xref: /openbmc/linux/Documentation/bpf/bpf_devel_QA.rst (revision 979ac5ef)

=================================
HOWTO interact with BPF subsystem
=================================

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

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

.. contents::
    :local:
    :depth: 2

Reporting bugs
==============

Q: How do I report bugs for BPF kernel code?
--------------------------------------------
A: Since all BPF kernel development as well as bpftool and iproute2 BPF
loader development happens through the bpf kernel mailing list,
please report any found issues around BPF to the following mailing
list:

 bpf@vger.kernel.org

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

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

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

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

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

Submitting patches
==================

Q: How do I run BPF CI on my changes before sending them out for review?
------------------------------------------------------------------------
A: BPF CI is GitHub based and hosted at https://github.com/kernel-patches/bpf.
While GitHub also provides a CLI that can be used to accomplish the same
results, here we focus on the UI based workflow.

The following steps lay out how to start a CI run for your patches:

- Create a fork of the aforementioned repository in your own account (one time
  action)

- Clone the fork locally, check out a new branch tracking either the bpf-next
  or bpf branch, and apply your to-be-tested patches on top of it

- Push the local branch to your fork and create a pull request against
  kernel-patches/bpf's bpf-next_base or bpf_base branch, respectively

Shortly after the pull request has been created, the CI workflow will run. Note
that capacity is shared with patches submitted upstream being checked and so
depending on utilization the run can take a while to finish.

Note furthermore that both base branches (bpf-next_base and bpf_base) will be
updated as patches are pushed to the respective upstream branches they track. As
such, your patch set will automatically (be attempted to) be rebased as well.
This behavior can result in a CI run being aborted and restarted with the new
base line.

Q: To which mailing list do I need to submit my BPF patches?
------------------------------------------------------------
A: Please submit your BPF patches to the bpf kernel mailing list:

 bpf@vger.kernel.org

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

Q: Where can I find patches currently under discussion for BPF subsystem?
-------------------------------------------------------------------------
A: All patches that are Cc'ed to netdev are queued for review under netdev
patchwork project:

  https://patchwork.kernel.org/project/netdevbpf/list/

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

  https://patchwork.kernel.org/project/netdevbpf/list/?delegate=121173

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

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

Q: How do the changes make their way into Linux?
------------------------------------------------
A: There are two BPF kernel trees (git repositories). Once patches have
been accepted by the BPF maintainers, they will be applied to one
of the two BPF trees:

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

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

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



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

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

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

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

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

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

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

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

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

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

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

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

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

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

Q: What does it mean when a patch gets applied to bpf or bpf-next tree?
-----------------------------------------------------------------------
A: It means that the patch looks good for mainline inclusion from
a BPF point of view.

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

On the bpf kernel mailing list reviews can come in at any point
in time. If discussions around a patch conclude that they cannot
get included as-is, we will either apply a follow-up fix or drop
them from the trees entirely. Therefore, we also reserve to rebase
the trees when deemed necessary. After all, the purpose of the tree
is to:

i) accumulate and stage BPF patches for integration into trees
   like net and net-next, and

ii) run extensive BPF test suite and
    workloads on the patches before they make their way any further.

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

Q: How long do I need to wait for feedback on my BPF patches?
-------------------------------------------------------------
A: We try to keep the latency low. The usual time to feedback will
be around 2 or 3 business days. It may vary depending on the
complexity of changes and current patch load.

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

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

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

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

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

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

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

Q: Verifier changes and test cases
----------------------------------
Q: I made a BPF verifier change, do I need to add test cases for
BPF kernel selftests_?

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

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

Q: samples/bpf preference vs selftests?
---------------------------------------
Q: When should I add code to ``samples/bpf/`` and when to BPF kernel
selftests_?

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

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

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

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

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

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

Q: Do you accept patches as well for iproute2's BPF loader?
-----------------------------------------------------------
A: Patches for the iproute2's BPF loader have to be sent to:

  netdev@vger.kernel.org

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Stable submission
=================

Q: I need a specific BPF commit in stable kernels. What should I do?
--------------------------------------------------------------------
A: In case you need a specific fix in stable kernels, first check whether
the commit has already been applied in the related ``linux-*.y`` branches:

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

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

  netdev@vger.kernel.org

The process in general is the same as on netdev itself, see also the
:ref:`netdev-FAQ`.

Q: Do you also backport to kernels not currently maintained as stable?
----------------------------------------------------------------------
A: No. If you need a specific BPF commit in kernels that are currently not
maintained by the stable maintainers, then you are on your own.

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

  https://www.kernel.org/

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

A: The same rules apply as with netdev patch submissions in general, see
the :ref:`netdev-FAQ`.

