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1 This document provides information for the BPF subsystem about various
2 workflows related to reporting bugs, submitting patches, and queueing
3 patches for stable kernels.
5 For general information about submitting patches, please refer to
6 Documentation/process/. This document only describes additional specifics
7 related to BPF.
9 Reporting bugs:
10 ---------------
12 Q: How do I report bugs for BPF kernel code?
14 A: Since all BPF kernel development as well as bpftool and iproute2 BPF
15 loader development happens through the netdev kernel mailing list,
16 please report any found issues around BPF to the following mailing
17 list:
19 netdev@vger.kernel.org
21 This may also include issues related to XDP, BPF tracing, etc.
23 Given netdev has a high volume of traffic, please also add the BPF
24 maintainers to Cc (from kernel MAINTAINERS file):
26 Alexei Starovoitov <ast@kernel.org>
27 Daniel Borkmann <daniel@iogearbox.net>
29 In case a buggy commit has already been identified, make sure to keep
30 the actual commit authors in Cc as well for the report. They can
31 typically be identified through the kernel's git tree.
33 Please do *not* report BPF issues to bugzilla.kernel.org since it
34 is a guarantee that the reported issue will be overlooked.
36 Submitting patches:
37 -------------------
39 Q: To which mailing list do I need to submit my BPF patches?
41 A: Please submit your BPF patches to the netdev kernel mailing list:
43 netdev@vger.kernel.org
45 Historically, BPF came out of networking and has always been maintained
46 by the kernel networking community. Although these days BPF touches
47 many other subsystems as well, the patches are still routed mainly
48 through the networking community.
50 In case your patch has changes in various different subsystems (e.g.
51 tracing, security, etc), make sure to Cc the related kernel mailing
52 lists and maintainers from there as well, so they are able to review
53 the changes and provide their Acked-by's to the patches.
55 Q: Where can I find patches currently under discussion for BPF subsystem?
57 A: All patches that are Cc'ed to netdev are queued for review under netdev
58 patchwork project:
60 http://patchwork.ozlabs.org/project/netdev/list/
62 Those patches which target BPF, are assigned to a 'bpf' delegate for
63 further processing from BPF maintainers. The current queue with
64 patches under review can be found at:
66 https://patchwork.ozlabs.org/project/netdev/list/?delegate=77147
68 Once the patches have been reviewed by the BPF community as a whole
69 and approved by the BPF maintainers, their status in patchwork will be
70 changed to 'Accepted' and the submitter will be notified by mail. This
71 means that the patches look good from a BPF perspective and have been
72 applied to one of the two BPF kernel trees.
74 In case feedback from the community requires a respin of the patches,
75 their status in patchwork will be set to 'Changes Requested', and purged
76 from the current review queue. Likewise for cases where patches would
77 get rejected or are not applicable to the BPF trees (but assigned to
78 the 'bpf' delegate).
80 Q: How do the changes make their way into Linux?
82 A: There are two BPF kernel trees (git repositories). Once patches have
83 been accepted by the BPF maintainers, they will be applied to one
84 of the two BPF trees:
86 https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/
87 https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/
89 The bpf tree itself is for fixes only, whereas bpf-next for features,
90 cleanups or other kind of improvements ("next-like" content). This is
91 analogous to net and net-next trees for networking. Both bpf and
92 bpf-next will only have a master branch in order to simplify against
93 which branch patches should get rebased to.
95 Accumulated BPF patches in the bpf tree will regularly get pulled
96 into the net kernel tree. Likewise, accumulated BPF patches accepted
97 into the bpf-next tree will make their way into net-next tree. net and
98 net-next are both run by David S. Miller. From there, they will go
99 into the kernel mainline tree run by Linus Torvalds. To read up on the
100 process of net and net-next being merged into the mainline tree, see
101 the netdev FAQ under:
103 Documentation/networking/netdev-FAQ.txt
105 Occasionally, to prevent merge conflicts, we might send pull requests
106 to other trees (e.g. tracing) with a small subset of the patches, but
107 net and net-next are always the main trees targeted for integration.
109 The pull requests will contain a high-level summary of the accumulated
110 patches and can be searched on netdev kernel mailing list through the
111 following subject lines (yyyy-mm-dd is the date of the pull request):
113 pull-request: bpf yyyy-mm-dd
114 pull-request: bpf-next yyyy-mm-dd
116 Q: How do I indicate which tree (bpf vs. bpf-next) my patch should be
117 applied to?
