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

[Joanne Koong <joannelkoong@gmail.com>]

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?

>
>
>
> > 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/
> >
> > --