Re: [RFC PATCH v7 1/8] net_sched: Introduce eBPF based Qdisc

[Kui-Feng Lee <sinquersw@gmail.com>]

On 1/30/24 17:01, Martin KaFai Lau wrote:
> On 1/30/24 9:49 AM, Kui-Feng Lee wrote:
>>>> 2. Returning a kptr from a program and treating it as releasing the 
>>>> reference.
>>>
>>> e.g. for dequeue:
>>>
>>> struct Qdisc_ops {
>>>      /* ... */
>>>      struct sk_buff *        (*dequeue)(struct Qdisc *);
>>> };
>>>
>>>
>>> Right now the verifier should complain on check_reference_leak() if 
>>> the struct_ops bpf prog is returning a referenced kptr.
>>>
>>> Unlike an argument, the return type of a function does not have a 
>>> name to tag. It is the first case that a struct_ops bpf_prog returning a 
>>
>> We may tag the stub functions instead, right?
> 
> What is the suggestion on how to tag the return type?
> 
> I was suggesting it doesn't need to tag and it should by default require 
> a trusted ptr for the pointer returned by struct_ops. The pointer 
> argument and the return pointer of a struct_ops should be a trusted ptr.


That make sense to me. Should we also allow operators to return a null
pointer?

> 
>> Is the purpose here to return a referenced pointer from a struct_ops
>> operator without verifier complaining?
> 
> Yes, basically need to teach the verifier the kernel will do the 
> reference release.
> 
>>
>>> pointer. One idea is to assume it must be a trusted pointer 
>>> (PTR_TRUSTED) and the verifier should check it is indeed with 
>>> PTR_TRUSTED flag.
>>>
>>> May be release_reference_state() can be called to assume the kernel 
>>> will release it as long as the return pointer type is PTR_TRUSTED and 
>>> the type matches the return type of the ops. Take a look at 
>>> check_return_code(). 
>