patches that are Cc'ed to netdev are queued for review under netdev
66patchwork project:
67
68 http://patchwork.ozlabs.org/project/netdev/list/
69
70Those patches which target BPF, are assigned to a 'bpf' delegate for
71further processing from BPF maintainers. The current queue with
72patches under review can be found at:
73
74 https://patchwork.ozlabs.org/project/netdev/list/?delegate=77147
75
76Once the patches have been reviewed by the BPF community as a whole
77and approved by the BPF maintainers, their status in patchwork will be
78changed to 'Accepted' and the submitter will be notified by mail. This
79means that the patches look good from a BPF perspective and have been
80applied to one of the two BPF kernel trees.
81
82In case feedback from the community requires a respin of the patches,
83their status in patchwork will be set to 'Changes Requested', and purged
84from the current review queue. Likewise for cases where patches would
85get rejected or are not applicable to the BPF trees (but assigned to
86the 'bpf' delegate).
87
88Q: How do the changes make their way into Linux?
89------------------------------------------------
90A: There are two BPF kernel trees (git repositories). Once patches have
91been accepted by the BPF maintainers, they will be applied to one
92of the two BPF trees:
93
94 * https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/
95 * https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/
96
97The bpf tree itself is for fixes only, whereas bpf-next for features,
98cleanups or other kind of improvements ("next-like" content). This is
99analogous to net and net-next trees for networking. Both bpf and
100bpf-next will only have a master branch in order to simplify against
101which branch patches should get rebased to.
102
103Accumulated BPF patches in the bpf tree will regularly get pulled
104into the net kernel tree. Likewise, accumulated BPF patches accepted
105into the bpf-next tree will make their way into net-next tree. net and
106net-next are both run by David S. Miller. From there, they will go
107into the kernel mainline tree run by Linus Torvalds. To read up on the
108process of net and net-next being merged into the mainline tree, see
109the :ref:`netdev-FAQ`
110
111
112
113Occasionally, to prevent merge conflicts, we might send pull requests
114to other trees (e.g. tracing) with a small subset of the patches, but
115net and net-next are always the main trees targeted for integration.
116
117The pull requests will contain a high-level summary of the accumulated
118patches and can be searched on netdev kernel mailing list through the
119following subject lines (``yyyy-mm-dd`` is the date of the pull
120request)::
121
122 pull-request: bpf yyyy-mm-dd
123 pull-request: bpf-next yyyy-mm-dd
124
125Q: How do I indicate which tree (bpf vs. bpf-next) my patch should be applied to?
126---------------------------------------------------------------------------------
127
128A: The process is the very same as described in the :ref:`netdev-FAQ`,
129so please read up on it. The subject line must indicate whether the
130patch is a fix or rather "next-like" content in order to let the
131maintainers know whether it is targeted at bpf or bpf-next.
132
133For fixes eventually landing in bpf -> net tree, the subject must
134look like::
135
136 git format-patch --subject-prefix='PATCH bpf' start..finish
137
138For features/improvements/etc that should eventually land in
139bpf-next -> net-next, the subject must look like::
140
141 git format-patch --subject-prefix='PATCH bpf-next' start..finish
142
143If unsure whether the patch or patch series should go into bpf
144or net directly, or bpf-next or net-next directly, it is not a
145problem either if the subject line says net or net-next as target.
146It is eventually up to the maintainers to do the delegation of
147the patches.
148
149If it is clear that patches should go into bpf or bpf-next tree,
150please make sure to rebase the patches against those trees in
151order to reduce potential conflicts.
152
153In case the patch or patch series has to be reworked and sent out
154again in a second or later revision, it is also required to add a
155version number (``v2``, ``v3``, ...) into the subject prefix::
156
157 git format-patch --subject-prefix='PATCH net-next v2' start..finish
158
159When changes have been requested to the patch series, always send the
160whole patch series again with the feedback incorporated (never send
161individual diffs on top of the old series).
162
163Q: What does it mean when a patch gets applied to bpf or bpf-next tree?
164-----------------------------------------------------------------------
165A: It means that the patch looks good for mainline inclusion from
166a BPF point of view.
167
168Be aware that this is not a final verdict that the patch will
169automatically get accepted into net or net-next trees eventually:
170
171On the netdev kernel mailing list reviews can come in at any point
172in time. If discussions around a patch conclude that they cannot
173get included as-is, we will either apply a follow-up fix or drop
174them from the trees entirely. Therefore, we also reserve to rebase
175the trees when deemed necessary. After all, the purpose of the tree
176is to:
177
178i) accumulate and stage BPF patches for integration into trees
179 like net and net-next, and
180
181ii) run extensive BPF test suite and
182 workloads on the patches before they make their way any further.
