Re: [PATCH bpf-next v4 0/4] bpf: Implement two type cast kfuncs

[Alexei Starovoitov <ast@meta.com>]

On 11/21/22 5:48 PM, John Fastabend wrote:
> Yonghong Song wrote:
>>
>>
>> On 11/20/22 6:10 PM, John Fastabend wrote:
>>> Yonghong Song wrote:
>>>> Currenty, a non-tracing bpf program typically has a single 'context' argument
>>>> with predefined uapi struct type. Following these uapi struct, user is able
>>>> to access other fields defined in uapi header. Inside the kernel, the
>>>> user-seen 'context' argument is replaced with 'kernel context' (or 'kctx'
>>>> in short) which can access more information than what uapi header provides.
>>>> To access other info not in uapi header, people typically do two things:
>>>>     (1). extend uapi to access more fields rooted from 'context'.
>>>>     (2). use bpf_probe_read_kernl() helper to read particular field based on
>>>>       kctx.
> 
> [...]
> 
>>>   From myside this allows us to pull in the dev info and from that get
>>> netns so fixes a gap we had to split into a kprobe + xdp.
>>>
>>> If we can get a pointer to the recv queue then with a few reads we
>>> get the hash, vlan, etc. (see timestapm thread)
>>
>> Thanks, John. Glad to see it is useful.
>>
>>>
>>> And then last bit is if we can get a ptr to the net ns list, plus
>>
>> Unfortunately, currently vmlinux btf does not have non-percpu global
>> variables, so net_namespace_list is not available to bpf programs.
>> But I think we could do the following with a little bit user space
>> initial involvement as a workaround.
> 
> What would you think of another kfunc, bpf_get_global_var() to fetch
> the global reference and cast it with a type? I think even if you
> had it in BTF you would still need some sort of helper otherwise
> how would you know what scope of the var should be and get it
> correct in type checker as a TRUSTED arg? I think for my use case
> UNTRUSTED is find, seeing we do it with probe_reads already, but
> getting a TRUSTED arg seems nicer given it can be known correct
> from kernel side.
> 
> I was thinking something like,
> 
>    struct net *head = bpf_get_global_var(net_namespace_list,
> 				bpf_core_type_id_kernel(struct *net));

We cannot do this as ptr_trusted, since it's an unknown cast.
The verifier cannot trust bpf prog to do the right thing.
But we can enable this buy adding export_symbol_gpl global vars to BTF.
Then they will be trusted and their types correct.
Pretty much like per-cpu variables.