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

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Date:   Mon, 7 Sep 2020 15:49:35 +0100
From:   Quentin Monnet <quentin@...valent.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.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 04/09/2020 23:03, Andrii Nakryiko wrote:
> 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?

Ok, the function _is_ confusing and _I_ totally got confused. Dumping
outer maps is already supported and this patch is not needed (and as you
mentioned, it does not do what I expected and wrote). Sorry about that.

I'll send a clean-up for that function instead in v2, to avoid confusion
in the future. Thanks for the review!

Quentin