183
184Once the BPF pull request was accepted by David S. Miller, then
185the patches end up in net or net-next tree, respectively, and
186make their way from there further into mainline. Again, see the
187:ref:`netdev-FAQ` for additional information e.g. on how often they are
188merged to mainline.
189
190Q: How long do I need to wait for feedback on my BPF patches?
191-------------------------------------------------------------
192A: We try to keep the latency low. The usual time to feedback will
193be around 2 or 3 business days. It may vary depending on the
194complexity of changes and current patch load.
195
196Q: How often do you send pull requests to major kernel trees like net or net-next?
197----------------------------------------------------------------------------------
198
199A: Pull requests will be sent out rather often in order to not
200accumulate too many patches in bpf or bpf-next.
201
202As a rule of thumb, expect pull requests for each tree regularly
203at the end of the week. In some cases pull requests could additionally
204come also in the middle of the week depending on the current patch
205load or urgency.
206
207Q: Are patches applied to bpf-next when the merge window is open?
208-----------------------------------------------------------------
209A: For the time when the merge window is open, bpf-next will not be
210processed. This is roughly analogous to net-next patch processing,
211so feel free to read up on the :ref:`netdev-FAQ` about further details.
212
213During those two weeks of merge window, we might ask you to resend
214your patch series once bpf-next is open again. Once Linus released
215a ``v*-rc1`` after the merge window, we continue processing of bpf-next.
216
217For non-subscribers to kernel mailing lists, there is also a status
218page run by David S. Miller on net-next that provides guidance:
219
220 http://vger.kernel.org/~davem/net-next.html
221
222Q: Verifier changes and test cases
223----------------------------------
224Q: I made a BPF verifier change, do I need to add test cases for
225BPF kernel selftests_?
226
227A: If the patch has changes to the behavior of the verifier, then yes,
228it is absolutely necessary to add test cases to the BPF kernel
229selftests_ suite. If they are not present and we think they are
230needed, then we might ask for them before accepting any changes.
231
232In particular, test_verifier.c is tracking a high number of BPF test
233cases, including a lot of corner cases that LLVM BPF back end may
234generate out of the restricted C code. Thus, adding test cases is
235absolutely crucial to make sure future changes do not accidentally
236affect prior use-cases. Thus, treat those test cases as: verifier
237behavior that is not tracked in test_verifier.c could potentially
238be subject to change.
239
240Q: samples/bpf preference vs selftests?
241---------------------------------------
242Q: When should I add code to `samples/bpf/`_ and when to BPF kernel
243selftests_ ?
244
245A: In general, we prefer additions to BPF kernel selftests_ rather than
246`samples/bpf/`_. The rationale is very simple: kernel selftests are
247regularly run by various bots to test for kernel regressions.
248
249The more test cases we add to BPF selftests, the better the coverage
250and the less likely it is that those could accidentally break. It is
251not that BPF kernel selftests cannot demo how a specific feature can
252be used.
253
254That said, `samples/bpf/`_ may be a good place for people to get started,
255so it might be advisable that simple demos of features could go into
256`samples/bpf/`_, but advanced functional and corner-case testing rather
257into kernel selftests.
258
259If your sample looks like a test case, then go for BPF kernel selftests
260instead!
261
262Q: When should I add code to the bpftool?
263-----------------------------------------
264A: The main purpose of bpftool (under tools/bpf/bpftool/) is to provide
265a central user space tool for debugging and introspection of BPF programs
266and maps that are active in the kernel. If UAPI changes related to BPF
267enable for dumping additional information of programs or maps, then
268bpftool should be extended as well to support dumping them.
269
270Q: When should I add code to iproute2's BPF loader?
271---------------------------------------------------
272A: For UAPI changes related to the XDP or tc layer (e.g. ``cls_bpf``),
273the convention is that those control-path related changes are added to
274iproute2's BPF loader as well from user space side. This is not only
275useful to have UAPI changes properly designed to be usable, but also
276to make those changes available to a wider user base of major
277downstream distributions.
278
279Q: Do you accept patches as well for iproute2's BPF loader?
280-----------------------------------------------------------
281A: Patches for the iproute2's BPF loader have to be sent to:
282
283 netdev@vger.kernel.org
284
285While those patches are not processed by the BPF kernel maintainers,
286please keep them in Cc as well, so they can be reviewed.