119 A: The process is the very same as described in the netdev FAQ, so
120 please read up on it. The subject line must indicate whether the
121 patch is a fix or rather "next-like" content in order to let the
122 maintainers know whether it is targeted at bpf or bpf-next.
124 For fixes eventually landing in bpf -> net tree, the subject must
125 look like:
127 git format-patch --subject-prefix='PATCH bpf' start..finish
129 For features/improvements/etc that should eventually land in
130 bpf-next -> net-next, the subject must look like:
132 git format-patch --subject-prefix='PATCH bpf-next' start..finish
134 If unsure whether the patch or patch series should go into bpf
135 or net directly, or bpf-next or net-next directly, it is not a
136 problem either if the subject line says net or net-next as target.
137 It is eventually up to the maintainers to do the delegation of
138 the patches.
140 If it is clear that patches should go into bpf or bpf-next tree,
141 please make sure to rebase the patches against those trees in
142 order to reduce potential conflicts.
144 In case the patch or patch series has to be reworked and sent out
145 again in a second or later revision, it is also required to add a
146 version number (v2, v3, ...) into the subject prefix:
148 git format-patch --subject-prefix='PATCH net-next v2' start..finish
150 When changes have been requested to the patch series, always send the
151 whole patch series again with the feedback incorporated (never send
152 individual diffs on top of the old series).
154 Q: What does it mean when a patch gets applied to bpf or bpf-next tree?
156 A: It means that the patch looks good for mainline inclusion from
157 a BPF point of view.
159 Be aware that this is not a final verdict that the patch will
160 automatically get accepted into net or net-next trees eventually:
162 On the netdev kernel mailing list reviews can come in at any point
163 in time. If discussions around a patch conclude that they cannot
164 get included as-is, we will either apply a follow-up fix or drop
165 them from the trees entirely. Therefore, we also reserve to rebase
166 the trees when deemed necessary. After all, the purpose of the tree
167 is to i) accumulate and stage BPF patches for integration into trees
168 like net and net-next, and ii) run extensive BPF test suite and
169 workloads on the patches before they make their way any further.
171 Once the BPF pull request was accepted by David S. Miller, then
172 the patches end up in net or net-next tree, respectively, and
173 make their way from there further into mainline. Again, see the
174 netdev FAQ for additional information e.g. on how often they are
175 merged to mainline.
177 Q: How long do I need to wait for feedback on my BPF patches?
179 A: We try to keep the latency low. The usual time to feedback will
180 be around 2 or 3 business days. It may vary depending on the
181 complexity of changes and current patch load.
183 Q: How often do you send pull requests to major kernel trees like
184 net or net-next?
186 A: Pull requests will be sent out rather often in order to not
187 accumulate too many patches in bpf or bpf-next.
189 As a rule of thumb, expect pull requests for each tree regularly
190 at the end of the week. In some cases pull requests could additionally
191 come also in the middle of the week depending on the current patch
192 load or urgency.
194 Q: Are patches applied to bpf-next when the merge window is open?
196 A: For the time when the merge window is open, bpf-next will not be
197 processed. This is roughly analogous to net-next patch processing,
198 so feel free to read up on the netdev FAQ about further details.
200 During those two weeks of merge window, we might ask you to resend
201 your patch series once bpf-next is open again. Once Linus released
202 a v*-rc1 after the merge window, we continue processing of bpf-next.
204 For non-subscribers to kernel mailing lists, there is also a status
205 page run by David S. Miller on net-next that provides guidance:
207 http://vger.kernel.org/~davem/net-next.html
209 Q: I made a BPF verifier change, do I need to add test cases for
210 BPF kernel selftests?
212 A: If the patch has changes to the behavior of the verifier, then yes,
213 it is absolutely necessary to add test cases to the BPF kernel
214 selftests suite. If they are not present and we think they are
215 needed, then we might ask for them before accepting any changes.
217 In particular, test_verifier.c is tracking a high number of BPF test
218 cases, including a lot of corner cases that LLVM BPF back end may
219 generate out of the restricted C code. Thus, adding test cases is
220 absolutely crucial to make sure future changes do not accidentally
221 affect prior use-cases. Thus, treat those test cases as: verifier
222 behavior that is not tracked in test_verifier.c could potentially
223 be subject to change.
