Re: [RFC PATCH bpf-next v2] bpf: Relax tracing prog recursive attach rules

[Yonghong Song <yonghong.song@linux.dev>]

On 11/25/23 3:40 PM, Song Liu wrote:
> On Sat, Nov 25, 2023 at 11:55 AM Yonghong Song <yonghong.song@linux.dev> wrote:
>>
>> On 11/24/23 4:16 PM, Dmitry Dolgov wrote:
>>>> On Thu, Nov 23, 2023 at 11:24:34PM -0800, Song Liu wrote:
>>>>> Following the corresponding discussion [1], the reason for that is to
>>>>> avoid tracing progs call cycles without introducing more complex
>>>>> solutions. Relax "no same type" requirement to "no progs that are
>>>>> already an attach target themselves" for the tracing type. In this way
>>>>> only a standalone tracing program (without any other progs attached to
>>>>> it) could be attached to another one, and no cycle could be formed. To
>>>> If prog B attached to prog A, and prog C attached to prog B, then we
>>>> detach B. At this point, can we re-attach B to A?
>>> Nope, with the proposed changes it still wouldn't be possible to
>>> reattach B to A (if we're talking about tracing progs of course),
>>> because this time B is an attachment target on its own.
>> IIUC, the 'prog B attached to prog A, and prog C attached to prog B'
>> not really possible.
>>      After prog B attached to prog A, we have
>>        prog B follower_cnt = 1
>>        prog A attach_depth = 1
> I think prog A has follower_cnt=1, while prog B has follow_cnt=0, no?

I checked bpf_tracing_prog_attach() again. Yes, I made a dumb mistake
and indeed
    After prog B attached to prog A, we should have
       prog A follower_cnt = 1 and prog B attach_depth = 1

>
> Thanks,
> Song
>
>>      Then prog C wants to attach to prog B,
>>        since we have prog B follower_cnt = 1, then attaching will fail.
>>
>> If we do have A <- B <- C chain by
>>      first prog C attached to prog B, and then prog B attached to A
>>      now we have
>>       prog B/C follower_cnt = 1
>>       prog A/B attach_depth = 1
>> after detaching B from A, we have
>>       prog B follower_cnt = 0
>>       prog A attach_depth = 0
>>
>> In this particular case, prog B attaching to prog A should succeed
>> since prog B follower_cnt = 0.
>>
>> Did I miss anything?
>>
>> In the commit message, 'falcosecurity libs project' is mentioned as a use
>> case for chained fentry/fexit bpf programs. I think you should expand the
>> use case in more details. It is possible with use case description, people
>> might find better/alternative solutions for your use case.
>>
>> Also, if you can have a test case to exercise your commit logic,
>> it will be even better.

[...]