netdev - Re: [PATCH bpf-next 1/3] bpftool: add extra CO-RE mode to btf dump command

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Date:   Thu, 19 Dec 2019 09:06:04 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andriin@...com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org, ast@...com,
        daniel@...earbox.net, andrii.nakryiko@...il.com, kernel-team@...com
Subject: Re: [PATCH bpf-next 1/3] bpftool: add extra CO-RE mode to btf dump
 command

On Wed, Dec 18, 2019 at 11:06:56PM -0800, Andrii Nakryiko wrote:
> +	if (core_mode) {
> +		printf("#if defined(__has_attribute) && __has_attribute(preserve_access_index)\n");
> +		printf("#define __CLANG_BPF_CORE_SUPPORTED\n");
> +		printf("#pragma clang attribute push (__attribute__((preserve_access_index)), apply_to = record)\n");
> +		printf("#endif\n\n");

I think it's dangerous to automatically opt-out when clang is not new enough.
bpf prog will compile fine, but it will be missing co-re relocations.
How about doing something like:
  printf("#ifdef NEEDS_CO_RE\n");
  printf("#pragma clang attribute push (__attribute__((preserve_access_index)), apply_to = record)\n");
  printf("#endif\n\n");
and emit it always when 'format c'.
Then on the program side it will look:
#define NEEDS_CO_RE
#include "vmlinux.h"
If clang is too old there will be a compile time error which is a good thing.
Future features will have different NEEDS_ macros.