Subject: Re: [PATCH v5 tip 1/7] bpf: make internal bpf API independent of CONFIG_BPF_SYSCALL ifdefs

From: Daniel Borkmann <[email-masked]>
Date: Mon, 02 Mar 2015 13:26:17 +0100

On 03/02/2015 12:51 PM, Masami Hiramatsu wrote:
> (2015/03/02 20:10), Daniel Borkmann wrote:
>> On 03/02/2015 11:53 AM, Masami Hiramatsu wrote:
>> ...
>>> Hmm, it seems that this still doesn't hide some APIs which is provided
>>> only when CONFIG_BPF_SYSCALL. For example bpf_register_map_type etc.
>>> I think all those APIs should be hidden in #ifdef or at least be commented
>>> so that the user doesn't refer that without the kconfig.
>>> (I don't think we need to provide dummy functions for those APIs,
>>>    but it's better to clarify which API we can use with which kconfig)
>>
>> Well, currently all possible map types (hash table, array map) that
>> would actually call into bpf_register_map_type() are only built when
>> CONFIG_BPF_SYSCALL is enabled (see kernel/bpf/Makefile). I don't think
>> new map additions should be added that are not under kernel/bpf/ and/or
>> enabled outside the CONFIG_BPF_SYSCALL, as it should be considered
>> part of the eBPF core code.
>>
>> The difference here (this patch) is simply that we don't want users to
>> build ifdef spaghetti constructs in user code, so the API that is
>> actually used by eBPF _users_ is being properly ifdef'ed in the headers.
>>
>> So, I don't think this is a big problem, but we could move these bits
>> under the ifdef CONFIG_BPF_SYSCALL w/o providing a dummy in the else part.
>> I can do that outside of the scope of this set.
> 
> Or, maybe we'd better move them into new include/linux/bpf_prog.h which
> includes basic include/linux/bpf.h. Then, user can include the bpf_prog.h
> instead of bpf.h. Also, we can check CONFIG_BPF_SYSCAL=y at the top of
> bpf_prog.h. This makes things clearer :)

I'm preferring the 1st variant, though. We have currently two native eBPF
users, that is, socket filters and tc's cls_bpf (queued in net-next) and
looking at the code/API usage, it's really not that hard, where it would
justify to move this to an extra header file, really.

I'm cooking a patch for net-next right now with the first variant (which is
on top of this patch that resides in net-next as-is as well).

Thanks,
Daniel