Re: [PATCH v3 bpf-next 18/18] selftests/bpf: Add a test for attaching BPF prog to another BPF prog and subprog

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

On Thu, Nov 7, 2019 at 10:43 PM Alexei Starovoitov <ast@kernel.org> wrote:
>
> Add a test that attaches one FEXIT program to main sched_cls networking program
> and two other FEXIT programs to subprograms. All three tracing programs
> access return values and skb->len of networking program and subprograms.
>
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> ---

Acked-by: Andrii Nakryiko <andriin@fb.com>

>  .../selftests/bpf/prog_tests/fexit_bpf2bpf.c  | 76 ++++++++++++++++
>  .../selftests/bpf/progs/fexit_bpf2bpf.c       | 91 +++++++++++++++++++
>  2 files changed, 167 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c
>  create mode 100644 tools/testing/selftests/bpf/progs/fexit_bpf2bpf.c
>

[...]

> +SEC("fexit/test_pkt_access_subprog2")
> +int test_subprog2(struct args_subprog2 *ctx)
> +{
> +       struct sk_buff *skb = (void *)ctx->args[0];
> +       __u64 ret;
> +       int len;
> +
> +       bpf_probe_read(&len, sizeof(len),
> +                      __builtin_preserve_access_index(&skb->len));

nit: we have bpf_core_read() for this, but I suspect you may have
wanted __builtin spelled out explicitly


> +
> +       ret = ctx->ret;
> +       /* bpf_prog_load() loads "test_pkt_access.o" with BPF_F_TEST_RND_HI32
> +        * which randomizes upper 32 bits after BPF_ALU32 insns.
> +        * Hence after 'w0 <<= 1' upper bits of $rax are random.
> +        * That is expected and correct. Trim them.
> +        */
> +       ret = (__u32) ret;
> +       if (len != 74 || ret != 148)
> +               return 0;
> +       test_result_subprog2 = 1;
> +       return 0;
> +}
> +char _license[] SEC("license") = "GPL";
> --
> 2.23.0
>