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! 696