selftests/bpf: Test CGROUP_STORAGE map can't be used by multiple progs

The current assumption is that the lifetime of a cgroup storage
is tied to the program's attachment. The storage is created in
cgroup_bpf_attach, and released upon cgroup_bpf_detach and
cgroup_bpf_release.

Because the current semantics is that each attachment gets a
completely independent cgroup storage, and you can have multiple
programs attached to the same (cgroup, attach type) pair, the key
of the CGROUP_STORAGE map, looking up the map with this pair could
yield multiple storages, and that is not permitted. Therefore,
the kernel verifier checks that two programs cannot share the same
CGROUP_STORAGE map, even if they have different expected attach
types, considering that the actual attach type does not always
have to be equal to the expected attach type.

The test creates a CGROUP_STORAGE map and make it shared across
two different programs, one cgroup_skb/egress and one /ingress.
It asserts that the two programs cannot be both loaded, due to
verifier failure from the above reason.

Signed-off-by: YiFei Zhu <zhuyifei@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/30a6b0da67ae6b0296c4d511bfb19c5f3d035916.1595565795.git.zhuyifei@google.com

selftests/bpf: Add test for CGROUP_STORAGE map on multiple attaches

This test creates a parent cgroup, and a child of that cgroup.
It attaches a cgroup_skb/egress program that simply counts packets,
to a global variable (ARRAY map), and to a CGROUP_STORAGE map.
The program is first attached to the parent cgroup only, then to
parent and child.

The test cases sends a message within the child cgroup, and because
the program is inherited across parent / child cgroups, it will
trigger the egress program for both the parent and child, if they
exist. The program, when looking up a CGROUP_STORAGE map, uses the
cgroup and attach type of the attachment parameters; therefore,
both attaches uses different cgroup storages.

We assert that all packet counts returns what we expects.

Signed-off-by: YiFei Zhu <zhuyifei@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/5a20206afa4606144691c7caa0d1b997cd60dec0.1595565795.git.zhuyifei@google.com