netdev - Re: [PATCH iproute2 5/5] bpf: implement btf handling and map annotation

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Message-ID: <d52ff3cf-b12e-e3bf-cf04-956bdf8c3272@iogearbox.net>
Date:   Thu, 19 Jul 2018 17:43:11 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Martin KaFai Lau <kafai@...com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     dsahern@...il.com, alexei.starovoitov@...il.com,
        netdev@...r.kernel.org
Subject: Re: [PATCH iproute2 5/5] bpf: implement btf handling and map
 annotation

On 07/19/2018 02:11 AM, Martin KaFai Lau wrote:
> On Wed, Jul 18, 2018 at 11:13:37AM -0700, Jakub Kicinski wrote:
>> On Wed, 18 Jul 2018 11:33:22 +0200, Daniel Borkmann wrote:
>>> On 07/18/2018 10:42 AM, Daniel Borkmann wrote:
>>>> On 07/18/2018 02:27 AM, Jakub Kicinski wrote:  
>>>>> On Wed, 18 Jul 2018 01:31:22 +0200, Daniel Borkmann wrote:  
>>>>>>   # bpftool map dump id 386
>>>>>>    [{
>>>>>>         "key": 0,
>>>>>>         "value": {
>>>>>>             "": {
>>>>>>                 "value": 0,
>>>>>>                 "ifindex": 0,
>>>>>>                 "mac": []
>>>>>>             }
>>>>>>         }
>>>>>>     },{
>>>>>>         "key": 1,
>>>>>>         "value": {
>>>>>>             "": {
>>>>>>                 "value": 0,
>>>>>>                 "ifindex": 0,
>>>>>>                 "mac": []
>>>>>>             }
>>>>>>         }
>>>>>>     },{
>>>>>>   [...]  
>>>>>
>>>>> Ugh, the empty keys ("") look worrying, we should probably improve
>>>>> handling of anonymous structs in bpftool :S  
>>>>
>>>> Yeah agree, I think it would be nice to see a more pahole style dump
>>>> where we have types and member names along with the value as otherwise
>>>> it might be a bit confusing.  
>>>
>>> Another feature that would be super useful imho would be in the /single/
>>> map view e.g. 'bpftool map show id 123' to have a detailed BTF key+value
>>> type dump, so in addition to the basic map info we show pahole like info
>>> of the structs with length/offsets.
>>
>> That sounds good!  We could also consider adding a btf object and
>> commands to interrogate BTF types in the kernel in general..  Perhaps
>> then we could add something like bpftool btf describe map id 123.
> +1 on the btf subcommand.

That would also work, I think both might be useful to have. Former would
all sit under a single command to show map details. With 'bpftool btf' you
would also allow for a full BTF dump when a specific BTF obj id is provided?

>> Having the single map view show more information seems interesting, but
>> I wonder if it could be surprising.  Is there precedent for such
>> behaviour?
> Having everything in one page (map show id 123) could be interesting.
> One thing is the pahole-like output may be quite long?
> e.g. the member of a struct could itself be another struct.

Right, though probably fine when you want to see all information specific
to one map. Of course the 'bpftool map' list view would need to hide this
information.

> Not sure how the pahole-like output may look like in json though.

Would the existing map k/v dump have more or less the same 'issue'?