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Date:   Thu, 18 Jun 2020 00:38:27 -0700
From:   Hao Luo <haoluo@...gle.com>
To:     Andrii Nakryiko <andriin@...com>
Cc:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Kernel Team <kernel-team@...com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Song Liu <songliubraving@...com>,
        Quentin Monnet <quentin@...valent.com>
Subject: Re: [PATCH bpf-next 1/9] libbpf: generalize libbpf externs support

On Wed, Jun 17, 2020 at 9:21 AM Andrii Nakryiko <andriin@...com> wrote:
>
> Switch existing Kconfig externs to be just one of few possible kinds of more
> generic externs. This refactoring is in preparation for ksymbol extern
> support, added in the follow up patch. There are no functional changes
> intended.
>
> Signed-off-by: Andrii Nakryiko <andriin@...com>
> ---

[...]

> @@ -5572,30 +5635,33 @@ static int bpf_object__resolve_externs(struct bpf_object *obj,
>  {
>         bool need_config = false;
>         struct extern_desc *ext;
> +       void *kcfg_data;
>         int err, i;
> -       void *data;
>
>         if (obj->nr_extern == 0)
>                 return 0;
>
> -       data = obj->maps[obj->kconfig_map_idx].mmaped;
> +       if (obj->kconfig_map_idx >= 0)
> +               kcfg_data = obj->maps[obj->kconfig_map_idx].mmaped;
>
>         for (i = 0; i < obj->nr_extern; i++) {
>                 ext = &obj->externs[i];
>
> -               if (strcmp(ext->name, "LINUX_KERNEL_VERSION") == 0) {
> -                       void *ext_val = data + ext->data_off;
> +               if (ext->type == EXT_KCFG &&
> +                   strcmp(ext->name, "LINUX_KERNEL_VERSION") == 0) {
> +                       void *ext_val = kcfg_data + ext->kcfg.data_off;
>                         __u32 kver = get_kernel_version();
>
>                         if (!kver) {
>                                 pr_warn("failed to get kernel version\n");
>                                 return -EINVAL;
>                         }
> -                       err = set_ext_value_num(ext, ext_val, kver);
> +                       err = set_kcfg_value_num(ext, ext_val, kver);
>                         if (err)
>                                 return err;
> -                       pr_debug("extern %s=0x%x\n", ext->name, kver);
> -               } else if (strncmp(ext->name, "CONFIG_", 7) == 0) {
> +                       pr_debug("extern (kcfg) %s=0x%x\n", ext->name, kver);
> +               } else if (ext->type == EXT_KCFG &&
> +                          strncmp(ext->name, "CONFIG_", 7) == 0) {
>                         need_config = true;
>                 } else {
>                         pr_warn("unrecognized extern '%s'\n", ext->name);

Ah, we need to initialize kcfg_data, otherwise the compiler will give
a warning on potentially uninitialized data.

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