On Thu, Nov 04, 2021 at 06:25:03PM +0530, Kumar Kartikeya Dwivedi wrote: > On Wed, Nov 03, 2021 at 04:46:42AM IST, Alexei Starovoitov wrote: > > On Sat, Oct 30, 2021 at 08:16:03PM +0530, Kumar Kartikeya Dwivedi wrote: > > > This series adds unstable conntrack lookup helpers using BPF kfunc support. The > > > patch adding the lookup helper is based off of Maxim's recent patch to aid in > > > rebasing their series on top of this, all adjusted to work with kfunc support > > > [0]. > > > > > > This is an RFC series, as I'm unsure whether the reference tracking for > > > PTR_TO_BTF_ID will be accepted. > > > > Yes. The patches look good overall. > > Please don't do __BPF_RET_TYPE_MAX signalling. It's an ambiguous name. > > _MAX is typically used for a different purpose. Just give it an explicit name. > > I don't fully understand why that skip is needed though. > > I needed a sentinel to skip return type checking (otherwise check that return > type and prototype match) since existing kfunc don't have a > get_kfunc_return_type callback, but if we add bpf_func_proto support to kfunc > then we can probably convert existing kfuncs to that as well and skip all this > logic. Mostly needed it for RET_PTR_TO_BTF_ID_OR_NULL. So it's just to special case r0=PTR_TO_BTF_ID_OR_NULL instead of PTR_TO_BTF_ID that it's doing by default now? Then could you use a btf_id list to whitelist all such funcs that needs _OR_NULL variant and just do a search in that list in check_kfunc_call() ? Instead of adding get_kfunc_return_type() callback. > Extending to support bpf_func_proto seemed like a bit of work so I wanted to get > some feedback first on all this, before working on it. No need to hack into bpf_func_proto. All kernel funcs have BTF. It's all we need. The _OR_NULL part we will eventually be able to express with btf_tag when it's supported by both gcc and clang. > > > Also, I want to understand whether it would make sense to introduce > > > check_helper_call style bpf_func_proto based argument checking for kfuncs, or > > > continue with how it is right now, since it doesn't seem correct that PTR_TO_MEM > > > can be passed where PTR_TO_BTF_ID may be expected. Only PTR_TO_CTX is enforced. > > > > Do we really allow to pass PTR_TO_MEM argument into a function that expects PTR_TO_BTF_ID ? > > Sorry, that's poorly phrased. Current kfunc doesn't support PTR_TO_MEM. I meant > it would be allowed now, with the way I implemented things, but there also isn't > a way to signal whether PTR_TO_BTF_ID is expected (hence the question about > bpf_func_proto). I did not understand why that was not done originally (maybe it > was lack of usecase). PTR_TO_CTX works because the type is matched with prog > type, so you can't pass something else there. For other cases the type of > register is considered. Right. btf_check_kfunc_arg_match doesn't allow ptr_to_mem yet. There is no signalling needed. All args passed by the program into kfunc have to be either exact PTR_TO_BTF_ID or conversions from PTR_TO_SOCK*. Passing rX=PTR_TO_CTX into kfunc should not work. If I'm reading the code correctly it's not allowed. I'm not sure why you're saying it can be done. It's possible to pass PTR_TO_CTX into another bpf prog's global function. The same btf_check_func_arg_match() helper checks both cases (global funcs and kfuncs). Maybe that's where the confusion comes from? Same with if (ptr_to_mem_ok). It's only for passing PTR_TO_MEM into bpf prog's global function. We can extend the verifier and allow PTR_TO_MEM into kfunc that has 'long *' prototype, for example. But it doesn't sound like the use case you have in mind.
