Re: [PATCH bpf-next v1 0/5] Introduce bpf_packet_pointer helper

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:

> On Tue, Mar 8, 2022 at 5:40 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>>
>> Kumar Kartikeya Dwivedi <memxor@gmail.com> writes:
>>
>> > On Tue, Mar 08, 2022 at 11:18:52AM IST, Andrii Nakryiko wrote:
>> >> On Sun, Mar 6, 2022 at 3:43 PM Kumar Kartikeya Dwivedi <memxor@gmail.com> wrote:
>> >> >
>> >> > Expose existing 'bpf_xdp_pointer' as a BPF helper named 'bpf_packet_pointer'
>> >> > returning a packet pointer with a fixed immutable range. This can be useful to
>> >> > enable DPA without having to use memcpy (currently the case in
>> >> > bpf_xdp_load_bytes and bpf_xdp_store_bytes).
>> >> >
>> >> > The intended usage to read and write data for multi-buff XDP is:
>> >> >
>> >> >         int err = 0;
>> >> >         char buf[N];
>> >> >
>> >> >         off &= 0xffff;
>> >> >         ptr = bpf_packet_pointer(ctx, off, sizeof(buf), &err);
>> >> >         if (unlikely(!ptr)) {
>> >> >                 if (err < 0)
>> >> >                         return XDP_ABORTED;
>> >> >                 err = bpf_xdp_load_bytes(ctx, off, buf, sizeof(buf));
>> >> >                 if (err < 0)
>> >> >                         return XDP_ABORTED;
>> >> >                 ptr = buf;
>> >> >         }
>> >> >         ...
>> >> >         // Do some stores and loads in [ptr, ptr + N) region
>> >> >         ...
>> >> >         if (unlikely(ptr == buf)) {
>> >> >                 err = bpf_xdp_store_bytes(ctx, off, buf, sizeof(buf));
>> >> >                 if (err < 0)
>> >> >                         return XDP_ABORTED;
>> >> >         }
>> >> >
>> >> > Note that bpf_packet_pointer returns a PTR_TO_PACKET, not PTR_TO_MEM, because
>> >> > these pointers need to be invalidated on clear_all_pkt_pointers invocation, and
>> >> > it is also more meaningful to the user to see return value as R0=pkt.
>> >> >
>> >> > This series is meant to collect feedback on the approach, next version can
>> >> > include a bpf_skb_pointer and exposing it as bpf_packet_pointer helper for TC
>> >> > hooks, and explore not resetting range to zero on r0 += rX, instead check access
>> >> > like check_mem_region_access (var_off + off < range), since there would be no
>> >> > data_end to compare against and obtain a new range.
>> >> >
>> >> > The common name and func_id is supposed to allow writing generic code using
>> >> > bpf_packet_pointer that works for both XDP and TC programs.
>> >> >
>> >> > Please see the individual patches for implementation details.
>> >> >
>> >>
>> >> Joanne is working on a "bpf_dynptr" framework that will support
>> >> exactly this feature, in addition to working with dynamically
>> >> allocated memory, working with memory of statically unknown size (but
>> >> safe and checked at runtime), etc. And all that within a generic
>> >> common feature implemented uniformly within the verifier. E.g., it
>> >> won't need any of the custom bits of logic added in patch #2 and #3.
>> >> So I'm thinking that instead of custom-implementing a partial case of
>> >> bpf_dynptr just for skb and xdp packets, let's maybe wait for dynptr
>> >> and do it only once there?
>> >>
>> >
>> > Interesting stuff, looking forward to it.
>> >
>> >> See also my ARG_CONSTANT comment. It seems like a pretty common thing
>> >> where input constant is used to characterize some pointer returned
>> >> from the helper (e.g., bpf_ringbuf_reserve() case), and we'll need
>> >> that for bpf_dynptr for exactly this "give me direct access of N
>> >> bytes, if possible" case. So improving/generalizing it now before
>> >> dynptr lands makes a lot of sense, outside of bpf_packet_pointer()
>> >> feature itself.
>> >
>> > No worries, we can continue the discussion in patch 1, I'll split out the arg
>> > changes into a separate patch, and wait for dynptr to be posted before reworking
>> > this.
>>
>> This does raise the question of what we do in the meantime, though? Your
>> patch includes a change to bpf_xdp_{load,store}_bytes() which, if we're
>> making it, really has to go in before those hit a release and become
>> UAPI.
>>
>> One option would be to still make the change to those other helpers;
>> they'd become a bit slower, but if we have a solution for that coming,
>> that may be OK for a single release? WDYT?
>
> I must have missed important changes to bpf_xdp_{load,store}_bytes().
> Does anything change about its behavior? If there are some fixes
> specific to those helpers, we should fix them as well as a separate
> patch. My main objection is adding a bpf_packet_pointer() special case
> when we have a generic mechanism in the works that will come this use
> case (among other use cases).

Well it's not a functional change per se, but Kartikeya's patch is
removing an optimisation from bpf_xdp_{load_store}_bytes() (i.e., the
use of the bpf_xdp_pointer()) in favour of making it available directly
to BPF. So if we don't do that change before those helpers are
finalised, we will end up either introducing a performance regression
for code using those helpers, or being stuck with the bpf_xdp_pointer()
use inside them even though it makes more sense to move it out to BPF.

So the "safe" thing to do would do the change to the store/load helpers
now, and get rid of the bpf_xdp_pointer() entirely until it can be
introduced as a BPF helper in a generic way. Of course this depends on
whether you consider performance regressions to be something to avoid,
but this being XDP IMO we should :)

-Toke