bpf: Track borrowed iterator results by default

An open-coded iterator can return a struct pointer borrowed from its
current element. The verifier has not tied such results to iterator
state since the task_vma iterator was introduced, so a saved result
can still be used after destroy. Commit bee9ef4a40a27 ("bpf: switch
task_vma iterator from mmap_lock to per-VMA locks") also made next
release the protection of the prior element. A later snapshot
conversion made vm_file explicitly borrowed from that element.

Associate non-RCU, non-acquired struct results with the iterator
reference by default. Invalidate dependents before next forks its
NULL and non-NULL paths, while retaining the iterator itself. Let
destroy propagate child-reference errors. Add KF_ITER_RET_INDEPENDENT
for results that outlive the current element, preserving existing
RCU and acquired-reference rules.

Trusted pointer field loads also need to preserve the source lifetime.
Loading a trusted field does not acquire a reference to the pointed-to
object. Set the field pointer's parent_id to the source's reference ID
if the source is referenced, or inherit its parent_id if it is borrowed.
This is a generic rule, including accesses unrelated to iterators. For
example, the following access is now rejected:

  file = bpf_get_task_exe_file(task);
  if (!file)
          return 0;
  inode = file->f_inode;
  bpf_put_file(file);
  ino = inode->i_ino;

The inode pointer borrows the file's lifetime. Releasing the acquired
file reference removes that lifetime guarantee, so the verifier must
invalidate the derived inode pointer as well. This does not imply that
bpf_put_file() immediately frees the file or inode. Reading the inode
while the file reference is held remains valid. Fields with independent
RCU protection retain their existing lifetime rules.

For an iterator result, the same propagation keeps trusted fields such
as vm_file and f_inode tied to the current element. Propagate the backing
object's lifetime through dynptr construction too, so its descendants
remain in the invalidation graph. Share the invalidation traversal
between next and release, so advancing the iterator preserves its own
reference.

Fixes: 4ac454682158 ("bpf: Introduce task_vma open-coded iterator kfuncs")
Suggested-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Xu Yunxiang <xyx2021@mail.ustc.edu.cn>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Amery Hung <ameryhung@gmail.com>
Link: https://patch.msgid.link/20261024100647.2579244-2-xyx2021@mail.ustc.edu.cn

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