Re: [PATCH bpf-next 2/7] bpf: introduce bpf subcommand BPF_PERF_EVENT_QUERY

[Yonghong Song <yhs@fb.com>]

On 5/16/18 4:27 AM, Peter Zijlstra wrote:
> On Tue, May 15, 2018 at 04:45:16PM -0700, Yonghong Song wrote:
>> Currently, suppose a userspace application has loaded a bpf program
>> and attached it to a tracepoint/kprobe/uprobe, and a bpf
>> introspection tool, e.g., bpftool, wants to show which bpf program
>> is attached to which tracepoint/kprobe/uprobe. Such attachment
>> information will be really useful to understand the overall bpf
>> deployment in the system.
>>
>> There is a name field (16 bytes) for each program, which could
>> be used to encode the attachment point. There are some drawbacks
>> for this approaches. First, bpftool user (e.g., an admin) may not
>> really understand the association between the name and the
>> attachment point. Second, if one program is attached to multiple
>> places, encoding a proper name which can imply all these
>> attachments becomes difficult.
>>
>> This patch introduces a new bpf subcommand BPF_PERF_EVENT_QUERY.
>> Given a pid and fd, if the <pid, fd> is associated with a
>> tracepoint/kprobe/uprobea perf event, BPF_PERF_EVENT_QUERY will return
>>     . prog_id
>>     . tracepoint name, or
>>     . k[ret]probe funcname + offset or kernel addr, or
>>     . u[ret]probe filename + offset
>> to the userspace.
>> The user can use "bpftool prog" to find more information about
>> bpf program itself with prog_id.
>>
>> Signed-off-by: Yonghong Song <yhs@fb.com>
>> ---
>>   include/linux/trace_events.h |  15 ++++++
>>   include/uapi/linux/bpf.h     |  25 ++++++++++
>>   kernel/bpf/syscall.c         | 113 +++++++++++++++++++++++++++++++++++++++++++
>>   kernel/trace/bpf_trace.c     |  53 ++++++++++++++++++++
>>   kernel/trace/trace_kprobe.c  |  29 +++++++++++
>>   kernel/trace/trace_uprobe.c  |  22 +++++++++
>>   6 files changed, 257 insertions(+)
> 
> Why is the command called *_PERF_EVENT_* ? Are there not a lot of !perf
> places to attach BPF proglets?

Just gave a complete picture, the below are major places to attach
BPF programs:
    . perf based (through perf ioctl)
    . raw tracepoint based (through bpf interface)

    . netlink interface for tc, xdp, tunneling
    . setsockopt for socket filters
    . cgroup based (bpf attachment subcommand)
      mostly networking and io devices
    . some other networking socket related (sk_skb stream/parser/verdict,
      sk_msg verdict) through bpf attachment subcommand.

Currently, for cgroup based attachment, we have BPF_PROG_QUERY with 
input cgroup file descriptor. For other networking based queries, we
may need to enumerate tc filters, networking devices, open sockets, etc.
to get the attachment information.

So to have one BPF_QUERY command line may be too complex to
cover all cases.

But you are right that BPF_PERF_EVENT_QUERY name is too narrow since
it should be used for other (pid, fd) based queries as well (e.g., 
socket, or other potential uses in the future).

How about the subcommand name BPF_TASK_FD_QUERY and make 
bpf_attr.task_fd_query extensible?

Thanks!