bpf: Support access to bpf map fields

This patch set adds support to access bpf map fields from bpf programs
using btf_struct_access().

That way a program can cast a pointer to map to either `struct bpf_map *`
or map type specific struct pointer such as `struct bpf_array *` or
`struct bpf_htab *`, and access necessary fields, e.g. map->max_entries.

The fields, in turn, should be defined by a user provided struct with
preserve_access_index attribute or included from vmlinux.h.

Please see patch 4 for more details on the feature and use-cases.

Other patches:

Patch 1 is refactoring to simplify btf_parse_vmlinux().
Patch 2 introduces a new function to simplify iteration over btf.
Patch 3 is a rename to avoid having two different `struct bpf_htab`.

Patch 5 enables access to map fields for all map types.
Patch 6 adds selftests.


Andrey Ignatov (6):
  bpf: Switch btf_parse_vmlinux to btf_find_by_name_kind
  bpf: Introduce btf_find_by_name_kind_next()
  bpf: Rename bpf_htab to bpf_shtab in sock_map
  bpf: Support access to bpf map fields
  bpf: Set map_btf_name for all map types
  selftests/bpf: Test access to bpf map pointer

 include/linux/bpf.h                           |   8 +
 include/linux/bpf_verifier.h                  |   1 +
 include/linux/btf.h                           |   2 +
 kernel/bpf/arraymap.c                         |   6 +
 kernel/bpf/bpf_struct_ops.c                   |   1 +
 kernel/bpf/btf.c                              |  97 ++-
 kernel/bpf/cpumap.c                           |   1 +
 kernel/bpf/devmap.c                           |   2 +
 kernel/bpf/hashtab.c                          |   5 +
 kernel/bpf/local_storage.c                    |   1 +
 kernel/bpf/lpm_trie.c                         |   1 +
 kernel/bpf/queue_stack_maps.c                 |   2 +
 kernel/bpf/reuseport_array.c                  |   1 +
 kernel/bpf/ringbuf.c                          |   1 +
 kernel/bpf/stackmap.c                         |   1 +
 kernel/bpf/verifier.c                         |  77 +-
 net/core/bpf_sk_storage.c                     |   1 +
 net/core/sock_map.c                           |  84 +--
 net/xdp/xskmap.c                              |   1 +
 .../selftests/bpf/prog_tests/map_ptr.c        |  32 +
 .../selftests/bpf/progs/map_ptr_kern.c        | 686 ++++++++++++++++++
 .../testing/selftests/bpf/verifier/map_ptr.c  |  62 ++
 .../selftests/bpf/verifier/map_ptr_mixing.c   |   2 +-
 23 files changed, 1007 insertions(+), 68 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/map_ptr.c
 create mode 100644 tools/testing/selftests/bpf/progs/map_ptr_kern.c
 create mode 100644 tools/testing/selftests/bpf/verifier/map_ptr.c

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2.24.1