bpf: Remove bpf_cgroup_kptr_get() kfunc

Now that bpf_cgroup_acquire() is KF_RCU | KF_RET_NULL,
bpf_cgroup_kptr_get() is redundant. Let's remove it, and update
selftests to instead use bpf_cgroup_acquire() where appropriate. The
next patch will update the BPF documentation to not mention
bpf_cgroup_kptr_get().

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230411041633.179404-2-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

bpf: Make bpf_cgroup_acquire() KF_RCU | KF_RET_NULL

struct cgroup is already an RCU-safe type in the verifier. We can
therefore update bpf_cgroup_acquire() to be KF_RCU | KF_RET_NULL, and
subsequently remove bpf_cgroup_kptr_get(). This patch does the first of
these by updating bpf_cgroup_acquire() to be KF_RCU | KF_RET_NULL, and
also updates selftests accordingly.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230411041633.179404-1-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

selftests/bpf: Tweak cgroup kfunc test.

Adjust cgroup kfunc test to dereference RCU protected cgroup pointer
as PTR_TRUSTED and pass into KF_TRUSTED_ARGS kfunc.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/20230303041446.3630-6-alexei.starovoitov@gmail.com

selftests/bpf: Add a test case for bpf_cgroup_from_id()

Add a test case for bpf_cgroup_from_id.

Signed-off-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/Y/bBlt+tPozcQgws@slm.duckdns.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

selftests/bpf: Add selftests for bpf_cgroup_ancestor() kfunc

bpf_cgroup_ancestor() allows BPF programs to access the ancestor of a
struct cgroup *. This patch adds selftests that validate its expected
behavior.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20221122055458.173143-5-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

selftests/bpf: Add cgroup kfunc / kptr selftests

This patch adds a selftest suite to validate the cgroup kfuncs that were
added in the prior patch.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20221122055458.173143-3-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>