netdev - Re: [PATCH RFC bpf-next v1 0/6] Introduce unstable CT lookup helpers

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Date:   Fri, 5 Nov 2021 13:49:08 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        Maxim Mikityanskiy <maximmi@...dia.com>,
        Florian Westphal <fw@...len.de>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        netdev@...r.kernel.org, netfilter-devel@...r.kernel.org
Subject: Re: [PATCH RFC bpf-next v1 0/6] Introduce unstable CT lookup helpers

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.