Subject: Re: [PATCH v3 3/6] bpf: add BPF_PROG_ATTACH and BPF_PROG_DETACH commands

From: Daniel Borkmann <[email-masked]>
Date: Mon, 05 Sep 2016 15:56:29 +0200

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

>> #define BPF_PROG_ATTACH_LAST_FIELD attach_type
>> #define BPF_PROG_DETACH_LAST_FIELD BPF_PROG_ATTACH_LAST_FIELD
>
> ...
>
>>
>> Correct way would be to:
>>
>> 	if (CHECK_ATTR(BPF_PROG_ATTACH))
>> 		return -EINVAL;
>
> Will add - thanks!
>
>
> Daniel
>