selftests/bpf: Check consistency between bpftool source, doc, completion

Whenever the eBPF subsystem gains new elements, such as new program or
map types, it is necessary to update bpftool if we want it able to
handle the new items.

In addition to the main arrays containing the names of these elements in
the source code, there are also multiple locations to update:

- The help message in the do_help() functions in bpftool's source code.
- The RST documentation files.
- The bash completion file.

This has led to omissions multiple times in the past. This patch
attempts to address this issue by adding consistency checks for all
these different locations. It also verifies that the bpf_prog_type,
bpf_map_type and bpf_attach_type enums from the UAPI BPF header have all
their members present in bpftool.

The script requires no argument to run, it reads and parses the
different files to check, and prints the mismatches, if any. It
currently reports a number of missing elements, which will be fixed in a
later patch:

  $ ./test_bpftool_synctypes.py
  Comparing [...]/linux/tools/bpf/bpftool/map.c (map_type_name) and [...]/linux/tools/bpf/bpftool/bash-completion/bpftool (BPFTOOL_MAP_CREATE_TYPES): {'ringbuf'}
  Comparing BPF header (enum bpf_attach_type) and [...]/linux/tools/bpf/bpftool/common.c (attach_type_name): {'BPF_TRACE_ITER', 'BPF_XDP_DEVMAP', 'BPF_XDP', 'BPF_SK_REUSEPORT_SELECT', 'BPF_XDP_CPUMAP', 'BPF_SK_REUSEPORT_SELECT_OR_MIGRATE'}
  Comparing [...]/linux/tools/bpf/bpftool/prog.c (attach_type_strings) and [...]/linux/tools/bpf/bpftool/prog.c (do_help() ATTACH_TYPE): {'skb_verdict'}
  Comparing [...]/linux/tools/bpf/bpftool/prog.c (attach_type_strings) and [...]/linux/tools/bpf/bpftool/Documentation/bpftool-prog.rst (ATTACH_TYPE): {'skb_verdict'}
  Comparing [...]/linux/tools/bpf/bpftool/prog.c (attach_type_strings) and [...]/linux/tools/bpf/bpftool/bash-completion/bpftool (BPFTOOL_PROG_ATTACH_TYPES): {'skb_verdict'}

Note that the script does NOT check for consistency between the list of
program types that bpftool claims it accepts and the actual list of
keywords that can be used. This is because bpftool does not "see" them,
they are ELF section names parsed by libbpf. It is not hard to parse the
section_defs[] array in libbpf, but some section names are associated
with program types that bpftool cannot load at the moment. For example,
some programs require a BTF target and an attach target that bpftool
cannot handle. The script may be extended to parse the array and check
only relevant values in the future.

The script is not added to the selftests' Makefile, because doing so
would require all patches with BPF UAPI change to also update bpftool.
Instead it is to be added to the CI.
Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210730215435.7095-3-quentin@isovalent.com