287
288The official git repository for iproute2 is run by Stephen Hemminger
289and can be found at:
290
291 https://git.kernel.org/pub/scm/linux/kernel/git/shemminger/iproute2.git/
292
293The patches need to have a subject prefix of '``[PATCH iproute2
294master]``' or '``[PATCH iproute2 net-next]``'. '``master``' or
295'``net-next``' describes the target branch where the patch should be
296applied to. Meaning, if kernel changes went into the net-next kernel
297tree, then the related iproute2 changes need to go into the iproute2
298net-next branch, otherwise they can be targeted at master branch. The
299iproute2 net-next branch will get merged into the master branch after
300the current iproute2 version from master has been released.
301
302Like BPF, the patches end up in patchwork under the netdev project and
303are delegated to 'shemminger' for further processing:
304
305 http://patchwork.ozlabs.org/project/netdev/list/?delegate=389
306
307Q: What is the minimum requirement before I submit my BPF patches?
308------------------------------------------------------------------
309A: When submitting patches, always take the time and properly test your
310patches *prior* to submission. Never rush them! If maintainers find
311that your patches have not been properly tested, it is a good way to
312get them grumpy. Testing patch submissions is a hard requirement!
313
314Note, fixes that go to bpf tree *must* have a ``Fixes:`` tag included.
315The same applies to fixes that target bpf-next, where the affected
316commit is in net-next (or in some cases bpf-next). The ``Fixes:`` tag is
317crucial in order to identify follow-up commits and tremendously helps
318for people having to do backporting, so it is a must have!
319
320We also don't accept patches with an empty commit message. Take your
321time and properly write up a high quality commit message, it is
322essential!
323
324Think about it this way: other developers looking at your code a month
325from now need to understand *why* a certain change has been done that
326way, and whether there have been flaws in the analysis or assumptions
327that the original author did. Thus providing a proper rationale and
328describing the use-case for the changes is a must.
329
330Patch submissions with >1 patch must have a cover letter which includes
331a high level description of the series. This high level summary will
332then be placed into the merge commit by the BPF maintainers such that
333it is also accessible from the git log for future reference.
334
335Q: Features changing BPF JIT and/or LLVM
336----------------------------------------
337Q: What do I need to consider when adding a new instruction or feature
338that would require BPF JIT and/or LLVM integration as well?
339
340A: We try hard to keep all BPF JITs up to date such that the same user
341experience can be guaranteed when running BPF programs on different
342architectures without having the program punt to the less efficient
343interpreter in case the in-kernel BPF JIT is enabled.
344
345If you are unable to implement or test the required JIT changes for
346certain architectures, please work together with the related BPF JIT
347developers in order to get the feature implemented in a timely manner.
348Please refer to the git log (``arch/*/net/``) to locate the necessary
349people for helping out.
350
351Also always make sure to add BPF test cases (e.g. test_bpf.c and
352test_verifier.c) for new instructions, so that they can receive
353broad test coverage and help run-time testing the various BPF JITs.
354
355In case of new BPF instructions, once the changes have been accepted
356into the Linux kernel, please implement support into LLVM's BPF back
357end. See LLVM_ section below for further information.
358
359Stable submission
360=================
361
362Q: I need a specific BPF commit in stable kernels. What should I do?
363--------------------------------------------------------------------
364A: In case you need a specific fix in stable kernels, first check whether
365the commit has already been applied in the related ``linux-*.y`` branches:
366
367 https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/
368
369If not the case, then drop an email to the BPF maintainers with the
370netdev kernel mailing list in Cc and ask for the fix to be queued up:
371
372 netdev@vger.kernel.org
373
374The process in general is the same as on netdev itself, see also the
375:ref:`netdev-FAQ`.
376
377Q: Do you also backport to kernels not currently maintained as stable?
378----------------------------------------------------------------------
379A: No. If you need a specific BPF commit in kernels that are currently not
380maintained by the stable maintainers, then you are on your own.
381
382The current stable and longterm stable kernels are all listed here:
383
384 https://www.kernel.org/
385
386Q: The BPF patch I am about to submit needs to go to stable as well
387-------------------------------------------------------------------
388What should I do?
389
390A: The same rules apply as with netdev patch submissions in general, see
391the :ref:`netdev-FAQ`.
392
393Never add "``Cc: stable@vger.kernel.org``" to the patch description, but
394ask the BPF maintainers to queue the patches instead. This can be done
395with a note, for example, under the ``---`` part of the patch which does
396not go into the git log. Alternatively, this can be done as a simple
397request by mail instead.
398
399Q: Queue stable patches
400-----------------------
401Q: Where do I find currently queued BPF patches that will be submitted
402to stable?
