Re: [RFC PATCH v7 1/8] net_sched: Introduce eBPF based Qdisc

[Amery Hung <ameryhung@gmail.com>]

On Wed, Jan 31, 2024 at 8:49 AM Kui-Feng Lee <sinquersw@gmail.com> wrote:
>
>
>
> On 1/30/24 17:01, Martin KaFai Lau wrote:
> > On 1/30/24 9:49 AM, Kui-Feng Lee wrote:
> >>>> 2. Returning a kptr from a program and treating it as releasing the
> >>>> reference.
> >>>
> >>> e.g. for dequeue:
> >>>
> >>> struct Qdisc_ops {
> >>>      /* ... */
> >>>      struct sk_buff *        (*dequeue)(struct Qdisc *);
> >>> };
> >>>
> >>>
> >>> Right now the verifier should complain on check_reference_leak() if
> >>> the struct_ops bpf prog is returning a referenced kptr.
> >>>
> >>> Unlike an argument, the return type of a function does not have a
> >>> name to tag. It is the first case that a struct_ops bpf_prog returning a
> >>
> >> We may tag the stub functions instead, right?
> >
> > What is the suggestion on how to tag the return type?
> >
> > I was suggesting it doesn't need to tag and it should by default require
> > a trusted ptr for the pointer returned by struct_ops. The pointer
> > argument and the return pointer of a struct_ops should be a trusted ptr.
>
>
> That make sense to me. Should we also allow operators to return a null
> pointer?
>

.dequeue in Qdisc_ops can return a null pointer when there is no skb
to be dequeued so I think that should be allowed.

> >
> >> Is the purpose here to return a referenced pointer from a struct_ops
> >> operator without verifier complaining?
> >
> > Yes, basically need to teach the verifier the kernel will do the
> > reference release.
> >
> >>
> >>> pointer. One idea is to assume it must be a trusted pointer
> >>> (PTR_TRUSTED) and the verifier should check it is indeed with
> >>> PTR_TRUSTED flag.
> >>>
> >>> May be release_reference_state() can be called to assume the kernel
> >>> will release it as long as the return pointer type is PTR_TRUSTED and
> >>> the type matches the return type of the ops. Take a look at
> >>> check_return_code().
> >