netdev - Re: [PATCH v3 3/6] bpf: add BPF_PROG_ATTACH and BPF_PROG

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Date:   Mon, 5 Sep 2016 16:09:38 +0200
From:   Daniel Mack <daniel@...que.org>
To:     Daniel Borkmann <daniel@...earbox.net>, htejun@...com, ast@...com
Cc:     davem@...emloft.net, kafai@...com, fw@...len.de,
        pablo@...filter.org, harald@...hat.com, netdev@...r.kernel.org,
        sargun@...gun.me
Subject: Re: [PATCH v3 3/6] bpf: add BPF_PROG_ATTACH and BPF_PROG_DETACH
 commands

On 09/05/2016 03:56 PM, Daniel Borkmann wrote:
> On 09/05/2016 02:54 PM, Daniel Mack wrote:
>> On 08/30/2016 01:00 AM, Daniel Borkmann wrote:
>>> On 08/26/2016 09:58 PM, Daniel Mack wrote:
>>
>>>>    enum bpf_map_type {
>>>> @@ -147,6 +149,13 @@ union bpf_attr {
>>>>    		__aligned_u64	pathname;
>>>>    		__u32		bpf_fd;
>>>>    	};
>>>> +
>>>> +	struct { /* anonymous struct used by BPF_PROG_ATTACH/DETACH commands */
>>>> +		__u32		target_fd;	/* container object to attach to */
>>>> +		__u32		attach_bpf_fd;	/* eBPF program to attach */
>>>> +		__u32		attach_type;	/* BPF_ATTACH_TYPE_* */
>>>> +		__u64		attach_flags;
>>>
>>> Could we just do ...
>>>
>>> __u32 dst_fd;
>>> __u32 src_fd;
>>> __u32 attach_type;
>>>
>>> ... and leave flags out, since unused anyway? Also see below.
>>
>> I'd really like to keep the flags, even if they're unused right now.
>> This only adds 8 bytes during the syscall operation, so it doesn't harm.
>> However, we cannot change the userspace API after the fact, and who
>> knows what this (rather generic) interface will be used for later on.
> 
> With the below suggestion added, then flags doesn't need to be
> added currently as it can be done safely at a later point in time
> with respecting old binaries. See also the syscall handling code
> in kernel/bpf/syscall.c +825 and the CHECK_ATTR() macro. The
> underlying idea of this was taken from perf_event_open() syscall
> back then, see [1] for a summary.
> 
>    [1] https://lkml.org/lkml/2014/8/26/116

Yes, I know that's possible, and I like the idea, but I don't think any
new interface should come without flags really, as flags are something
that will most certainly be needed at some point anyway. I didn't have
them in my first shot, but Alexei pointed out that they should be added,
and I agree.

Also, this optimization wouldn't make the transported struct payload any
smaller anyway, because the member of that union used by BPF_PROG_LOAD
is still by far the biggest.

I really don't think it's worth sparing 8 bytes here and then do the
binary compat dance after flags are added, for no real gain.



Thanks,
Daniel