225 Q: When should I add code to samples/bpf/ and when to BPF kernel
226 selftests?
228 A: In general, we prefer additions to BPF kernel selftests rather than
229 samples/bpf/. The rationale is very simple: kernel selftests are
230 regularly run by various bots to test for kernel regressions.
232 The more test cases we add to BPF selftests, the better the coverage
233 and the less likely it is that those could accidentally break. It is
234 not that BPF kernel selftests cannot demo how a specific feature can
235 be used.
237 That said, samples/bpf/ may be a good place for people to get started,
238 so it might be advisable that simple demos of features could go into
239 samples/bpf/, but advanced functional and corner-case testing rather
240 into kernel selftests.
242 If your sample looks like a test case, then go for BPF kernel selftests
243 instead!
245 Q: When should I add code to the bpftool?
247 A: The main purpose of bpftool (under tools/bpf/bpftool/) is to provide
248 a central user space tool for debugging and introspection of BPF programs
249 and maps that are active in the kernel. If UAPI changes related to BPF
250 enable for dumping additional information of programs or maps, then
251 bpftool should be extended as well to support dumping them.
253 Q: When should I add code to iproute2's BPF loader?
255 A: For UAPI changes related to the XDP or tc layer (e.g. cls_bpf), the
256 convention is that those control-path related changes are added to
257 iproute2's BPF loader as well from user space side. This is not only
258 useful to have UAPI changes properly designed to be usable, but also
259 to make those changes available to a wider user base of major
260 downstream distributions.
262 Q: Do you accept patches as well for iproute2's BPF loader?
264 A: Patches for the iproute2's BPF loader have to be sent to:
266 netdev@vger.kernel.org
268 While those patches are not processed by the BPF kernel maintainers,
269 please keep them in Cc as well, so they can be reviewed.
271 The official git repository for iproute2 is run by Stephen Hemminger
272 and can be found at:
274 https://git.kernel.org/pub/scm/linux/kernel/git/shemminger/iproute2.git/
276 The patches need to have a subject prefix of '[PATCH iproute2 master]'
277 or '[PATCH iproute2 net-next]'. 'master' or 'net-next' describes the
278 target branch where the patch should be applied to. Meaning, if kernel
279 changes went into the net-next kernel tree, then the related iproute2
280 changes need to go into the iproute2 net-next branch, otherwise they
281 can be targeted at master branch. The iproute2 net-next branch will get
282 merged into the master branch after the current iproute2 version from
283 master has been released.
285 Like BPF, the patches end up in patchwork under the netdev project and
286 are delegated to 'shemminger' for further processing:
288 http://patchwork.ozlabs.org/project/netdev/list/?delegate=389
290 Q: What is the minimum requirement before I submit my BPF patches?
292 A: When submitting patches, always take the time and properly test your
293 patches *prior* to submission. Never rush them! If maintainers find
294 that your patches have not been properly tested, it is a good way to
295 get them grumpy. Testing patch submissions is a hard requirement!
297 Note, fixes that go to bpf tree *must* have a Fixes: tag included. The
298 same applies to fixes that target bpf-next, where the affected commit
299 is in net-next (or in some cases bpf-next). The Fixes: tag is crucial
300 in order to identify follow-up commits and tremendously helps for people
301 having to do backporting, so it is a must have!
303 We also don't accept patches with an empty commit message. Take your
304 time and properly write up a high quality commit message, it is
305 essential!
307 Think about it this way: other developers looking at your code a month
308 from now need to understand *why* a certain change has been done that
309 way, and whether there have been flaws in the analysis or assumptions
310 that the original author did. Thus providing a proper rationale and
311 describing the use-case for the changes is a must.
313 Patch submissions with >1 patch must have a cover letter which includes
314 a high level description of the series. This high level summary will
315 then be placed into the merge commit by the BPF maintainers such that
316 it is also accessible from the git log for future reference.
318 Q: What do I need to consider when adding a new instruction or feature
319 that would require BPF JIT and/or LLVM integration as well?
