[RFC PATCH bpf-next 0/5] Allow storage of flexible metadata information for eBPF programs

[YiFei Zhu <zhuyifei1999@gmail.com>]

From: YiFei Zhu <zhuyifei@google.com>

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_ADD_MAP to attach 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 attached. 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 moves the used_maps array and count into an RCU-protected
struct.

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 adds a test to check the metadata loading and dumping.

YiFei Zhu (5):
  bpf: RCU protect used_maps array and count
  bpf: Add BPF_PROG_ADD_MAP syscall
  libbpf: Add BPF_PROG_ADD_MAP syscall and use it on .metadata section
  bpftool: support dumping metadata
  selftests/bpf: Test bpftool loading and dumping metadata

 .../net/ethernet/netronome/nfp/bpf/offload.c  |  33 +++--
 include/linux/bpf.h                           |  11 +-
 include/uapi/linux/bpf.h                      |   7 +
 kernel/bpf/core.c                             |  25 +++-
 kernel/bpf/syscall.c                          | 102 +++++++++++++-
 kernel/bpf/verifier.c                         |  37 +++--
 net/core/dev.c                                |  12 +-
 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                           |  11 ++
 tools/lib/bpf/bpf.h                           |   1 +
 tools/lib/bpf/libbpf.c                        |  97 ++++++++++++-
 tools/lib/bpf/libbpf.map                      |   1 +
 tools/testing/selftests/bpf/Makefile          |   3 +-
 .../selftests/bpf/progs/metadata_unused.c     |  15 ++
 .../selftests/bpf/progs/metadata_used.c       |  15 ++
 .../selftests/bpf/test_bpftool_metadata.sh    |  82 +++++++++++
 21 files changed, 572 insertions(+), 39 deletions(-)
 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

-- 
2.28.0