[PATCH bpf-next v4 6/7] bpf: Add split iteration support to task_vma iterator

[Puranjay Mohan <puranjay@kernel.org>]

Wire up the task_vma iterator to use the KF_ACQUIRE/KF_RELEASE and
KF_FORBID_SLEEP infrastructure from the previous commits.

Annotate bpf_iter_task_vma_next with KF_ACQUIRE | KF_FORBID_SLEEP so
each returned VMA pointer becomes a tracked reference that prevents
sleeping. Add bpf_iter_task_vma_release(it__iter) with KF_RELEASE that
releases the mmap read lock, drops the acquired reference, and
re-enables sleeping. The next call to _next will re-acquire the lock
via mmap_read_trylock().

This enables a split iteration pattern:

  bpf_for_each(task_vma, vma, task, 0) {
      u64 start = vma->vm_start;
      /* vma access allowed, sleeping forbidden */
      bpf_iter_task_vma_release(&___it);
      /* vma access forbidden, sleeping allowed */
      bpf_copy_from_user(&buf, sizeof(buf), (void *)start);
  }

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 kernel/bpf/helpers.c   |  3 ++-
 kernel/bpf/task_iter.c | 10 ++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 6eb6c82ed2ee..2663280c92c6 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -4605,8 +4605,9 @@ BTF_ID_FLAGS(func, bpf_iter_num_new, KF_ITER_NEW)
 BTF_ID_FLAGS(func, bpf_iter_num_next, KF_ITER_NEXT | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_iter_num_destroy, KF_ITER_DESTROY)
 BTF_ID_FLAGS(func, bpf_iter_task_vma_new, KF_ITER_NEW | KF_RCU)
-BTF_ID_FLAGS(func, bpf_iter_task_vma_next, KF_ITER_NEXT | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_iter_task_vma_next, KF_ITER_NEXT | KF_RET_NULL | KF_ACQUIRE | KF_FORBID_SLEEP)
 BTF_ID_FLAGS(func, bpf_iter_task_vma_destroy, KF_ITER_DESTROY)
+BTF_ID_FLAGS(func, bpf_iter_task_vma_release, KF_RELEASE)
 #ifdef CONFIG_CGROUPS
 BTF_ID_FLAGS(func, bpf_iter_css_task_new, KF_ITER_NEW)
 BTF_ID_FLAGS(func, bpf_iter_css_task_next, KF_ITER_NEXT | KF_RET_NULL)
diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
index a85115c191e4..55b264d634db 100644
--- a/kernel/bpf/task_iter.c
+++ b/kernel/bpf/task_iter.c
@@ -879,6 +879,16 @@ __bpf_kfunc struct vm_area_struct *bpf_iter_task_vma_next(struct bpf_iter_task_v
 	return vma;
 }
 
+__bpf_kfunc void bpf_iter_task_vma_release(struct bpf_iter_task_vma *it__iter)
+{
+	struct bpf_iter_task_vma_kern *kit = (void *)it__iter;
+
+	if (kit->data && kit->data->locked) {
+		bpf_mmap_unlock_mm(kit->data->work, kit->data->mm);
+		kit->data->locked = false;
+	}
+}
+
 __bpf_kfunc void bpf_iter_task_vma_destroy(struct bpf_iter_task_vma *it)
 {
 	struct bpf_iter_task_vma_kern *kit = (void *)it;
-- 
2.47.3