netdev - Re: [PATCH bpf-next 1/2] tools: bpftool: dump outer maps content

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Date:   Fri, 4 Sep 2020 15:03:28 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Quentin Monnet <quentin@...valent.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Martynas Pumputis <m@...bda.lt>
Subject: Re: [PATCH bpf-next 1/2] tools: bpftool: dump outer maps content

On Fri, Sep 4, 2020 at 9:14 AM Quentin Monnet <quentin@...valent.com> wrote:
>
> Although user space can lookup and dump the content of an outer map
> (hash-of-maps or array-of-maps), bpftool does not allow to do so.
>
> It seems that the only reason for that is historical. Lookups for outer
> maps was added in commit 14dc6f04f49d ("bpf: Add syscall lookup support
> for fd array and htab"), and although the relevant code in bpftool had
> not been merged yet, I suspect it had already been written with the
> assumption that user space could not read outer maps.
>
> Let's remove the restriction, dump for outer maps works with no further
> change.
>
> Reported-by: Martynas Pumputis <m@...bda.lt>
> Signed-off-by: Quentin Monnet <quentin@...valent.com>
> ---
>  tools/bpf/bpftool/map.c | 4 ----
>  1 file changed, 4 deletions(-)
>
> diff --git a/tools/bpf/bpftool/map.c b/tools/bpf/bpftool/map.c
> index bc0071228f88..cb3a75eb5531 100644
> --- a/tools/bpf/bpftool/map.c
> +++ b/tools/bpf/bpftool/map.c
> @@ -739,10 +739,6 @@ static int dump_map_elem(int fd, void *key, void *value,
>         /* lookup error handling */
>         lookup_errno = errno;
>
> -       if (map_is_map_of_maps(map_info->type) ||
> -           map_is_map_of_progs(map_info->type))
> -               return 0;
> -

this code path handles the error case when lookup fails, or am I
misreading it? It's fine to remove this restriction, but the commit
message is completely misleading. That whole dump_map_elem() code is a
bit confusing. E.g., what's the purpose of num_elems there?..

Also, can you please update the commit message with how the output
looks like for map-of-maps with your change?

>         if (json_output) {
>                 jsonw_start_object(json_wtr);
>                 jsonw_name(json_wtr, "key");
> --
> 2.25.1
>