bpf/libbpf/libbpf_overview.rst

.. SPDX-License-Identifier: GPL-2.0

===============
libbpf Overview
===============

libbpf is a C-based library containing a BPF loader that takes compiled BPF
object files and prepares and loads them into the Linux kernel. libbpf takes the
heavy lifting of loading, verifying, and attaching BPF programs to various
kernel hooks, allowing BPF application developers to focus only on BPF program
correctness and performance.

The following are the high-level features supported by libbpf:

* Provides high-level and low-level APIs for user space programs to interact
  with BPF programs. The low-level APIs wrap all the bpf system call
  functionality, which is useful when users need more fine-grained control
  over the interactions between user space and BPF programs.
* Provides overall support for the BPF object skeleton generated by bpftool.
  The skeleton file simplifies the process for the user space programs to access
  global variables and work with BPF programs.
* Provides BPF-side APIS, including BPF helper definitions, BPF maps support,
  and tracing helpers, allowing developers to simplify BPF code writing.
* Supports BPF CO-RE mechanism, enabling BPF developers to write portable
  BPF programs that can be compiled once and run across different kernel
  versions.

This document will delve into the above concepts in detail, providing a deeper
understanding of the capabilities and advantages of libbpf and how it can help
you develop BPF applications efficiently.

BPF App Lifecycle and libbpf APIs
==================================

A BPF application consists of one or more BPF programs (either cooperating or
completely independent), BPF maps, and global variables. The global
variables are shared between all BPF programs, which allows them to cooperate on
a common set of data. libbpf provides APIs that user space programs can use to
manipulate the BPF programs by triggering different phases of a BPF application
lifecycle.

The following section provides a brief overview of each phase in the BPF life
cycle:

* **Open phase**: In this phase, libbpf parses the BPF
  object file and discovers BPF maps, BPF programs, and global variables. After
  a BPF app is opened, user space apps can make additional adjustments
  (setting BPF program types, if necessary; pre-setting initial values for
  global variables, etc.) before all the entities are created and loaded.

* **Load phase**: In the load phase, libbpf creates BPF
  maps, resolves various relocations, and verifies and loads BPF programs into
  the kernel. At this point, libbpf validates all the parts of a BPF application
  and loads the BPF program into the kernel, but no BPF program has yet been
  executed. After the load phase, it’s possible to set up the initial BPF map
  state without racing with the BPF program code execution.

* **Attachment phase**: In this phase, libbpf
  attaches BPF programs to various BPF hook points (e.g., tracepoints, kprobes,
  cgroup hooks, network packet processing pipeline, etc.). During this
  phase, BPF programs perform useful work such as processing
  packets, or updating BPF maps and global variables that can be read from user
  space.

* **Tear down phase**: In the tear down phase,
  libbpf detaches BPF programs and unloads them from the kernel. BPF maps are
  destroyed, and all the resources used by the BPF app are freed.

BPF Object Skeleton File
========================

BPF skeleton is an alternative interface to libbpf APIs for working with BPF
objects. Skeleton code abstract away generic libbpf APIs to significantly
simplify code for manipulating BPF programs from user space. Skeleton code
includes a bytecode representation of the BPF object file, simplifying the
process of distributing your BPF code. With BPF bytecode embedded, there are no
extra files to deploy along with your application binary.

You can generate the skeleton header file ``(.skel.h)`` for a specific object
file by passing the BPF object to the bpftool. The generated BPF skeleton
provides the following custom functions that correspond to the BPF lifecycle,
each of them prefixed with the specific object name:

* ``<name>__open()`` – creates and opens BPF application (``<name>`` stands for
  the specific bpf object name)
* ``<name>__load()`` – instantiates, loads,and verifies BPF application parts
* ``<name>__attach()`` – attaches all auto-attachable BPF programs (it’s
  optional, you can have more control by using libbpf APIs directly)
* ``<name>__destroy()`` – detaches all BPF programs and
  frees up all used resources

Using the skeleton code is the recommended way to work with bpf programs. Keep
in mind, BPF skeleton provides access to the underlying BPF object, so whatever
was possible to do with generic libbpf APIs is still possible even when the BPF
skeleton is used. It's an additive convenience feature, with no syscalls, and no
cumbersome code.

Other Advantages of Using Skeleton File
---------------------------------------

* BPF skeleton provides an interface for user space programs to work with BPF
  global variables. The skeleton code memory maps global variables as a struct
  into user space. The struct interface allows user space programs to initialize
  BPF programs before the BPF load phase and fetch and update data from user
  space afterward.

* The ``skel.h`` file reflects the object file structure by listing out the
  available maps, programs, etc. BPF skeleton provides direct access to all the
  BPF maps and BPF programs as struct fields. This eliminates the need for
  string-based lookups with ``bpf_object_find_map_by_name()`` and
  ``bpf_object_find_program_by_name()`` APIs, reducing errors due to BPF source
  code and user-space code getting out of sync.

* The embedded bytecode representation of the object file ensures that the
  skeleton and the BPF object file are always in sync.

