On Wed, Nov 6, 2024 at 2:13 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> 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.
I see, it makes sense. I guess we'll need tryget_task_struct() here
and just return an error if we failed to get it.
