[bpf-next,v3,0/8] Allow storage of flexible metadata information for eBPF programs

Currently, if a user wants to store arbitrary metadata for an eBPF
program, for example, the program build commit hash or version, they
could store it in a map, and conveniently libbpf uses .data section to
populate an internal map. However, if the program does not actually
reference the map, then the map would be de-refcounted and freed.

This patch set introduces a new syscall BPF_PROG_BIND_MAP to add a map
to a program's used_maps, even if the program instructions does not
reference the map. libbpf is extended to recognize the .metadata section
and load it as an internal map, and use the new syscall to ensure the
map is bound. bpftool is also extended to have a new flag to prog
subcommand, "--metadata" to dump the contents of the metadata section
without a separate map dump call.

An example use of this would be BPF C file declaring:

  char commit_hash[] SEC(".metadata") = "abcdef123456";

and bpftool would emit:

  $ bpftool prog --metadata
  [...]
        metadata:
                commit_hash = "abcdef123456"

Patch 1 protects the used_maps array and count with a mutex.

Patch 2 implements the new syscall.

Patch 3 extends libbpf to have a wrapper around the syscall, probe the
kernel for support of this new syscall, and use it on .metadata section
if supported and the section exists.

Patch 4 extends bpftool so that it is able to dump metadata from prog
show.

Patch 5 extends bpftool gen skeleton to treat the metadata section like
an rodata section so that it mmaps the map read-only at load time.

Patch 6 adds a test to check the metadata loading and dumping.

Changes since RFC:
* Fixed a few missing unlocks, and missing close while iterating map fds.
* Move mutex initialization to right after prog aux allocation, and mutex
  destroy to right after prog aux free.
* s/ADD_MAP/BIND_MAP/
* Use mutex only instead of RCU to protect the used_map array & count.

Changes since v1:
* Made struct bpf_prog_bind_opts in libbpf so flags is optional.
* Deduped probe_kern_global_data and probe_prog_bind_map into a common
  helper.
* Added comment regarding why EEXIST is ignored in libbpf bind map.
* Froze all LIBBPF_MAP_METADATA internal maps.
* Moved bpf_prog_bind_map into new LIBBPF_0.1.1 in libbpf.map.
* Added p_err() calls on error cases in bpftool show_prog_metadata.
* Reverse christmas tree coding style in bpftool show_prog_metadata.
* Made bpftool gen skeleton recognize .metadata as an internal map and
  generate datasec definition in skeleton.
* Added C test using skeleton to see asset that the metadata is what we
  expect and rebinding causes EEXIST.

Cc: YiFei Zhu <zhuyifei1999@gmail.com>

Stanislav Fomichev (2):
  libbpf: implement bpf_prog_find_metadata
  bpftool: mention --metadata in the documentation

YiFei Zhu (6):
  bpf: Mutex protect used_maps array and count
  bpf: Add BPF_PROG_BIND_MAP syscall
  libbpf: Add BPF_PROG_BIND_MAP syscall and use it on .metadata section
  bpftool: support dumping metadata
  bpftool: support metadata internal map in gen skeleton
  selftests/bpf: Test load and dump metadata with btftool and skel

 .../net/ethernet/netronome/nfp/bpf/offload.c  |  18 ++-
 include/linux/bpf.h                           |   1 +
 include/uapi/linux/bpf.h                      |   7 +
 kernel/bpf/core.c                             |  15 +-
 kernel/bpf/syscall.c                          |  81 ++++++++++-
 net/core/dev.c                                |  11 +-
 .../bpftool/Documentation/bpftool-prog.rst    |   5 +-
 tools/bpf/bpftool/gen.c                       |   5 +
 tools/bpf/bpftool/json_writer.c               |   6 +
 tools/bpf/bpftool/json_writer.h               |   3 +
 tools/bpf/bpftool/main.c                      |  10 ++
 tools/bpf/bpftool/main.h                      |   1 +
 tools/bpf/bpftool/prog.c                      | 132 +++++++++++++++++-
 tools/include/uapi/linux/bpf.h                |   7 +
 tools/lib/bpf/bpf.c                           |  87 ++++++++++++
 tools/lib/bpf/bpf.h                           |   9 ++
 tools/lib/bpf/libbpf.c                        | 130 ++++++++++++++---
 tools/lib/bpf/libbpf.map                      |   2 +
 tools/testing/selftests/bpf/Makefile          |   3 +-
 .../selftests/bpf/prog_tests/metadata.c       |  83 +++++++++++
 .../selftests/bpf/progs/metadata_unused.c     |  15 ++
 .../selftests/bpf/progs/metadata_used.c       |  15 ++
 .../selftests/bpf/test_bpftool_metadata.sh    |  82 +++++++++++
 23 files changed, 687 insertions(+), 41 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/metadata.c
 create mode 100644 tools/testing/selftests/bpf/progs/metadata_unused.c
 create mode 100644 tools/testing/selftests/bpf/progs/metadata_used.c
 create mode 100755 tools/testing/selftests/bpf/test_bpftool_metadata.sh