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

From: Martin KaFai Lau <[email-masked]>
Date: Tue, 30 Jan 2024 17:01:14 -0800

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.

> 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().