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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, Sep 18, 2019 at 10:37 AM Daniel T. Lee <danieltimlee@gmail.com> wrote:
>
> On Tue, Sep 17, 2019 at 1:04 PM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
> >
> > On Wed, Sep 11, 2019 at 2:33 PM Daniel T. Lee <danieltimlee@gmail.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@gmail.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

just what it says: use static global variable (also volatile to
prevent compiler optimizations) :)

static volatile __u32 pcktsz; /* this should work */

>
> 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);
> > >  }
> > >
> >
> > [...]