bpf_link observability APIs

This patch series adds various observability APIs to bpf_link:
  - each bpf_link now gets ID, similar to bpf_map and bpf_prog, by which
    user-space can iterate over all existing bpf_links and create limited FD
    from ID;
  - allows to get extra object information with bpf_link general and
    type-specific information;
  - makes LINK_UPDATE operation allowed only for writable bpf_links and allows
    to pin bpf_link as read-only file;
  - implements `bpf link show` command which lists all active bpf_links in the
    system;
  - implements `bpf link pin` allowing to pin bpf_link by ID or from other
    pinned path.

This RFC series is missing selftests and only limited amount of manual testing
was performed. But kernel implementation is hopefully in a good shape and
won't change much (unless some big issues are identified with the current
approach). It would be great to get feedback on approach and implementation,
before I invest more time in writing tests.

Andrii Nakryiko (8):
  bpf: refactor bpf_link update handling
  bpf: allow bpf_link pinning as read-only and enforce LINK_UPDATE
  bpf: allocate ID for bpf_link
  bpf: support GET_FD_BY_ID and GET_NEXT_ID for bpf_link
  bpf: add support for BPF_OBJ_GET_INFO_BY_FD for bpf_link
  libbpf: add low-level APIs for new bpf_link commands
  bpftool: expose attach_type-to-string array to non-cgroup code
  bpftool: add bpf_link show and pin support

 include/linux/bpf-cgroup.h     |  14 --
 include/linux/bpf.h            |  34 ++-
 include/linux/bpf_types.h      |   6 +
 include/uapi/linux/bpf.h       |  31 +++
 kernel/bpf/btf.c               |   2 +
 kernel/bpf/cgroup.c            |  89 +++++++-
 kernel/bpf/inode.c             |  30 ++-
 kernel/bpf/syscall.c           | 387 +++++++++++++++++++++++++------
 kernel/bpf/verifier.c          |   2 +
 kernel/cgroup/cgroup.c         |  27 ---
 tools/bpf/bpftool/cgroup.c     |  28 +--
 tools/bpf/bpftool/link.c       | 403 +++++++++++++++++++++++++++++++++
 tools/bpf/bpftool/main.c       |   2 +
 tools/bpf/bpftool/main.h       |  37 +++
 tools/include/uapi/linux/bpf.h |  31 +++
 tools/lib/bpf/bpf.c            |  19 +-
 tools/lib/bpf/bpf.h            |   4 +-
 tools/lib/bpf/libbpf.map       |   6 +
 18 files changed, 983 insertions(+), 169 deletions(-)
 create mode 100644 tools/bpf/bpftool/link.c

-- 
2.24.1