On 2024/11/6 22:13, Alexei Starovoitov wrote:
On Wed, Nov 6, 2024 at 2:10 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
+__bpf_kfunc int bpf_iter_task_file_new(struct bpf_iter_task_file *it,
+ struct task_struct *task)
+{
+ struct bpf_iter_task_file_kern *kit = (void *)it;
+
+ BUILD_BUG_ON(sizeof(struct bpf_iter_task_file_kern) > sizeof(struct bpf_iter_task_file));
+ BUILD_BUG_ON(__alignof__(struct bpf_iter_task_file_kern) !=
+ __alignof__(struct bpf_iter_task_file));
+
+ kit->task = task;
This is broken, since task refcnt can drop while iter is running.
I noticed this as well, but I thought that given KF_TRUSTED_ARGS we
should have a guarantee that the task survives the iteration? Am I
mistaken?
KF_TRUSTED_ARGS will only guarantee that the task is valid when it's
passed into this kfunc. Right after the prog can call
bpf_task_release() to release the ref and kit->task will become
dangling.
If this object was RCU protected we could have marked this iter
as KF_RCU_PROTECTED, then the verifier would make sure that
RCU unlock doesn't happen between iter_new and iter_destroy.
Thanks for pointing this out.
I will fix it in the next version of the patch series.
