netdev - Re: [bpf-next,v3] samples: bpf: add max_pckt_size option at xdp_adjust

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Date:   Thu, 19 Sep 2019 02:37:36 +0900
From:   "Daniel T. Lee" <danieltimlee@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [bpf-next,v3] samples: bpf: add max_pckt_size option at xdp_adjust_tail

On Tue, Sep 17, 2019 at 1:04 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Wed, Sep 11, 2019 at 2:33 PM Daniel T. Lee <danieltimlee@...il.com> wrote:
> >
> > Currently, at xdp_adjust_tail_kern.c, MAX_PCKT_SIZE is limited
> > to 600. To make this size flexible, a new map 'pcktsz' is added.
> >
> > By updating new packet size to this map from the userland,
> > xdp_adjust_tail_kern.o will use this value as a new max_pckt_size.
> >
> > If no '-P <MAX_PCKT_SIZE>' option is used, the size of maximum packet
> > will be 600 as a default.
> >
> > Signed-off-by: Daniel T. Lee <danieltimlee@...il.com>
> >
> > ---
> > Changes in v2:
> >     - Change the helper to fetch map from 'bpf_map__next' to
> >     'bpf_object__find_map_fd_by_name'.
> >
> >  samples/bpf/xdp_adjust_tail_kern.c | 23 +++++++++++++++++++----
> >  samples/bpf/xdp_adjust_tail_user.c | 28 ++++++++++++++++++++++------
> >  2 files changed, 41 insertions(+), 10 deletions(-)
> >
> > diff --git a/samples/bpf/xdp_adjust_tail_kern.c b/samples/bpf/xdp_adjust_tail_kern.c
> > index 411fdb21f8bc..d6d84ffe6a7a 100644
> > --- a/samples/bpf/xdp_adjust_tail_kern.c
> > +++ b/samples/bpf/xdp_adjust_tail_kern.c
> > @@ -25,6 +25,13 @@
> >  #define ICMP_TOOBIG_SIZE 98
> >  #define ICMP_TOOBIG_PAYLOAD_SIZE 92
> >
> > +struct bpf_map_def SEC("maps") pcktsz = {
> > +       .type = BPF_MAP_TYPE_ARRAY,
> > +       .key_size = sizeof(__u32),
> > +       .value_size = sizeof(__u32),
> > +       .max_entries = 1,
> > +};
> > +
>
> Hey Daniel,
>
> This looks like an ideal use case for global variables on BPF side. I
> think it's much cleaner and will make BPF side of things simpler.
> Would you mind giving global data a spin instead of adding this map?
>

Sure thing!
But, I'm not sure there is global variables for BPF?
AFAIK, there aren't any support for global variables yet in BPF
program (_kern.c).

    # when defining global variable at _kern.c
    libbpf: bpf: relocation: not yet supported relo for non-static
global '<var>' variable found in insns[39].code 0x18

By the way, thanks for the review.

Thanks,
Daniel


> >  struct bpf_map_def SEC("maps") icmpcnt = {
> >         .type = BPF_MAP_TYPE_ARRAY,
> >         .key_size = sizeof(__u32),
> > @@ -64,7 +71,8 @@ static __always_inline void ipv4_csum(void *data_start, int data_size,
> >         *csum = csum_fold_helper(*csum);
> >  }
> >
>
> [...]