Re: [PATCH bpf-next 03/11] tools, bpf: fix bpftool build with !CONFIG_BPF_EVENTS

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

On Thu, Apr 14, 2022 at 3:45 PM Alexander Lobakin <alobakin@pm.me> wrote:
>
> Fix the following error when building bpftool:
>
>   CLANG   profiler.bpf.o
>   CLANG   pid_iter.bpf.o
> skeleton/profiler.bpf.c:18:21: error: invalid application of 'sizeof' to an incomplete type 'struct bpf_perf_event_value'
>         __uint(value_size, sizeof(struct bpf_perf_event_value));
>                            ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> tools/bpf/bpftool/bootstrap/libbpf/include/bpf/bpf_helpers.h:13:39: note: expanded from macro '__uint'
>                                       ^~~
> tools/bpf/bpftool/bootstrap/libbpf/include/bpf/bpf_helper_defs.h:7:8: note: forward declaration of 'struct bpf_perf_event_value'
> struct bpf_perf_event_value;
>        ^
>
> struct bpf_perf_event_value is being used in the kernel only when
> CONFIG_BPF_EVENTS is enabled, so it misses a BTF entry then.
> Emit the type unconditionally to fix the problem.
>
> Fixes: 47c09d6a9f67 ("bpftool: Introduce "prog profile" command")
> Signed-off-by: Alexander Lobakin <alobakin@pm.me>
> ---
>  kernel/bpf/syscall.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 34fdf27d14cf..dd8284a60a8e 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -4286,6 +4286,7 @@ static int link_create(union bpf_attr *attr, bpfptr_t uattr)
>                 goto out;
>         case BPF_PROG_TYPE_PERF_EVENT:
>         case BPF_PROG_TYPE_TRACEPOINT:
> +               BTF_TYPE_EMIT(struct bpf_perf_event_value);

same as for previous two patches, if there are types that bpftool
expects and might not be in vmlinux.h due to different kernel
configurations, it's cleaner to just define their minimal local
definitions with __attribute__((preserve_access_index))

>                 if (attr->link_create.attach_type != BPF_PERF_EVENT) {
>                         ret = -EINVAL;
>                         goto out;
> --
> 2.35.2
>
>