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

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

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

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

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

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

Testing patches
===============

Q: How to run BPF selftests
---------------------------
A: After you have booted into the newly compiled kernel, navigate to
the BPF selftests_ suite in order to test BPF functionality (current
working directory points to the root of the cloned git tree)::

  $ cd tools/testing/selftests/bpf/
  $ make

To run the verifier tests::

  $ sudo ./test_verifier

The verifier tests print out all the current checks being
performed. The summary at the end of running all tests will dump
information of test successes and failures::

  Summary: 418 PASSED, 0 FAILED

In order to run through all BPF selftests, the following command is
needed::

  $ sudo make run_tests

See the kernels selftest `Documentation/dev-tools/kselftest.rst`_
document for further documentation.

To maximize the number of tests passing, the .config of the kernel
under test should match the config file fragment in
tools/testing/selftests/bpf as closely as possible.

Finally to ensure support for latest BPF Type Format features -
discussed in `Documentation/bpf/btf.rst`_ - pahole version 1.16
is required for kernels built with CONFIG_DEBUG_INFO_BTF=y.
pahole is delivered in the dwarves package or can be built
from source at

https://github.com/acmel/dwarves

pahole starts to use libbpf definitions and APIs since v1.13 after the
commit 21507cd3e97b ("pahole: add libbpf as submodule under lib/bpf").
It works well with the git repository because the libbpf submodule will
use "git submodule update --init --recursive" to update.

Unfortunately, the default github release source code does not contain
libbpf submodule source code and this will cause build issues, the tarball
from https://git.kernel.org/pub/scm/devel/pahole/pahole.git/ is same with
github, you can get the source tarball with corresponding libbpf submodule
codes from

https://fedorapeople.org/~acme/dwarves

Some distros have pahole version 1.16 packaged already, e.g.
Fedora, Gentoo.

Q: Which BPF kernel selftests version should I run my kernel against?
---------------------------------------------------------------------
A: If you run a kernel ``xyz``, then always run the BPF kernel selftests
from that kernel ``xyz`` as well. Do not expect that the BPF selftest
from the latest mainline tree will pass all the time.

In particular, test_bpf.c and test_verifier.c have a large number of
test cases and are constantly updated with new BPF test sequences, or
existing ones are adapted to verifier changes e.g. due to verifier
becoming smarter and being able to better track certain things.

LLVM
====

Q: Where do I find LLVM with BPF support?
-----------------------------------------
A: The BPF back end for LLVM is upstream in LLVM since version 3.7.1.

All major distributions these days ship LLVM with BPF back end enabled,
so for the majority of use-cases it is not required to compile LLVM by
hand anymore, just install the distribution provided package.

LLVM's static compiler lists the supported targets through
``llc --version``, make sure BPF targets are listed. Example::

     $ llc --version
     LLVM (http://llvm.org/):
       LLVM version 10.0.0
       Optimized build.
       Default target: x86_64-unknown-linux-gnu
       Host CPU: skylake

       Registered Targets:
         aarch64    - AArch64 (little endian)
         bpf        - BPF (host endian)
         bpfeb      - BPF (big endian)
         bpfel      - BPF (little endian)
         x86        - 32-bit X86: Pentium-Pro and above
         x86-64     - 64-bit X86: EM64T and AMD64

For developers in order to utilize the latest features added to LLVM's
BPF back end, it is advisable to run the latest LLVM releases. Support
for new BPF kernel features such as additions to the BPF instruction
set are often developed together.

All LLVM releases can be found at: http://releases.llvm.org/

Q: Got it, so how do I build LLVM manually anyway?
--------------------------------------------------
A: We recommend that developers who want the fastest incremental builds
use the Ninja build system, you can find it in your system's package
manager, usually the package is ninja or ninja-build.

You need ninja, cmake and gcc-c++ as build requisites for LLVM. Once you
have that set up, proceed with building the latest LLVM and clang version
from the git repositories::

     $ git clone https://github.com/llvm/llvm-project.git
     $ mkdir -p llvm-project/llvm/build
     $ cd llvm-project/llvm/build
     $ cmake .. -G "Ninja" -DLLVM_TARGETS_TO_BUILD="BPF;X86" \
                -DLLVM_ENABLE_PROJECTS="clang"    \
                -DCMAKE_BUILD_TYPE=Release        \
                -DLLVM_BUILD_RUNTIME=OFF
     $ ninja

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

Set ``-DLLVM_TARGETS_TO_BUILD`` equal to the target you wish to build, you
will find a full list of targets within the llvm-project/llvm/lib/Target
directory.

Q: Reporting LLVM BPF issues
----------------------------
Q: Should I notify BPF kernel maintainers about issues in LLVM's BPF code
generation back end or about LLVM generated code that the verifier
refuses to accept?

