libbpf: add non-mmapable data section selftest

libbpf: add non-mmapable data section selftest

Add non-mmapable data section to test_skeleton selftest and make sure it
really isn't mmapable by trying to mmap() it anyways.

Also make sure that libbpf doesn't report BPF_F_MMAPABLE flag to users.

Additional, some more manual testing was performed that this feature
works as intended.

Looking at created map through bpftool shows that flags passed to kernel are
indeed zero:

  $ bpftool map show
  ...
  1782: array  name .data.non_mmapa  flags 0x0
          key 4B  value 16B  max_entries 1  memlock 4096B
          btf_id 1169
          pids test_progs(8311)
  ...

Checking BTF uploaded to kernel for this map shows that zero_key and
zero_value are indeed marked as static, even though zero_key is actually
original global (but STV_HIDDEN) variable:

  $ bpftool btf dump id 1169
  ...
  [51] VAR 'zero_key' type_id=2, linkage=static
  [52] VAR 'zero_value' type_id=7, linkage=static
  ...
  [62] DATASEC '.data.non_mmapable' size=16 vlen=2
          type_id=51 offset=0 size=4 (VAR 'zero_key')
          type_id=52 offset=4 size=12 (VAR 'zero_value')
  ...

And original BTF does have zero_key marked as linkage=global:

  $ bpftool btf dump file test_skeleton.bpf.linked3.o
  ...
  [51] VAR 'zero_key' type_id=2, linkage=global
  [52] VAR 'zero_value' type_id=7, linkage=static
  ...
  [62] DATASEC '.data.non_mmapable' size=16 vlen=2
          type_id=51 offset=0 size=4 (VAR 'zero_key')
          type_id=52 offset=4 size=12 (VAR 'zero_value')

Bpftool didn't require any changes at all because it checks whether internal
map is mmapable already, but just to double-check generated skeleton, we
see that .data.non_mmapable neither sets mmaped pointer nor has
a corresponding field in the skeleton:

  $ grep non_mmapable test_skeleton.skel.h
                  struct bpf_map *data_non_mmapable;
          s->maps[7].name = ".data.non_mmapable";
          s->maps[7].map = &obj->maps.data_non_mmapable;

But .data.read_mostly has all of those things:

  $ grep read_mostly test_skeleton.skel.h
                  struct bpf_map *data_read_mostly;
          struct test_skeleton__data_read_mostly {
                  int read_mostly_var;
          } *data_read_mostly;
          s->maps[6].name = ".data.read_mostly";
          s->maps[6].map = &obj->maps.data_read_mostly;
          s->maps[6].mmaped = (void **)&obj->data_read_mostly;
          _Static_assert(sizeof(s->data_read_mostly->read_mostly_var) == 4, "unexpected size of 'read_mostly_var'");

Acked-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Dave Marchevsky <davemarchevsky@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20221019002816.359650-4-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>