Re: [PATCH bpf-next 6/6] selftests/bpf: test detaching struct_ops links.

[Martin KaFai Lau <martin.lau@linux.dev>]

On 5/3/24 11:34 AM, Kui-Feng Lee wrote:
> 
> 
> On 5/2/24 11:15, Martin KaFai Lau wrote:
>> On 4/29/24 2:36 PM, Kui-Feng Lee wrote:
>>> @@ -572,6 +576,12 @@ static int bpf_dummy_reg(void *kdata)
>>>       if (ops->test_2)
>>>           ops->test_2(4, ops->data);
>>> +    if (ops->do_unreg) {
>>> +        rcu_read_lock();
>>> +        bpf_struct_ops_kvalue_unreg(kdata);
>>
>> Instead of unreg() immediately before the reg() has returned, the test should 
>> reflect more on how the subsystem can use it in practice. The subsystem does 
>> not do unreg() during reg().
>>
>> It also needs to test a case when the link is created and successfully 
>> registered to the subsystem. The user space does BPF_LINK_DETACH first and  >> then the subsystem does link->ops->detach() by itself later.

> 
> agree
> 
>>
>> It can create a kfunc in bpf_testmod.c to trigger the subsystem to do 
>> link->ops->detach(). The kfunc can be called by a SEC("syscall") bpf prog 
>> which is run by bpf_prog_test_run_opts(). The test_progs can then decide on 
>> the timing when to do link->ops->detach() to test different cases.
> 
> What is the purpose of this part?
> If it goes through link->ops->detach(), it should work just like to call
> bpf_link_detach() twice on the same link from the user space. Do you
> want to make sure detaching a link twice work?

It is not quite what I meant and apparently link detach twice on the same valid 
(i.e. refcnt non zero) link won't work.

Anyhow, the idea is to show how the racing case may work in patch 3 (when 
userspace tries to detach and the subsystem tries to detach/unreg itself also). 
I was suggesting the kfunc idea such that the test_progs can have better control 
on the timing on when to ask the subsystem to unreg/detach itself instead of 
having to do the unreg() during the reg() as in patch 6 here. If kfunc does not 
make sense and there is a better way to do this, feel free to ignore.