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

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Tue, 08 Mar 2022 14:40:23 +0100

Kumar Kartikeya Dwivedi <[email-masked]> 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 <[email-masked]> 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?

-Toke