netdev - Re: [PATCH bpf-next v1 0/5] Introduce bpf_packet

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Message-ID: <20220308070828.4tjiuvvyqwmhru6a@apollo.legion>
Date:   Tue, 8 Mar 2022 12:38:28 +0530
From:   Kumar Kartikeya Dwivedi <memxor@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Jakub Kicinski <kuba@...nel.org>, Lorenz Bauer <linux@....io>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v1 0/5] Introduce bpf_packet_pointer helper

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@...il.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.

>
> > Kumar Kartikeya Dwivedi (5):
> >   bpf: Add ARG_SCALAR and ARG_CONSTANT
> >   bpf: Introduce pkt_uid concept for PTR_TO_PACKET
> >   bpf: Introduce bpf_packet_pointer helper to do DPA
> >   selftests/bpf: Add verifier tests for pkt pointer with pkt_uid
> >   selftests/bpf: Update xdp_adjust_frags to use bpf_packet_pointer
> >
> >  include/linux/bpf.h                           |   4 +
> >  include/linux/bpf_verifier.h                  |   9 +-
> >  include/uapi/linux/bpf.h                      |  12 ++
> >  kernel/bpf/verifier.c                         |  97 ++++++++++--
> >  net/core/filter.c                             |  48 +++---
> >  tools/include/uapi/linux/bpf.h                |  12 ++
> >  .../bpf/prog_tests/xdp_adjust_frags.c         |  46 ++++--
> >  .../bpf/progs/test_xdp_update_frags.c         |  46 ++++--
> >  tools/testing/selftests/bpf/verifier/xdp.c    | 146 ++++++++++++++++++
> >  9 files changed, 358 insertions(+), 62 deletions(-)
> >
> > --
> > 2.35.1
> >

--
Kartikeya