[bpf-next,0/9] libbpf ksym support and bpftool show PIDs

This patch set implements libbpf support for a second kind of special externs,
kernel symbols, in addition to existing Kconfig externs. 

Right now, only untyped (const void) externs are supported, which, in
C language, allow only to take their address. In the future, with kernel BTF
getting type info about its own global and per-cpu variables, libbpf will
extend this support with BTF type info, which will allow to also directly
access variable's contents and follow its internal pointers, similarly to how
it's possible today in fentry/fexit programs.

As a first practical use of this functionality, bpftool gained ability to show
PIDs of processes that have open file descriptors for BPF map/program/link/BTF
object. It relies on iter/task_file BPF iterator program to extract this
information efficiently.

There was a bunch of bpftool refactoring (especially Makefile) necessary to
generalize bpftool's internal BPF program use. This includes generalization of
BPF skeletons support, addition of a vmlinux.h generation, extracting and
building minimal subset of bpftool for bootstrapping.

rfc -> v1:
- show pids, if supported by kernel, always (Alexei);
- switched iter output to binary to support showing process names;
- update man pages;
- fix few minor bugs in libbpf w.r.t. extern iteration.

Cc: Hao Luo <haoluo@google.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Quentin Monnet <quentin@isovalent.com>

Andrii Nakryiko (9):
  libbpf: generalize libbpf externs support
  libbpf: add support for extracting kernel symbol addresses
  selftests/bpf: add __ksym extern selftest
  tools/bpftool: move map/prog parsing logic into common
  tools/bpftool: minimize bootstrap bpftool
  tools/bpftool: generalize BPF skeleton support and generate vmlinux.h
  libbpf: wrap source argument of BPF_CORE_READ macro in parentheses
  tools/bpftool: show info for processes holding BPF map/prog/link/btf
    FDs
  tools/bpftool: add documentation and sample output for process info

 tools/bpf/bpftool/.gitignore                  |   5 +-
 .../bpf/bpftool/Documentation/bpftool-btf.rst |   5 +
 .../bpftool/Documentation/bpftool-link.rst    |  13 +-
 .../bpf/bpftool/Documentation/bpftool-map.rst |   8 +-
 .../bpftool/Documentation/bpftool-prog.rst    |  11 +
 tools/bpf/bpftool/Makefile                    |  60 ++-
 tools/bpf/bpftool/btf.c                       |   6 +
 tools/bpf/bpftool/common.c                    | 308 +++++++++++
 tools/bpf/bpftool/link.c                      |   7 +
 tools/bpf/bpftool/main.c                      |  12 +-
 tools/bpf/bpftool/main.h                      |  56 +-
 tools/bpf/bpftool/map.c                       | 163 +-----
 tools/bpf/bpftool/pids.c                      | 229 +++++++++
 tools/bpf/bpftool/prog.c                      | 159 +-----
 tools/bpf/bpftool/skeleton/pid_iter.bpf.c     |  80 +++
 tools/bpf/bpftool/skeleton/pid_iter.h         |  12 +
 tools/bpf/bpftool/skeleton/profiler.bpf.c     |   3 +-
 tools/bpf/bpftool/skeleton/profiler.h         |  46 --
 tools/build/feature/Makefile                  |   4 +-
 tools/build/feature/test-clang-bpf-co-re.c    |   9 +
 .../build/feature/test-clang-bpf-global-var.c |   4 -
 tools/lib/bpf/bpf_core_read.h                 |   8 +-
 tools/lib/bpf/bpf_helpers.h                   |   1 +
 tools/lib/bpf/btf.h                           |   5 +
 tools/lib/bpf/libbpf.c                        | 484 ++++++++++++------
 .../testing/selftests/bpf/prog_tests/ksyms.c  |  71 +++
 .../testing/selftests/bpf/progs/test_ksyms.c  |  32 ++
 27 files changed, 1253 insertions(+), 548 deletions(-)
 create mode 100644 tools/bpf/bpftool/pids.c
 create mode 100644 tools/bpf/bpftool/skeleton/pid_iter.bpf.c
 create mode 100644 tools/bpf/bpftool/skeleton/pid_iter.h
 delete mode 100644 tools/bpf/bpftool/skeleton/profiler.h
 create mode 100644 tools/build/feature/test-clang-bpf-co-re.c
 delete mode 100644 tools/build/feature/test-clang-bpf-global-var.c
 create mode 100644 tools/testing/selftests/bpf/prog_tests/ksyms.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_ksyms.c