Re: [PATCH v2 bpf-next 7/7] bpf: add tests for %pT format specifier

[Andrii Nakryiko]

On Mon, May 11, 2020 at 10:59 PM Alan Maguire <alan.maguire@xxxxxxxxxx> wrote:
>
> tests verify we get > 0 return value from bpf_trace_print()
> using %pT format specifier with various modifiers/pointer
> values.
>
> Signed-off-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
> ---

There is no need to use perf buffer for returning results to
user-space. See prog_tests/skeleton.c and progs/test_skeleton.c for a
very minimalistic and simple way to do tests like this.

>  .../selftests/bpf/prog_tests/trace_printk_btf.c    | 83 ++++++++++++++++++++++
>  .../selftests/bpf/progs/netif_receive_skb.c        | 81 +++++++++++++++++++++
>  2 files changed, 164 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/trace_printk_btf.c
>  create mode 100644 tools/testing/selftests/bpf/progs/netif_receive_skb.c

[...]

> diff --git a/tools/testing/selftests/bpf/progs/netif_receive_skb.c b/tools/testing/selftests/bpf/progs/netif_receive_skb.c
> new file mode 100644
> index 0000000..b5148df
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/netif_receive_skb.c
> @@ -0,0 +1,81 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2020, Oracle and/or its affiliates. */
> +#include <linux/bpf.h>
> +#include <stdbool.h>
> +#include <bpf/bpf_helpers.h>
> +#include <bpf/bpf_endian.h>
> +#include <bpf/bpf_tracing.h>
> +
> +char _license[] SEC("license") = "GPL";
> +
> +struct {
> +       __uint(type, BPF_MAP_TYPE_PERF_EVENT_ARRAY);
> +       __uint(key_size, sizeof(int));
> +       __uint(value_size, sizeof(int));
> +} perf_buf_map SEC(".maps");
> +
> +struct result {
> +       int ret;
> +       int subtest;
> +       int num_subtest;
> +};
> +
> +typedef struct {
> +       int counter;
> +} atomic_t;
> +typedef struct refcount_struct {
> +       atomic_t refs;
> +} refcount_t;
> +
> +struct sk_buff {
> +       /* field names and sizes should match to those in the kernel */
> +       unsigned int len, data_len;
> +       __u16 mac_len, hdr_len, queue_mapping;
> +       struct net_device *dev;
> +       /* order of the fields doesn't matter */
> +       refcount_t users;
> +       unsigned char *data;
> +       char __pkt_type_offset[0];
> +       char cb[48];
> +};


please use vmlinux.h instead of duplicating these definitions (which
also will start failing, when sk_buff definition will change).

> +
> +#define CHECK_PRINTK(_fmt, _p, res)                                    \
> +       do {                                                            \
> +               char fmt[] = _fmt;                                      \
> +               ++(res)->num_subtest;                                   \
> +               if ((res)->ret >= 0) {                                  \
> +                       ++(res)->subtest;                               \
> +                       (res)->ret = bpf_trace_printk(fmt, sizeof(fmt), \
> +                                                     (_p));            \
> +               }                                                       \
> +       } while (0)
> +
> +/* TRACE_EVENT(netif_receive_skb,
> + *     TP_PROTO(struct sk_buff *skb),
> + */
> +SEC("tp_btf/netif_receive_skb")
> +int BPF_PROG(trace_netif_receive_skb, struct sk_buff *skb)
> +{
> +       char skb_type[] = "struct sk_buff";
> +       struct __btf_ptr nullp = { .ptr = 0, .type = skb_type };
> +       struct __btf_ptr p = { .ptr = skb, .type = skb_type };
> +       struct result res = { 0, 0 };
> +
> +       CHECK_PRINTK("%pT\n", &p, &res);
> +       CHECK_PRINTK("%pTc\n", &p, &res);
> +       CHECK_PRINTK("%pTN\n", &p, &res);
> +       CHECK_PRINTK("%pTx\n", &p, &res);
> +       CHECK_PRINTK("%pT0\n", &p, &res);
> +       CHECK_PRINTK("%pTcNx0\n", &p, &res);
> +       CHECK_PRINTK("%pT\n", &nullp, &res);
> +       CHECK_PRINTK("%pTc\n", &nullp, &res);
> +       CHECK_PRINTK("%pTN\n", &nullp, &res);
> +       CHECK_PRINTK("%pTx\n", &nullp, &res);
> +       CHECK_PRINTK("%pT0\n", &nullp, &res);
> +       CHECK_PRINTK("%pTcNx0\n", &nullp, &res);

with global variables this would be:

int pT = 0;
int pTc = 0;
/* and so on */

then inside BPF_PROG:

pT = bpf_printk("%pT\n", &p);
pTc = bpf_printk("%pTc\n", &p);
/* and so on */

CHECK_PRINTK isn't necessary, IMO. bpf_printk is defined in bpf_helpers.h

> +
> +       bpf_perf_event_output(ctx, &perf_buf_map, BPF_F_CURRENT_CPU,
> +                             &res, sizeof(res));
> +
> +       return 0;
> +}
> --
> 1.8.3.1
>