321 A: We try hard to keep all BPF JITs up to date such that the same user
322 experience can be guaranteed when running BPF programs on different
323 architectures without having the program punt to the less efficient
324 interpreter in case the in-kernel BPF JIT is enabled.
326 If you are unable to implement or test the required JIT changes for
327 certain architectures, please work together with the related BPF JIT
328 developers in order to get the feature implemented in a timely manner.
329 Please refer to the git log (arch/*/net/) to locate the necessary
330 people for helping out.
332 Also always make sure to add BPF test cases (e.g. test_bpf.c and
333 test_verifier.c) for new instructions, so that they can receive
334 broad test coverage and help run-time testing the various BPF JITs.
336 In case of new BPF instructions, once the changes have been accepted
337 into the Linux kernel, please implement support into LLVM's BPF back
338 end. See LLVM section below for further information.
340 Stable submission:
341 ------------------
343 Q: I need a specific BPF commit in stable kernels. What should I do?
345 A: In case you need a specific fix in stable kernels, first check whether
346 the commit has already been applied in the related linux-*.y branches:
348 https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/
350 If not the case, then drop an email to the BPF maintainers with the
351 netdev kernel mailing list in Cc and ask for the fix to be queued up:
353 netdev@vger.kernel.org
355 The process in general is the same as on netdev itself, see also the
356 netdev FAQ document.
358 Q: Do you also backport to kernels not currently maintained as stable?
360 A: No. If you need a specific BPF commit in kernels that are currently not
361 maintained by the stable maintainers, then you are on your own.
363 The current stable and longterm stable kernels are all listed here:
365 https://www.kernel.org/
367 Q: The BPF patch I am about to submit needs to go to stable as well. What
368 should I do?
370 A: The same rules apply as with netdev patch submissions in general, see
371 netdev FAQ under:
373 Documentation/networking/netdev-FAQ.txt
375 Never add "Cc: stable@vger.kernel.org" to the patch description, but
376 ask the BPF maintainers to queue the patches instead. This can be done
377 with a note, for example, under the "---" part of the patch which does
378 not go into the git log. Alternatively, this can be done as a simple
379 request by mail instead.
381 Q: Where do I find currently queued BPF patches that will be submitted
382 to stable?
384 A: Once patches that fix critical bugs got applied into the bpf tree, they
385 are queued up for stable submission under:
387 http://patchwork.ozlabs.org/bundle/bpf/stable/?state=*
389 They will be on hold there at minimum until the related commit made its
390 way into the mainline kernel tree.
392 After having been under broader exposure, the queued patches will be
393 submitted by the BPF maintainers to the stable maintainers.
395 Testing patches:
396 ----------------
398 Q: Which BPF kernel selftests version should I run my kernel against?
400 A: If you run a kernel xyz, then always run the BPF kernel selftests from
401 that kernel xyz as well. Do not expect that the BPF selftest from the
402 latest mainline tree will pass all the time.
404 In particular, test_bpf.c and test_verifier.c have a large number of
405 test cases and are constantly updated with new BPF test sequences, or
406 existing ones are adapted to verifier changes e.g. due to verifier
407 becoming smarter and being able to better track certain things.
409 LLVM:
410 -----
412 Q: Where do I find LLVM with BPF support?
414 A: The BPF back end for LLVM is upstream in LLVM since version 3.7.1.
416 All major distributions these days ship LLVM with BPF back end enabled,
417 so for the majority of use-cases it is not required to compile LLVM by
418 hand anymore, just install the distribution provided package.
420 LLVM's static compiler lists the supported targets through 'llc --version',
421 make sure BPF targets are listed. Example:
423 $ llc --version
424 LLVM (http://llvm.org/):
425 LLVM version 6.0.0svn
426 Optimized build.
427 Default target: x86_64-unknown-linux-gnu
428 Host CPU: skylake
430 Registered Targets:
431 bpf - BPF (host endian)
432 bpfeb - BPF (big endian)
433 bpfel - BPF (little endian)
434 x86 - 32-bit X86: Pentium-Pro and above
435 x86-64 - 64-bit X86: EM64T and AMD64
437 For developers in order to utilize the latest features added to LLVM's
438 BPF back end, it is advisable to run the latest LLVM releases. Support
439 for new BPF kernel features such as additions to the BPF instruction
440 set are often developed together.
