netdev - Re: [PATCH net-next v2 1/2] libbpf: parse maps sections of varying size

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Date:   Wed, 4 Oct 2017 12:54:15 -0700
From:   Alexei Starovoitov <ast@...com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Craig Gallek <kraigatgoog@...il.com>
CC:     Jesper Dangaard Brouer <brouer@...hat.com>,
        "David S . Miller" <davem@...emloft.net>,
        Chonggang Li <chonggangli@...gle.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v2 1/2] libbpf: parse maps sections of varying
 size

On 10/4/17 12:27 PM, Daniel Borkmann wrote:
> On 10/04/2017 03:59 PM, Craig Gallek wrote:
>> On Tue, Oct 3, 2017 at 10:39 AM, Daniel Borkmann
>> <daniel@...earbox.net> wrote:
>>> On 10/03/2017 01:07 AM, Alexei Starovoitov wrote:
>>>> On 10/2/17 9:41 AM, Craig Gallek wrote:
>>>>>
>>>>> +    /* Assume equally sized map definitions */
>>>>> +    map_def_sz = data->d_size / nr_maps;
>>>>> +    if (!data->d_size || (data->d_size % nr_maps) != 0) {
>>>>> +        pr_warning("unable to determine map definition size "
>>>>> +               "section %s, %d maps in %zd bytes\n",
>>>>> +               obj->path, nr_maps, data->d_size);
>>>>> +        return -EINVAL;
>>>>> +    }
>>>>
>>>> this approach is not as flexible as done by samples/bpf/bpf_load.c
>>>> where it looks at every map independently by walking symtab,
>>>> but I guess it's ok.
>>>
>>> Regarding different map spec structs in a single prog: unless
>>> we have a good use case why we would need it (and I'm not aware
>>> of anything in particular), I would just go with a fixed size.
>>> I did kind of similar sanity checks in bpf_fetch_maps_end() in
>>> iproute2 loader as well.
>>
>> Split vote?  I'm happy to try to make this work with varying sizes,
>> but I agree the usefulness seems low.  It would imply building map
>> sections with different definition structures and we would then need a
>> way to differentiate them.  If the goal is to allow for different map
>> definition structures, I don't think size alone is a sufficient
>> differentiator.
>
> They would still all need to go to maps section, but you would end
> up going with different structs for map specs, where they all would
> need to overlap for the members in order for this to work. E.g. you
> would have maps of both structs sitting in map section of a single
> prog ...
>
> struct bpf_map_spec_v1 {
>     __u32 type;
>     __u32 size_key;
>     __u32 size_value;
>     __u32 max_elem;
> };
>
> struct bpf_map_spec_v2 {
>     __u32 type;
>     __u32 size_key;
>     __u32 size_value;
>     __u32 max_elem;
>     __u32 flags;
> };
>
> ... rather than just using single spec everywhere consistently, and
> have the members zeroed that are unused or such, so if there's no
> real use-case for different structs (cannot think of any right now),
> lets not add complexity for it until it's really required.

makes sense to me :)