403
404A: Once patches that fix critical bugs got applied into the bpf tree, they
405are queued up for stable submission under:
406
407 http://patchwork.ozlabs.org/bundle/bpf/stable/?state=*
408
409They will be on hold there at minimum until the related commit made its
410way into the mainline kernel tree.
411
412After having been under broader exposure, the queued patches will be
413submitted by the BPF maintainers to the stable maintainers.
414
415Testing patches
416===============
417
418Q: How to run BPF selftests
419---------------------------
420A: After you have booted into the newly compiled kernel, navigate to
421the BPF selftests_ suite in order to test BPF functionality (current
422working directory points to the root of the cloned git tree)::
423
424 $ cd tools/testing/selftests/bpf/
425 $ make
426
427To run the verifier tests::
428
429 $ sudo ./test_verifier
430
431The verifier tests print out all the current checks being
432performed. The summary at the end of running all tests will dump
433information of test successes and failures::
434
435 Summary: 418 PASSED, 0 FAILED
436
437In order to run through all BPF selftests, the following command is
438needed::
439
440 $ sudo make run_tests
441
442See the kernels selftest `Documentation/dev-tools/kselftest.rst`_
443document for further documentation.
444
445Q: Which BPF kernel selftests version should I run my kernel against?
446---------------------------------------------------------------------
447A: If you run a kernel ``xyz``, then always run the BPF kernel selftests
448from that kernel ``xyz`` as well. Do not expect that the BPF selftest
449from the latest mainline tree will pass all the time.
450
451In particular, test_bpf.c and test_verifier.c have a large number of
452test cases and are constantly updated with new BPF test sequences, or
453existing ones are adapted to verifier changes e.g. due to verifier
454becoming smarter and being able to better track certain things.
455
456LLVM
457====
458
459Q: Where do I find LLVM with BPF support?
460-----------------------------------------
461A: The BPF back end for LLVM is upstream in LLVM since version 3.7.1.
462
463All major distributions these days ship LLVM with BPF back end enabled,
464so for the majority of use-cases it is not required to compile LLVM by
465hand anymore, just install the distribution provided package.
466
467LLVM's static compiler lists the supported targets through
468``llc --version``, make sure BPF targets are listed. Example::
469
470 $ llc --version
471 LLVM (http://llvm.org/):
472 LLVM version 6.0.0svn
473 Optimized build.
474 Default target: x86_64-unknown-linux-gnu
475 Host CPU: skylake
476
477 Registered Targets:
478 bpf - BPF (host endian)
479 bpfeb - BPF (big endian)
480 bpfel - BPF (little endian)
481 x86 - 32-bit X86: Pentium-Pro and above
482 x86-64 - 64-bit X86: EM64T and AMD64
483
484For developers in order to utilize the latest features added to LLVM's
485BPF back end, it is advisable to run the latest LLVM releases. Support
486for new BPF kernel features such as additions to the BPF instruction
487set are often developed together.
488
489All LLVM releases can be found at: http://releases.llvm.org/
490
491Q: Got it, so how do I build LLVM manually anyway?
492--------------------------------------------------
493A: You need cmake and gcc-c++ as build requisites for LLVM. Once you have
494that set up, proceed with building the latest LLVM and clang version
495from the git repositories::
496
497 $ git clone http://llvm.org/git/llvm.git
498 $ cd llvm/tools
499 $ git clone --depth 1 http://llvm.org/git/clang.git
500 $ cd ..; mkdir build; cd build
501 $ cmake .. -DLLVM_TARGETS_TO_BUILD="BPF;X86" \
502 -DBUILD_SHARED_LIBS=OFF \
503 -DCMAKE_BUILD_TYPE=Release \
504 -DLLVM_BUILD_RUNTIME=OFF
505 $ make -j $(getconf _NPROCESSORS_ONLN)
506
507The built binaries can then be found in the build/bin/ directory, where
508you can point the PATH variable to.
509
510Q: Reporting LLVM BPF issues
511----------------------------
512Q: Should I notify BPF kernel maintainers about issues in LLVM's BPF code
513generation back end or about LLVM generated code that the verifier
514refuses to accept?
515
516A: Yes, please do!
517
518LLVM's BPF back end is a key piece of the whole BPF
519infrastructure and it ties deeply into verification of programs from the
520kernel side. Therefore, any issues on either side need to be investigated
521and fixed whenever necessary.
