203 lines
6.9 KiB
ReStructuredText
203 lines
6.9 KiB
ReStructuredText
.. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
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API naming convention
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=====================
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libbpf API provides access to a few logically separated groups of
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functions and types. Every group has its own naming convention
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described here. It's recommended to follow these conventions whenever a
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new function or type is added to keep libbpf API clean and consistent.
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All types and functions provided by libbpf API should have one of the
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following prefixes: ``bpf_``, ``btf_``, ``libbpf_``, ``xsk_``,
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``btf_dump_``, ``ring_buffer_``, ``perf_buffer_``.
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System call wrappers
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--------------------
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System call wrappers are simple wrappers for commands supported by
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sys_bpf system call. These wrappers should go to ``bpf.h`` header file
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and map one to one to corresponding commands.
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For example ``bpf_map_lookup_elem`` wraps ``BPF_MAP_LOOKUP_ELEM``
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command of sys_bpf, ``bpf_prog_attach`` wraps ``BPF_PROG_ATTACH``, etc.
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Objects
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-------
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Another class of types and functions provided by libbpf API is "objects"
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and functions to work with them. Objects are high-level abstractions
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such as BPF program or BPF map. They're represented by corresponding
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structures such as ``struct bpf_object``, ``struct bpf_program``,
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``struct bpf_map``, etc.
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Structures are forward declared and access to their fields should be
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provided via corresponding getters and setters rather than directly.
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These objects are associated with corresponding parts of ELF object that
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contains compiled BPF programs.
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For example ``struct bpf_object`` represents ELF object itself created
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from an ELF file or from a buffer, ``struct bpf_program`` represents a
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program in ELF object and ``struct bpf_map`` is a map.
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Functions that work with an object have names built from object name,
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double underscore and part that describes function purpose.
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For example ``bpf_object__open`` consists of the name of corresponding
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object, ``bpf_object``, double underscore and ``open`` that defines the
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purpose of the function to open ELF file and create ``bpf_object`` from
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it.
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All objects and corresponding functions other than BTF related should go
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to ``libbpf.h``. BTF types and functions should go to ``btf.h``.
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Auxiliary functions
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-------------------
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Auxiliary functions and types that don't fit well in any of categories
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described above should have ``libbpf_`` prefix, e.g.
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``libbpf_get_error`` or ``libbpf_prog_type_by_name``.
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AF_XDP functions
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-------------------
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AF_XDP functions should have an ``xsk_`` prefix, e.g.
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``xsk_umem__get_data`` or ``xsk_umem__create``. The interface consists
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of both low-level ring access functions and high-level configuration
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functions. These can be mixed and matched. Note that these functions
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are not reentrant for performance reasons.
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ABI
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---
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libbpf can be both linked statically or used as DSO. To avoid possible
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conflicts with other libraries an application is linked with, all
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non-static libbpf symbols should have one of the prefixes mentioned in
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API documentation above. See API naming convention to choose the right
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name for a new symbol.
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Symbol visibility
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-----------------
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libbpf follow the model when all global symbols have visibility "hidden"
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by default and to make a symbol visible it has to be explicitly
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attributed with ``LIBBPF_API`` macro. For example:
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.. code-block:: c
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LIBBPF_API int bpf_prog_get_fd_by_id(__u32 id);
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This prevents from accidentally exporting a symbol, that is not supposed
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to be a part of ABI what, in turn, improves both libbpf developer- and
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user-experiences.
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ABI versionning
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---------------
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To make future ABI extensions possible libbpf ABI is versioned.
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Versioning is implemented by ``libbpf.map`` version script that is
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passed to linker.
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Version name is ``LIBBPF_`` prefix + three-component numeric version,
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starting from ``0.0.1``.
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Every time ABI is being changed, e.g. because a new symbol is added or
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semantic of existing symbol is changed, ABI version should be bumped.
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This bump in ABI version is at most once per kernel development cycle.
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For example, if current state of ``libbpf.map`` is:
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.. code-block:: none
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LIBBPF_0.0.1 {
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global:
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bpf_func_a;
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bpf_func_b;
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local:
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\*;
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};
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, and a new symbol ``bpf_func_c`` is being introduced, then
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``libbpf.map`` should be changed like this:
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.. code-block:: none
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LIBBPF_0.0.1 {
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global:
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bpf_func_a;
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bpf_func_b;
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local:
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\*;
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};
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LIBBPF_0.0.2 {
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global:
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bpf_func_c;
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} LIBBPF_0.0.1;
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, where new version ``LIBBPF_0.0.2`` depends on the previous
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``LIBBPF_0.0.1``.
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Format of version script and ways to handle ABI changes, including
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incompatible ones, described in details in [1].
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Stand-alone build
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-------------------
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Under https://github.com/libbpf/libbpf there is a (semi-)automated
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mirror of the mainline's version of libbpf for a stand-alone build.
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However, all changes to libbpf's code base must be upstreamed through
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the mainline kernel tree.
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API documentation convention
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============================
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The libbpf API is documented via comments above definitions in
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header files. These comments can be rendered by doxygen and sphinx
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for well organized html output. This section describes the
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convention in which these comments should be formated.
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Here is an example from btf.h:
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.. code-block:: c
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/**
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* @brief **btf__new()** creates a new instance of a BTF object from the raw
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* bytes of an ELF's BTF section
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* @param data raw bytes
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* @param size number of bytes passed in `data`
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* @return new BTF object instance which has to be eventually freed with
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* **btf__free()**
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*
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* On error, error-code-encoded-as-pointer is returned, not a NULL. To extract
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* error code from such a pointer `libbpf_get_error()` should be used. If
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* `libbpf_set_strict_mode(LIBBPF_STRICT_CLEAN_PTRS)` is enabled, NULL is
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* returned on error instead. In both cases thread-local `errno` variable is
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* always set to error code as well.
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*/
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The comment must start with a block comment of the form '/\*\*'.
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The documentation always starts with a @brief directive. This line is a short
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description about this API. It starts with the name of the API, denoted in bold
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like so: **api_name**. Please include an open and close parenthesis if this is a
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function. Follow with the short description of the API. A longer form description
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can be added below the last directive, at the bottom of the comment.
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Parameters are denoted with the @param directive, there should be one for each
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parameter. If this is a function with a non-void return, use the @return directive
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to document it.
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License
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-------------------
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libbpf is dual-licensed under LGPL 2.1 and BSD 2-Clause.
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Links
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-------------------
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[1] https://www.akkadia.org/drepper/dsohowto.pdf
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(Chapter 3. Maintaining APIs and ABIs).
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