442 All LLVM releases can be found at: http://releases.llvm.org/
444 Q: Got it, so how do I build LLVM manually anyway?
446 A: You need cmake and gcc-c++ as build requisites for LLVM. Once you have
447 that set up, proceed with building the latest LLVM and clang version
448 from the git repositories:
450 $ git clone http://llvm.org/git/llvm.git
451 $ cd llvm/tools
452 $ git clone --depth 1 http://llvm.org/git/clang.git
453 $ cd ..; mkdir build; cd build
454 $ cmake .. -DLLVM_TARGETS_TO_BUILD="BPF;X86" \
455 -DBUILD_SHARED_LIBS=OFF \
456 -DCMAKE_BUILD_TYPE=Release \
457 -DLLVM_BUILD_RUNTIME=OFF
458 $ make -j $(getconf _NPROCESSORS_ONLN)
460 The built binaries can then be found in the build/bin/ directory, where
461 you can point the PATH variable to.
463 Q: Should I notify BPF kernel maintainers about issues in LLVM's BPF code
464 generation back end or about LLVM generated code that the verifier
465 refuses to accept?
467 A: Yes, please do! LLVM's BPF back end is a key piece of the whole BPF
468 infrastructure and it ties deeply into verification of programs from the
469 kernel side. Therefore, any issues on either side need to be investigated
470 and fixed whenever necessary.
472 Therefore, please make sure to bring them up at netdev kernel mailing
473 list and Cc BPF maintainers for LLVM and kernel bits:
475 Yonghong Song <yhs@fb.com>
476 Alexei Starovoitov <ast@kernel.org>
477 Daniel Borkmann <daniel@iogearbox.net>
479 LLVM also has an issue tracker where BPF related bugs can be found:
481 https://bugs.llvm.org/buglist.cgi?quicksearch=bpf
483 However, it is better to reach out through mailing lists with having
484 maintainers in Cc.
486 Q: I have added a new BPF instruction to the kernel, how can I integrate
487 it into LLVM?
489 A: LLVM has a -mcpu selector for the BPF back end in order to allow the
490 selection of BPF instruction set extensions. By default the 'generic'
491 processor target is used, which is the base instruction set (v1) of BPF.
493 LLVM has an option to select -mcpu=probe where it will probe the host
494 kernel for supported BPF instruction set extensions and selects the
495 optimal set automatically.
497 For cross-compilation, a specific version can be select manually as well.
499 $ llc -march bpf -mcpu=help
500 Available CPUs for this target:
502 generic - Select the generic processor.
503 probe - Select the probe processor.
504 v1 - Select the v1 processor.
505 v2 - Select the v2 processor.
506 [...]
508 Newly added BPF instructions to the Linux kernel need to follow the same
509 scheme, bump the instruction set version and implement probing for the
510 extensions such that -mcpu=probe users can benefit from the optimization
511 transparently when upgrading their kernels.
513 If you are unable to implement support for the newly added BPF instruction
514 please reach out to BPF developers for help.
516 By the way, the BPF kernel selftests run with -mcpu=probe for better
517 test coverage.
519 Q: In some cases clang flag "-target bpf" is used but in other cases the
520 default clang target, which matches the underlying architecture, is used.
521 What is the difference and when I should use which?
523 A: Although LLVM IR generation and optimization try to stay architecture
524 independent, "-target <arch>" still has some impact on generated code:
526 - BPF program may recursively include header file(s) with file scope
527 inline assembly codes. The default target can handle this well,
528 while bpf target may fail if bpf backend assembler does not
529 understand these assembly codes, which is true in most cases.
531 - When compiled without -g, additional elf sections, e.g.,
532 .eh_frame and .rela.eh_frame, may be present in the object file
533 with default target, but not with bpf target.
535 - The default target may turn a C switch statement into a switch table
536 lookup and jump operation. Since the switch table is placed
537 in the global readonly section, the bpf program will fail to load.
538 The bpf target does not support switch table optimization.
539 The clang option "-fno-jump-tables" can be used to disable
540 switch table generation.
542 You should use default target when:
544 - Your program includes a header file, e.g., ptrace.h, which eventually
545 pulls in some header files containing file scope host assembly codes.
546 - You can add "-fno-jump-tables" to work around the switch table issue.
548 Otherwise, you can use bpf target.
550 Happy BPF hacking!