522
523Therefore, please make sure to bring them up at netdev kernel mailing
524list and Cc BPF maintainers for LLVM and kernel bits:
525
526* Yonghong Song <yhs@fb.com>
527* Alexei Starovoitov <ast@kernel.org>
528* Daniel Borkmann <daniel@iogearbox.net>
529
530LLVM also has an issue tracker where BPF related bugs can be found:
531
532 https://bugs.llvm.org/buglist.cgi?quicksearch=bpf
533
534However, it is better to reach out through mailing lists with having
535maintainers in Cc.
536
537Q: New BPF instruction for kernel and LLVM
538------------------------------------------
539Q: I have added a new BPF instruction to the kernel, how can I integrate
540it into LLVM?
541
542A: LLVM has a ``-mcpu`` selector for the BPF back end in order to allow
543the selection of BPF instruction set extensions. By default the
544``generic`` processor target is used, which is the base instruction set
545(v1) of BPF.
546
547LLVM has an option to select ``-mcpu=probe`` where it will probe the host
548kernel for supported BPF instruction set extensions and selects the
549optimal set automatically.
550
551For cross-compilation, a specific version can be select manually as well ::
552
553 $ llc -march bpf -mcpu=help
554 Available CPUs for this target:
555
556 generic - Select the generic processor.
557 probe - Select the probe processor.
558 v1 - Select the v1 processor.
559 v2 - Select the v2 processor.
560 [...]
561
562Newly added BPF instructions to the Linux kernel need to follow the same
563scheme, bump the instruction set version and implement probing for the
564extensions such that ``-mcpu=probe`` users can benefit from the
565optimization transparently when upgrading their kernels.
566
567If you are unable to implement support for the newly added BPF instruction
568please reach out to BPF developers for help.
569
570By the way, the BPF kernel selftests run with ``-mcpu=probe`` for better
571test coverage.
572
573Q: clang flag for target bpf?
574-----------------------------
575Q: In some cases clang flag ``-target bpf`` is used but in other cases the
576default clang target, which matches the underlying architecture, is used.
577What is the difference and when I should use which?
578
579A: Although LLVM IR generation and optimization try to stay architecture
580independent, ``-target <arch>`` still has some impact on generated code:
581
582- BPF program may recursively include header file(s) with file scope
583 inline assembly codes. The default target can handle this well,
584 while ``bpf`` target may fail if bpf backend assembler does not
585 understand these assembly codes, which is true in most cases.
586
587- When compiled without ``-g``, additional elf sections, e.g.,
588 .eh_frame and .rela.eh_frame, may be present in the object file
589 with default target, but not with ``bpf`` target.
590
591- The default target may turn a C switch statement into a switch table
592 lookup and jump operation. Since the switch table is placed
593 in the global readonly section, the bpf program will fail to load.
594 The bpf target does not support switch table optimization.
595 The clang option ``-fno-jump-tables`` can be used to disable
596 switch table generation.
597
598- For clang ``-target bpf``, it is guaranteed that pointer or long /
599 unsigned long types will always have a width of 64 bit, no matter
600 whether underlying clang binary or default target (or kernel) is
601 32 bit. However, when native clang target is used, then it will
602 compile these types based on the underlying architecture's conventions,
603 meaning in case of 32 bit architecture, pointer or long / unsigned
604 long types e.g. in BPF context structure will have width of 32 bit
605 while the BPF LLVM back end still operates in 64 bit. The native
606 target is mostly needed in tracing for the case of walking ``pt_regs``
607 or other kernel structures where CPU's register width matters.
608 Otherwise, ``clang -target bpf`` is generally recommended.
609
610You should use default target when:
611
612- Your program includes a header file, e.g., ptrace.h, which eventually
613 pulls in some header files containing file scope host assembly codes.
614
615- You can add ``-fno-jump-tables`` to work around the switch table issue.
616
617Otherwise, you can use ``bpf`` target. Additionally, you *must* use bpf target
618when:
619
620- Your program uses data structures with pointer or long / unsigned long
621 types that interface with BPF helpers or context data structures. Access
622 into these structures is verified by the BPF verifier and may result
623 in verification failures if the native architecture is not aligned with
624 the BPF architecture, e.g. 64-bit. An example of this is
625 BPF_PROG_TYPE_SK_MSG require ``-target bpf``
626
627
628.. Links
629.. _Documentation/process/: https://www.kernel.org/doc/html/latest/process/
630.. _MAINTAINERS: ../../MAINTAINERS
631.. _netdev-FAQ: ../networking/netdev-FAQ.rst
632.. _samples/bpf/: ../../samples/bpf/
633.. _selftests: ../../tools/testing/selftests/bpf/
634.. _Documentation/dev-tools/kselftest.rst:
635 https://www.kernel.org/doc/html/latest/dev-tools/kselftest.html
636
637Happy BPF hacking!