A: Yes, please do!

LLVM's BPF back end is a key piece of the whole BPF
infrastructure and it ties deeply into verification of programs from the
kernel side. Therefore, any issues on either side need to be investigated
and fixed whenever necessary.

Therefore, please make sure to bring them up at netdev kernel mailing
list and Cc BPF maintainers for LLVM and kernel bits:

* Yonghong Song <yhs@fb.com>
* Alexei Starovoitov <ast@kernel.org>
* Daniel Borkmann <daniel@iogearbox.net>

LLVM also has an issue tracker where BPF related bugs can be found:

  https://bugs.llvm.org/buglist.cgi?quicksearch=bpf

However, it is better to reach out through mailing lists with having
maintainers in Cc.

Q: New BPF instruction for kernel and LLVM
------------------------------------------
Q: I have added a new BPF instruction to the kernel, how can I integrate
it into LLVM?

A: LLVM has a ``-mcpu`` selector for the BPF back end in order to allow
the selection of BPF instruction set extensions. By default the
``generic`` processor target is used, which is the base instruction set
(v1) of BPF.

LLVM has an option to select ``-mcpu=probe`` where it will probe the host
kernel for supported BPF instruction set extensions and selects the
optimal set automatically.

For cross-compilation, a specific version can be select manually as well ::

     $ llc -march bpf -mcpu=help
     Available CPUs for this target:

       generic - Select the generic processor.
       probe   - Select the probe processor.
       v1      - Select the v1 processor.
       v2      - Select the v2 processor.
     [...]

Newly added BPF instructions to the Linux kernel need to follow the same
scheme, bump the instruction set version and implement probing for the
extensions such that ``-mcpu=probe`` users can benefit from the
optimization transparently when upgrading their kernels.

If you are unable to implement support for the newly added BPF instruction
please reach out to BPF developers for help.

By the way, the BPF kernel selftests run with ``-mcpu=probe`` for better
test coverage.

Q: clang flag for target bpf?
-----------------------------
Q: In some cases clang flag ``-target bpf`` is used but in other cases the
default clang target, which matches the underlying architecture, is used.
What is the difference and when I should use which?

A: Although LLVM IR generation and optimization try to stay architecture
independent, ``-target <arch>`` still has some impact on generated code:

- BPF program may recursively include header file(s) with file scope
  inline assembly codes. The default target can handle this well,
  while ``bpf`` target may fail if bpf backend assembler does not
  understand these assembly codes, which is true in most cases.

- When compiled without ``-g``, additional elf sections, e.g.,
  .eh_frame and .rela.eh_frame, may be present in the object file
  with default target, but not with ``bpf`` target.

- The default target may turn a C switch statement into a switch table
  lookup and jump operation. Since the switch table is placed
  in the global readonly section, the bpf program will fail to load.
  The bpf target does not support switch table optimization.
  The clang option ``-fno-jump-tables`` can be used to disable
  switch table generation.

- For clang ``-target bpf``, it is guaranteed that pointer or long /
  unsigned long types will always have a width of 64 bit, no matter
  whether underlying clang binary or default target (or kernel) is
  32 bit. However, when native clang target is used, then it will
  compile these types based on the underlying architecture's conventions,
  meaning in case of 32 bit architecture, pointer or long / unsigned
  long types e.g. in BPF context structure will have width of 32 bit
  while the BPF LLVM back end still operates in 64 bit. The native
  target is mostly needed in tracing for the case of walking ``pt_regs``
  or other kernel structures where CPU's register width matters.
  Otherwise, ``clang -target bpf`` is generally recommended.

You should use default target when:

- Your program includes a header file, e.g., ptrace.h, which eventually
  pulls in some header files containing file scope host assembly codes.

- You can add ``-fno-jump-tables`` to work around the switch table issue.

Otherwise, you can use ``bpf`` target. Additionally, you *must* use bpf target
when:

- Your program uses data structures with pointer or long / unsigned long
  types that interface with BPF helpers or context data structures. Access
  into these structures is verified by the BPF verifier and may result
  in verification failures if the native architecture is not aligned with
  the BPF architecture, e.g. 64-bit. An example of this is
  BPF_PROG_TYPE_SK_MSG require ``-target bpf``


.. Links
.. _Documentation/process/: https://www.kernel.org/doc/html/latest/process/
.. _netdev-FAQ: Documentation/process/maintainer-netdev.rst
.. _selftests:
   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/bpf/
.. _Documentation/dev-tools/kselftest.rst:
   https://www.kernel.org/doc/html/latest/dev-tools/kselftest.html
.. _Documentation/bpf/btf.rst: btf.rst

Happy BPF hacking!