Re: Trying the bpf trace a bpf xdp program

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Fri, Dec 6, 2019 at 5:05 AM Eelco Chaudron <echaudro@redhat.com> wrote:
>
> Thanks the hint that it should be the jitted arguments solved it… And
> you quick example worked, just in case some one else is playing with it,
> here is my working example:
>
> // SPDX-License-Identifier: GPL-2.0
> #include <linux/bpf.h>
> #include "bpf_helpers.h"
> #include "bpf_trace_helpers.h"
>
> #define bpf_debug(fmt, ...)                \
> {                                          \
>      char __fmt[] = fmt;                    \
>      bpf_trace_printk(__fmt, sizeof(__fmt), \
>                       ##__VA_ARGS__);       \
> }
>
> struct net_device {
>      /* Structure does not need to contain all entries,
>       * as "preserve_access_index" will use BTF to fix this... */
>      int                    ifindex;
> } __attribute__((preserve_access_index));
>
> struct xdp_rxq_info {
>      /* Structure does not need to contain all entries,
>       * as "preserve_access_index" will use BTF to fix this... */
>      struct net_device *dev;
>      __u32 queue_index;
> } __attribute__((preserve_access_index));
>
> struct xdp_buff {
>      void *data;
>      void *data_end;
>      void *data_meta;
>      void *data_hard_start;
>      unsigned long handle;
>      struct xdp_rxq_info *rxq;
> } __attribute__((preserve_access_index));
>
>
> BPF_TRACE_1("fentry/xdp_prog_simple", trace_on_entry,
>              struct xdp_buff *, xdp)
> {
>      bpf_debug("fentry: [ifindex = %u, queue =  %u]\n",
>                xdp->rxq->dev->ifindex, xdp->rxq->queue_index);
>      return 0;
> }
>
>
> BPF_TRACE_2("fexit/xdp_prog_simple", trace_on_exit,
>              struct xdp_buff*, xdp, int, ret)
> {
>      bpf_debug("fexit: [ifindex = %u, queue =  %u, ret = %d]\n",
>                xdp->rxq->dev->ifindex, xdp->rxq->queue_index, ret);

This is great. Could you submit it as selftests/bpf ?
It will help others trying to do the same.
May be instead of bpf_debug() use global variables so the test will be
self checking ?

Long term we should teach verifier to understand 'struct xdp_md*'
in addition to 'struct xdp_buff *'. That will help ease of use.