netdev - Re: [PATCH bpf-next] bpftool: Fix pretty print dump for maps without BTF loaded

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Date:   Wed, 16 Feb 2022 15:08:44 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Yinjun Zhang <yinjun.zhang@...igine.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [PATCH bpf-next] bpftool: Fix pretty print dump for maps without
 BTF loaded

On Wed, Feb 16, 2022 at 1:21 AM Jiri Olsa <jolsa@...nel.org> wrote:
>
> The commit e5043894b21f ("bpftool: Use libbpf_get_error() to check error")
> fails to dump map without BTF loaded in pretty mode (-p option).
>
> Fixing this by making sure get_map_kv_btf won't fail in case there's
> no BTF available for the map.
>
> Cc: Yinjun Zhang <yinjun.zhang@...igine.com>
> Fixes: e5043894b21f ("bpftool: Use libbpf_get_error() to check error")
> Suggested-by: Andrii Nakryiko <andrii@...nel.org>
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
>  tools/bpf/bpftool/map.c | 29 +++++++++++++----------------
>  1 file changed, 13 insertions(+), 16 deletions(-)
>
> diff --git a/tools/bpf/bpftool/map.c b/tools/bpf/bpftool/map.c
> index 7a341a472ea4..8562add7417d 100644
> --- a/tools/bpf/bpftool/map.c
> +++ b/tools/bpf/bpftool/map.c
> @@ -805,29 +805,28 @@ static int maps_have_btf(int *fds, int nb_fds)
>
>  static struct btf *btf_vmlinux;
>
> -static struct btf *get_map_kv_btf(const struct bpf_map_info *info)
> +static int get_map_kv_btf(const struct bpf_map_info *info, struct btf **btf)
>  {
> -       struct btf *btf = NULL;
> +       int err = 0;
>
>         if (info->btf_vmlinux_value_type_id) {
>                 if (!btf_vmlinux) {
>                         btf_vmlinux = libbpf_find_kernel_btf();
> -                       if (libbpf_get_error(btf_vmlinux))
> +                       err = libbpf_get_error(btf_vmlinux);
> +                       if (err) {
>                                 p_err("failed to get kernel btf");
> +                               return err;
> +                       }
>                 }
> -               return btf_vmlinux;
> +               *btf = btf_vmlinux;
>         } else if (info->btf_value_type_id) {
> -               int err;
> -
> -               btf = btf__load_from_kernel_by_id(info->btf_id);
> -               err = libbpf_get_error(btf);
> -               if (err) {
> +               *btf = btf__load_from_kernel_by_id(info->btf_id);
> +               err = libbpf_get_error(*btf);
> +               if (err)
>                         p_err("failed to get btf");
> -                       btf = ERR_PTR(err);
> -               }
>         }

get_map_kv_btf is supposed to set btf to NULL, otherwise you can get a
crash in the caller

I've added

else {
    *btf = NULL;
}

and force-pushed


>
> -       return btf;
> +       return err;
>  }
>
>  static void free_map_kv_btf(struct btf *btf)
> @@ -862,8 +861,7 @@ map_dump(int fd, struct bpf_map_info *info, json_writer_t *wtr,
>         prev_key = NULL;
>
>         if (wtr) {
> -               btf = get_map_kv_btf(info);
> -               err = libbpf_get_error(btf);
> +               err = get_map_kv_btf(info, &btf);
>                 if (err) {
>                         goto exit_free;
>                 }
> @@ -1054,8 +1052,7 @@ static void print_key_value(struct bpf_map_info *info, void *key,
>         json_writer_t *btf_wtr;
>         struct btf *btf;
>
> -       btf = get_map_kv_btf(info);
> -       if (libbpf_get_error(btf))
> +       if (get_map_kv_btf(info, &btf))
>                 return;
>
>         if (json_output) {
> --
> 2.35.1
>