BPF Helpers
===========

libbpf provides BPF-side APIs that BPF programs can use to interact with the
system. The BPF helpers definition allows developers to use them in BPF code as
any other plain C function. For example, there are helper functions to print
debugging messages, get the time since the system was booted, interact with BPF
maps, manipulate network packets, etc.

For a complete description of what the helpers do, the arguments they take, and
the return value, see the `bpf-helpers
<https://man7.org/linux/man-pages/man7/bpf-helpers.7.html>`_ man page.

BPF CO-RE (Compile Once – Run Everywhere)
=========================================

BPF programs work in the kernel space and have access to kernel memory and data
structures. One limitation that BPF applications come across is the lack of
portability across different kernel versions and configurations. `BCC
<https://github.com/iovisor/bcc/>`_ is one of the solutions for BPF
portability. However, it comes with runtime overhead and a large binary size
from embedding the compiler with the application.

libbpf steps up the BPF program portability by supporting the BPF CO-RE concept.
BPF CO-RE brings together BTF type information, libbpf, and the compiler to
produce a single executable binary that you can run on multiple kernel versions
and configurations.

To make BPF programs portable libbpf relies on the BTF type information of the
running kernel. Kernel also exposes this self-describing authoritative BTF
information through ``sysfs`` at ``/sys/kernel/btf/vmlinux``.

You can generate the BTF information for the running kernel with the following
command:

::

  $ bpftool btf dump file /sys/kernel/btf/vmlinux format c > vmlinux.h

The command generates a ``vmlinux.h`` header file with all kernel types
(:doc:`BTF types <../btf>`) that the running kernel uses. Including
``vmlinux.h`` in your BPF program eliminates dependency on system-wide kernel
headers.

libbpf enables portability of BPF programs by looking at the BPF program’s
recorded BTF type and relocation information and matching them to BTF
information (vmlinux) provided by the running kernel. libbpf then resolves and
matches all the types and fields, and updates necessary offsets and other
relocatable data to ensure that BPF program’s logic functions correctly for a
specific kernel on the host. BPF CO-RE concept thus eliminates overhead
associated with BPF development and allows developers to write portable BPF
applications without modifications and runtime source code compilation on the
target machine.

The following code snippet shows how to read the parent field of a kernel
``task_struct`` using BPF CO-RE and libbf. The basic helper to read a field in a
CO-RE relocatable manner is ``bpf_core_read(dst, sz, src)``, which will read
``sz`` bytes from the field referenced by ``src`` into the memory pointed to by
``dst``.

.. code-block:: C
   :emphasize-lines: 6

    //...
    struct task_struct *task = (void *)bpf_get_current_task();
    struct task_struct *parent_task;
    int err;

    err = bpf_core_read(&parent_task, sizeof(void *), &task->parent);
    if (err) {
      /* handle error */
    }

    /* parent_task contains the value of task->parent pointer */

In the code snippet, we first get a pointer to the current ``task_struct`` using
``bpf_get_current_task()``.  We then use ``bpf_core_read()`` to read the parent
field of task struct into the ``parent_task`` variable. ``bpf_core_read()`` is
just like ``bpf_probe_read_kernel()`` BPF helper, except it records information
about the field that should be relocated on the target kernel. i.e, if the
``parent`` field gets shifted to a different offset within
``struct task_struct`` due to some new field added in front of it, libbpf will
automatically adjust the actual offset to the proper value.

Getting Started with libbpf
===========================

Check out the `libbpf-bootstrap <https://github.com/libbpf/libbpf-bootstrap>`_
repository with simple examples of using libbpf to build various BPF
applications.

See also `libbpf API documentation
<https://libbpf.readthedocs.io/en/latest/api.html>`_.

libbpf and Rust
===============

If you are building BPF applications in Rust, it is recommended to use the
`Libbpf-rs <https://github.com/libbpf/libbpf-rs>`_ library instead of bindgen
bindings directly to libbpf. Libbpf-rs wraps libbpf functionality in
Rust-idiomatic interfaces and provides libbpf-cargo plugin to handle BPF code
compilation and skeleton generation. Using Libbpf-rs will make building user
space part of the BPF application easier. Note that the BPF program themselves
must still be written in plain C.

libbpf logging
==============

By default, libbpf logs informational and warning messages to stderr. The
verbosity of these messages can be controlled by setting the environment
variable LIBBPF_LOG_LEVEL to either warn, info, or debug. A custom log
callback can be set using ``libbpf_set_print()``.

Additional Documentation
========================

* `Program types and ELF Sections <https://libbpf.readthedocs.io/en/latest/program_types.html>`_
* `API naming convention <https://libbpf.readthedocs.io/en/latest/libbpf_naming_convention.html>`_
* `Building libbpf <https://libbpf.readthedocs.io/en/latest/libbpf_build.html>`_
* `API documentation Convention <https://libbpf.readthedocs.io/en/latest/libbpf_naming_convention.html#api-documentation-convention>`_