[PATCH v2 bpf-next 1/6] bpf: Add KF_RCU flag to bpf_refcount_acquire_impl

[Dave Marchevsky <davemarchevsky@fb.com>]

Refcounted local kptrs are kptrs to user-defined types with a
bpf_refcount field. Recent commits ([0], [1]) modified the lifetime of
refcounted local kptrs such that the underlying memory is not reused
until RCU grace period has elapsed.

Separately, verification of bpf_refcount_acquire calls currently
succeeds for MAYBE_NULL non-owning reference input, which is a problem
as bpf_refcount_acquire_impl has no handling for this case.

This patch takes advantage of aforementioned lifetime changes to tag
bpf_refcount_acquire_impl kfunc KF_RCU, thereby preventing MAYBE_NULL
input to the kfunc. The KF_RCU flag applies to all kfunc params; it's
fine for it to apply to the void *meta__ign param as that's populated by
the verifier and is tagged __ign regardless.

  [0]: commit 7e26cd12ad1c ("bpf: Use bpf_mem_free_rcu when
       bpf_obj_dropping refcounted nodes") is the actual change to
       allocation behaivor
  [1]: commit 0816b8c6bf7f ("bpf: Consider non-owning refs to refcounted
       nodes RCU protected") modified verifier understanding of
       refcounted local kptrs to match [0]'s changes

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Fixes: 7c50b1cb76ac ("bpf: Add bpf_refcount_acquire kfunc")
---
 kernel/bpf/helpers.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index e46ac288a108..6e1874cc9c13 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2515,7 +2515,7 @@ BTF_ID_FLAGS(func, bpf_obj_new_impl, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_percpu_obj_new_impl, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_obj_drop_impl, KF_RELEASE)
 BTF_ID_FLAGS(func, bpf_percpu_obj_drop_impl, KF_RELEASE)
-BTF_ID_FLAGS(func, bpf_refcount_acquire_impl, KF_ACQUIRE | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_refcount_acquire_impl, KF_ACQUIRE | KF_RET_NULL | KF_RCU)
 BTF_ID_FLAGS(func, bpf_list_push_front_impl)
 BTF_ID_FLAGS(func, bpf_list_push_back_impl)
 BTF_ID_FLAGS(func, bpf_list_pop_front, KF_ACQUIRE | KF_RET_NULL)
-- 
2.34.1