bpf: Mark cgroups and dfl_cgrp fields as trusted.

bpf programs sometimes do:
bpf_cgrp_storage_get(&map, task->cgroups->dfl_cgrp, ...);
It is safe to do, because cgroups->dfl_cgrp pointer is set diring init and
never changes. The task->cgroups is also never NULL. It is also set during init
and will change when task switches cgroups. For any trusted task pointer
dereference of cgroups and dfl_cgrp should yield trusted pointers. The verifier
wasn't aware of this. Hence in gcc compiled kernels task->cgroups dereference
was producing PTR_TO_BTF_ID without modifiers while in clang compiled kernels
the verifier recognizes __rcu tag in cgroups field and produces
PTR_TO_BTF_ID | MEM_RCU | MAYBE_NULL.
Tag cgroups and dfl_cgrp as trusted to equalize clang and gcc behavior.
When GCC supports btf_type_tag such tagging will done directly in the type.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: David Vernet <void@manifault.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/bpf/20230303041446.3630-3-alexei.starovoitov@gmail.com

kernel/bpf/verifier.c

@@ -5046,6 +5046,11 @@ static int bpf_map_direct_read(struct bpf_map *map, int off, int size, u64 *val)
 
 BTF_TYPE_SAFE_NESTED(struct task_struct) {
 	const cpumask_t *cpus_ptr;
+	struct css_set __rcu *cgroups;
+};
+
+BTF_TYPE_SAFE_NESTED(struct css_set) {
+	struct cgroup *dfl_cgrp;
 };
 
 static bool nested_ptr_is_trusted(struct bpf_verifier_env *env,
@@ -5057,6 +5062,7 @@ static bool nested_ptr_is_trusted(struct bpf_verifier_env *env,
 		return false;
 
 	BTF_TYPE_EMIT(BTF_TYPE_SAFE_NESTED(struct task_struct));
+	BTF_TYPE_EMIT(BTF_TYPE_SAFE_NESTED(struct css_set));
 
 	return btf_nested_type_is_trusted(&env->log, reg, off);
 }