bpftool: cgroup bpf operations

This patchset adds basic cgroup bpf operations to bpftool.

Right now there is no convenient way to perform these operations. The /samples/bpf/load_sock_ops.c implements attach/detacg operations, but only for BPF_CGROUP_SOCK_OPS programs. Bps (part of bcc) implements bpf introspection, but lacks any cgroup-related specific.

I find having a tool to perform these basic operations in the kernel tree very useful, as it can be used in the corresponding bpf documentation without creating additional dependencies. And bpftool seems to be a right tool to extend with such functionality.

v4:
- ATTACH_FLAGS and ATTACH_TYPE are listed and described in docs and usage
- ATTACH_FLAG names converted to "multi" and "override"
- do_attach() recognizes ATTACH_FLAG abbreviations, e.g "mul"
- Local variables sorted ("reverse Christmas tree")
- unknown attach flags value will be never truncated

v3:
- SRC replaced with OBJ in prog load docs
- Output unknown attach type in hex
- License header in SPDX format
- Minor style fixes (e.g. variable reordering)

v2:
- Added prog load operations
- All cgroup operations are looking like bpftool cgroup <command>
- All cgroup-related stuff is moved to a separate file
- Added support for attach flags
- Added support for attaching/detaching programs by id, pinned name, etc
- Changed cgroup detach arguments order
- Added empty json output for succesful programs
- Style fixed: includes order, strncmp and macroses, error handling
- Added man pages

v1:
https://lwn.net/Articles/740366/

Roman Gushchin (4):
libbpf: add ability to guess program type based on section name
libbpf: prefer global symbols as bpf program name source
bpftool: implement prog load command
bpftool: implement cgroup bpf operations

 tools/bpf/bpftool/Documentation/bpftool-cgroup.rst | 118 ++++++++
 tools/bpf/bpftool/Documentation/bpftool-map.rst    |   2 +-
 tools/bpf/bpftool/Documentation/bpftool-prog.rst   |  12 +-
 tools/bpf/bpftool/Documentation/bpftool.rst        |   8 +-
 tools/bpf/bpftool/cgroup.c                         | 307 +++++++++++++++++++++
 tools/bpf/bpftool/common.c                         |  71 ++---
 tools/bpf/bpftool/main.c                           |   3 +-
 tools/bpf/bpftool/main.h                           |   2 +
 tools/bpf/bpftool/prog.c                           |  29 +-
 tools/lib/bpf/libbpf.c                             |  53 ++++
 10 files changed, 566 insertions(+), 39 deletions(-)
 create mode 100644 tools/bpf/bpftool/Documentation/bpftool-cgroup.rst
 create mode 100644 tools/bpf/bpftool/cgroup.c

2.14.3