[PATCH bpf-next 1/2] bpf: mark mm->owner as __safe_rcu_or_null

[Yafang Shao <laoar.shao@gmail.com>]

When CONFIG_MEMCG is enabled, we can access mm->owner under RCU. The
owner can be NULL. With this change, BPF helpers can safely access
mm->owner to retrieve the associated task from the mm. We can then make
policy decision based on the task attribute.

The typical use case is as follows,

  bpf_rcu_read_lock(); // rcu lock must be held for rcu trusted field
  @owner = @mm->owner; // mm_struct::owner is rcu trusted or null
  if (!@owner)
      goto out;

  /* Do something based on the task attribute */

out:
  bpf_rcu_read_unlock();

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 kernel/bpf/verifier.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index c908015b2d34..d0adf5600c4d 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7054,6 +7054,9 @@ BTF_TYPE_SAFE_RCU(struct cgroup_subsys_state) {
 /* RCU trusted: these fields are trusted in RCU CS and can be NULL */
 BTF_TYPE_SAFE_RCU_OR_NULL(struct mm_struct) {
 	struct file __rcu *exe_file;
+#ifdef CONFIG_MEMCG
+	struct task_struct __rcu *owner;
+#endif
 };
 
 /* skb->sk, req->sk are not RCU protected, but we mark them as such
-- 
2.47.3