Re: [1/2 bpf-next] bpf: expose net_device from xdp for metadata

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

On 10/11/2022 18.02, John Fastabend wrote:
> Toke Høiland-Jørgensen wrote:
>> John Fastabend <john.fastabend@gmail.com> writes:
>>
>>> Yonghong Song wrote:
>>>>
>>>>
>>>> On 11/9/22 1:52 PM, John Fastabend wrote:
>>>>> Allow xdp progs to read the net_device structure. Its useful to extract
>>>>> info from the dev itself. Currently, our tracing tooling uses kprobes
>>>>> to capture statistics and information about running net devices. We use
>>>>> kprobes instead of other hooks tc/xdp because we need to collect
>>>>> information about the interface not exposed through the xdp_md structures.
>>>>> This has some down sides that we want to avoid by moving these into the
>>>>> XDP hook itself. First, placing the kprobes in a generic function in
>>>>> the kernel is after XDP so we miss redirects and such done by the
>>>>> XDP networking program. And its needless overhead because we are
>>>>> already paying the cost for calling the XDP program, calling yet
>>>>> another prog is a waste. Better to do everything in one hook from
>>>>> performance side.
>>>>>
>>>>> Of course we could one-off each one of these fields, but that would
>>>>> explode the xdp_md struct and then require writing convert_ctx_access
>>>>> writers for each field. By using BTF we avoid writing field specific
>>>>> convertion logic, BTF just knows how to read the fields, we don't
>>>>> have to add many fields to xdp_md, and I don't have to get every
>>>>> field we will use in the future correct.
>>>>>
>>>>> For reference current examples in our code base use the ifindex,
>>>>> ifname, qdisc stats, net_ns fields, among others. With this
>>>>> patch we can now do the following,
>>>>>
>>>>>           dev = ctx->rx_dev;
>>>>>           net = dev->nd_net.net;
>>>>>
>>>>> 	uid.ifindex = dev->ifindex;
>>>>> 	memcpy(uid.ifname, dev->ifname, NAME);
>>>>>           if (net)
>>>>> 		uid.inum = net->ns.inum;
>>>>>
>>>>> to report the name, index and ns.inum which identifies an
>>>>> interface in our system.
>>>>
>>>> In
>>>> https://lore.kernel.org/bpf/ad15b398-9069-4a0e-48cb-4bb651ec3088@meta.com/
>>>> Namhyung Kim wanted to access new perf data with a helper.
>>>> I proposed a helper bpf_get_kern_ctx() which will get
>>>> the kernel ctx struct from which the actual perf data
>>>> can be retrieved. The interface looks like
>>>> 	void *bpf_get_kern_ctx(void *)
>>>> the input parameter needs to be a PTR_TO_CTX and
>>>> the verifer is able to return the corresponding kernel
>>>> ctx struct based on program type.
>>>>
>>>> The following is really hacked demonstration with
>>>> some of change coming from my bpf_rcu_read_lock()
>>>> patch set https://lore.kernel.org/bpf/20221109211944.3213817-1-yhs@fb.com/
>>>>
>>>> I modified your test to utilize the
>>>> bpf_get_kern_ctx() helper in your test_xdp_md.c.
>>>>
>>>> With this single helper, we can cover the above perf
>>>> data use case and your use case and maybe others
>>>> to avoid new UAPI changes.
>>>
>>> hmm I like the idea of just accessing the xdp_buff directly
>>> instead of adding more fields. I'm less convinced of the
>>> kfunc approach. What about a terminating field *self in the
>>> xdp_md. Then we can use existing convert_ctx_access to make
>>> it BPF inlined and no verifier changes needed.
>>>
>>> Something like this quickly typed up and not compiled, but
>>> I think shows what I'm thinking.
>>>
>>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>>> index 94659f6b3395..10ebd90d6677 100644
>>> --- a/include/uapi/linux/bpf.h
>>> +++ b/include/uapi/linux/bpf.h
>>> @@ -6123,6 +6123,10 @@ struct xdp_md {
>>>          __u32 rx_queue_index;  /* rxq->queue_index  */
>>>   
>>>          __u32 egress_ifindex;  /* txq->dev->ifindex */
>>> +       /* Last xdp_md entry, for new types add directly to xdp_buff and use
>>> +        * BTF access. Reading this gives BTF access to xdp_buff.
>>> +        */
>>> +       __bpf_md_ptr(struct xdp_buff *, self);
>>>   };
>>
>> xdp_md is UAPI; I really don't think it's a good idea to add "unstable"
>> BTF fields like this to it, that's just going to confuse people. Tying
>> this to a kfunc for conversion is more consistent with the whole "kfunc
>> and BTF are its own thing" expectation.
> 
> hmm from my side self here would be stable. Whats behind it is not,
> but that seems fine to me.  Doing `ctx->self` feels more natural imo
> then doing a call. A bunch more work but could do btf casts maybe
> with annotations. I'm not sure its worth it though because only reason
> I can think to do this would be for this self reference from ctx.
> 
>     struct xdp_buff *xdp = __btf (struct xdp_buff *)ctx;
> 
> C++ has 'this' as well but thats confusing from C side. Could have
> a common syntax to do 'ctx->this' to get the pointer in BTF
> format.
> 
> Maybe see what Yonghong thinks.
> 
>>
>> The kfunc doesn't actually have to execute any instructions either, it
>> can just be collapsed into a type conversion to BTF inside the verifier,
>> no?
> 
> Agree either implementation can be made that same underneath its just
> a style question. I can probably do either but using the ctx keeps
> the existing machinery to go through is_valid_access and so on.
> 

What kind of access does the BPF-prog obtain with these different
proposals, e.g. read-only access to xdp_buff or also write access?

--Jesper