Re: [PATCH bpf-next v4 0/3] Add skb + xdp dynptrs

Kumar Kartikeya Dwivedi
2022-08-24T23:19:05Z

On Wed, 24 Aug 2022 at 20:02, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
>
> On Tue, Aug 23, 2022 at 11:52 AM Joanne Koong <joannelkoong@gmail.com> wrote:
> >
> > On Mon, Aug 22, 2022 at 7:32 PM Kumar Kartikeya Dwivedi
> > <memxor@gmail.com> wrote:
> > >
> > > On Tue, 23 Aug 2022 at 02:06, Joanne Koong <joannelkoong@gmail.com> wrote:
> > > >
> > > > This patchset is the 2nd in the dynptr series. The 1st can be found here [0].
> > > >
> > > > This patchset adds skb and xdp type dynptrs, which have two main benefits for
> > > > packet parsing:
> > > > * allowing operations on sizes that are not statically known at
> > > > compile-time (eg variable-sized accesses).
> > > > * more ergonomic and less brittle iteration through data (eg does not need
> > > > manual if checking for being within bounds of data_end)
> > > >
> > >
> > > Just curious: so would you be adding a dynptr interface for obtaining
> > > data_meta slices as well in the future? Since the same manual bounds
> > > checking is needed for data_meta vs data. How would that look in the
> > > generic dynptr interface of data/read/write this set is trying to fit
> > > things in?
> >
> > Oh cool, I didn't realize there is also a data_meta used in packet
> > parsing - thanks for bringing this up. I think there are 2 options for
> > how data_meta can be incorporated into the dynptr interface:
> >
> > 1) have a separate api "bpf_dynptr_from_{skb/xdp}_meta. We'll have to
> > have a function in the verifier that does something similar to
> > 'may_access_direct_pkt_data' but for pkt data meta, since skb progs
> > can have different access restrictions for data vs. data_meta.
> >
> > 2) ideally, the flags arg would be used to indicate whether the
> > parsing should be for data_meta. To support this though, I think we'd
> > need to do access type checking within the helper instead of at the
> > verifier level. One idea is to pass in the env->ops ptr as a 4th arg
> > (manually patching it from the verifier) to the helper, which can be
> > used to determine if data_meta access is permitted.
> >
> > In both options, there'll be a new BPF_DYNPTR_{SKB/XDP}_META dynptr
> > type and data/read/write will be supported for it.
> >
> > What are your thoughts?
>
> I think separate bpf_dynptr_from_skb_meta() and
> bpf_dynptr_from_xdp_meta() is cleaner than a flag. Also having a
> separate helper would make it easier to disable this helper for
> program types that don't have access to ctx->data_meta, right?
>

Agreed, and with flags then you also need to force them to be constant
(to be able to distinguish the return type from the flag's value),
which might be too restrictive.