libbpf: Document API and ABI conventions

Document API and ABI for libbpf: naming convention, symbol visibility,
ABI versioning.

This is just a starting point. Documentation can be significantly
extended in the future to cover more topics.

ABI versioning section touches only a few basic points with a link to
more comprehensive documentation from Ulrich Drepper. This section can
be extended in the future when there is better understanding what works
well and what not so well in libbpf development process and production
usage.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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+.. SPDX-License-Identifier: GPL-2.0
+
+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_``.
+
+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``.
+
+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.
+
+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].
+
+Links
+=====
+
+[1] https://www.akkadia.org/drepper/dsohowto.pdf
+    (Chapter 3. Maintaining APIs and ABIs).