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diff --git a/tools/lib/bpf/README.rst b/tools/lib/bpf/README.rst new file mode 100644 index 000000000..8928f7787 --- /dev/null +++ b/tools/lib/bpf/README.rst @@ -0,0 +1,168 @@ +.. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) + +libbpf API naming convention +============================ + +libbpf API provides access to a few logically separated groups of +functions and types. Every group has its own naming convention +described here. It's recommended to follow these conventions whenever a +new function or type is added to keep libbpf API clean and consistent. + +All types and functions provided by libbpf API should have one of the +following prefixes: ``bpf_``, ``btf_``, ``libbpf_``, ``xsk_``, +``perf_buffer_``. + +System call wrappers +-------------------- + +System call wrappers are simple wrappers for commands supported by +sys_bpf system call. These wrappers should go to ``bpf.h`` header file +and map one-on-one to corresponding commands. + +For example ``bpf_map_lookup_elem`` wraps ``BPF_MAP_LOOKUP_ELEM`` +command of sys_bpf, ``bpf_prog_attach`` wraps ``BPF_PROG_ATTACH``, etc. + +Objects +------- + +Another class of types and functions provided by libbpf API is "objects" +and functions to work with them. Objects are high-level abstractions +such as BPF program or BPF map. They're represented by corresponding +structures such as ``struct bpf_object``, ``struct bpf_program``, +``struct bpf_map``, etc. + +Structures are forward declared and access to their fields should be +provided via corresponding getters and setters rather than directly. + +These objects are associated with corresponding parts of ELF object that +contains compiled BPF programs. + +For example ``struct bpf_object`` represents ELF object itself created +from an ELF file or from a buffer, ``struct bpf_program`` represents a +program in ELF object and ``struct bpf_map`` is a map. + +Functions that work with an object have names built from object name, +double underscore and part that describes function purpose. + +For example ``bpf_object__open`` consists of the name of corresponding +object, ``bpf_object``, double underscore and ``open`` that defines the +purpose of the function to open ELF file and create ``bpf_object`` from +it. + +Another example: ``bpf_program__load`` is named for corresponding +object, ``bpf_program``, that is separated from other part of the name +by double underscore. + +All objects and corresponding functions other than BTF related should go +to ``libbpf.h``. BTF types and functions should go to ``btf.h``. + +Auxiliary functions +------------------- + +Auxiliary functions and types that don't fit well in any of categories +described above should have ``libbpf_`` prefix, e.g. +``libbpf_get_error`` or ``libbpf_prog_type_by_name``. + +AF_XDP functions +------------------- + +AF_XDP functions should have an ``xsk_`` prefix, e.g. +``xsk_umem__get_data`` or ``xsk_umem__create``. The interface consists +of both low-level ring access functions and high-level configuration +functions. These can be mixed and matched. Note that these functions +are not reentrant for performance reasons. + +Please take a look at Documentation/networking/af_xdp.rst in the Linux +kernel source tree on how to use XDP sockets and for some common +mistakes in case you do not get any traffic up to user space. + +libbpf ABI +========== + +libbpf can be both linked statically or used as DSO. To avoid possible +conflicts with other libraries an application is linked with, all +non-static libbpf symbols should have one of the prefixes mentioned in +API documentation above. See API naming convention to choose the right +name for a new symbol. + +Symbol visibility +----------------- + +libbpf follow the model when all global symbols have visibility "hidden" +by default and to make a symbol visible it has to be explicitly +attributed with ``LIBBPF_API`` macro. For example: + +.. code-block:: c + + LIBBPF_API int bpf_prog_get_fd_by_id(__u32 id); + +This prevents from accidentally exporting a symbol, that is not supposed +to be a part of ABI what, in turn, improves both libbpf developer- and +user-experiences. + +ABI versionning +--------------- + +To make future ABI extensions possible libbpf ABI is versioned. +Versioning is implemented by ``libbpf.map`` version script that is +passed to linker. + +Version name is ``LIBBPF_`` prefix + three-component numeric version, +starting from ``0.0.1``. + +Every time ABI is being changed, e.g. because a new symbol is added or +semantic of existing symbol is changed, ABI version should be bumped. +This bump in ABI version is at most once per kernel development cycle. + +For example, if current state of ``libbpf.map`` is: + +.. code-block:: + LIBBPF_0.0.1 { + global: + bpf_func_a; + bpf_func_b; + local: + \*; + }; + +, and a new symbol ``bpf_func_c`` is being introduced, then +``libbpf.map`` should be changed like this: + +.. code-block:: + LIBBPF_0.0.1 { + global: + bpf_func_a; + bpf_func_b; + local: + \*; + }; + LIBBPF_0.0.2 { + global: + bpf_func_c; + } LIBBPF_0.0.1; + +, where new version ``LIBBPF_0.0.2`` depends on the previous +``LIBBPF_0.0.1``. + +Format of version script and ways to handle ABI changes, including +incompatible ones, described in details in [1]. + +Stand-alone build +================= + +Under https://github.com/libbpf/libbpf there is a (semi-)automated +mirror of the mainline's version of libbpf for a stand-alone build. + +However, all changes to libbpf's code base must be upstreamed through +the mainline kernel tree. + +License +======= + +libbpf is dual-licensed under LGPL 2.1 and BSD 2-Clause. + +Links +===== + +[1] https://www.akkadia.org/drepper/dsohowto.pdf + (Chapter 3. Maintaining APIs and ABIs). |