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

Andrii Nakryiko
2022-08-24T18:03:12Z

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?

>
> >
> >
> >
> > > When comparing the differences in runtime for packet parsing without dynptrs
> > > vs. with dynptrs for the more simple cases, there is no noticeable difference.
> > > For the more complex cases where lengths are non-statically known at compile
> > > time, there can be a significant speed-up when using dynptrs (eg a 2x speed up
> > > for cls redirection). Patch 3 contains more details as well as examples of how
> > > to use skb and xdp dynptrs.
> > >
> > > [0] https://lore.kernel.org/bpf/20220523210712.3641569-1-joannelkoong@gmail.com/
> > >
> > > --