bpf: add bpf_for_each_map_elem() helper

This patch set introduced bpf_for_each_map_elem() helper.
The helper permits bpf program iterates through all elements
for a particular map.

The work originally inspired by an internal discussion where
firewall rules are kept in a map and bpf prog wants to
check packet 5 tuples against all rules in the map.
A bounded loop can be used but it has a few drawbacks.
As the loop iteration goes up, verification time goes up too.
For really large maps, verification may fail.
A helper which abstracts out the loop itself will not have
verification time issue.

A recent discussion in [1] involves to iterate all hash map
elements in bpf program. Currently iterating all hashmap elements
in bpf program is not easy if key space is really big.
Having a helper to abstract out the loop itself is even more
meaningful.

The proposed helper signature looks like:
  long bpf_for_each_map_elem(map, callback_fn, callback_ctx, flags)
where callback_fn is a static function and callback_ctx is
a piece of data allocated on the caller stack which can be
accessed by the callback_fn. The callback_fn signature might be
different for different maps. For example, for hash/array maps,
the signature is
  long callback_fn(map, key, val, callback_ctx)

In the rest of series, Patches 1/2/3 did some refactoring. Patch 4
implemented core kernel support for the helper. Patches 5 and 6
added hashmap and arraymap support. Patches 7/8 added libbpf
support. Patch 9 added bpftool support. Patches 10 and 11 added
selftests for hashmap and arraymap.

[1]: https://lore.kernel.org/bpf/20210122205415.113822-1-xiyou...

Yonghong Song (11):
  bpf: factor out visit_func_call_insn() in check_cfg()
  bpf: factor out verbose_invalid_scalar()
  bpf: refactor check_func_call() to allow callback function
  bpf: add bpf_for_each_map_elem() helper
  bpf: add hashtab support for bpf_for_each_map_elem() helper
  bpf: add arraymap support for bpf_for_each_map_elem() helper
  libbpf: move function is_ldimm64() earlier in libbpf.c
  libbpf: support local function pointer relocation
  bpftool: print local function pointer properly
  selftests/bpf: add hashmap test for bpf_for_each_map_elem() helper
  selftests/bpf: add arraymap test for bpf_for_each_map_elem() helper

 include/linux/bpf.h                           |  17 +
 include/linux/bpf_verifier.h                  |   3 +
 include/uapi/linux/bpf.h                      |  29 +-
 kernel/bpf/arraymap.c                         |  39 ++
 kernel/bpf/bpf_iter.c                         |  16 +
 kernel/bpf/hashtab.c                          |  63 ++++
 kernel/bpf/helpers.c                          |   2 +
 kernel/bpf/verifier.c                         | 346 +++++++++++++++---
 kernel/trace/bpf_trace.c                      |   2 +
 tools/bpf/bpftool/xlated_dumper.c             |   3 +
 tools/include/uapi/linux/bpf.h                |  29 +-
 tools/lib/bpf/libbpf.c                        |  52 ++-
 .../selftests/bpf/prog_tests/for_each.c       | 132 +++++++
 .../bpf/progs/for_each_array_map_elem.c       |  61 +++
 .../bpf/progs/for_each_hash_map_elem.c        |  95 +++++
 15 files changed, 829 insertions(+), 60 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/for_each.c
 create mode 100644 tools/testing/selftests/bpf/progs/for_each_array_map_elem.c
 create mode 100644 tools/testing/selftests/bpf/progs/for_each_hash_map_elem.c

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2.24.1