Re: [PATCH v2 bpf-next 3/4] net: netfilter: add bpf_ct_set_nat_info kfunc helper

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

On Wed, 7 Sept 2022 at 06:27, Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Mon, Sep 5, 2022 at 6:14 AM Lorenzo Bianconi <lorenzo@kernel.org> wrote:
> > +int bpf_ct_set_nat_info(struct nf_conn___init *nfct__ref,
> > +                       union nf_inet_addr *addr, __be16 *port,
> > +                       enum nf_nat_manip_type manip)
> > +{
> ...
> > @@ -437,6 +483,7 @@ BTF_ID_FLAGS(func, bpf_ct_set_timeout, KF_TRUSTED_ARGS)
> >  BTF_ID_FLAGS(func, bpf_ct_change_timeout, KF_TRUSTED_ARGS)
> >  BTF_ID_FLAGS(func, bpf_ct_set_status, KF_TRUSTED_ARGS)
> >  BTF_ID_FLAGS(func, bpf_ct_change_status, KF_TRUSTED_ARGS)
> > +BTF_ID_FLAGS(func, bpf_ct_set_nat_info)
> >  BTF_SET8_END(nf_ct_kfunc_set)
>
> Instead of __ref and patch 1 and 2 it would be better to
> change the meaning of "trusted_args".
> In this case "addr" and "port" are just as "trusted".
> They're not refcounted per verifier definition,
> but they need to be "trusted" by the helper.
> At the end the "trusted_args" flags would mean
> "this helper can assume that all pointers can be safely
> accessed without worrying about lifetime".

So you mean it only forces PTR_TO_BTF_ID to have reg->ref_obj_id > 0?

But suppose in the future you have a type that has scalars only.

struct foo { int a; int b; ... };
Just data, and this is acquired from a kfunc and released using another kfunc.
Now with this new definition you are proposing, verifier ends up
allowing PTR_TO_MEM to also be passed to such helpers for the struct
foo *.

I guess even reg->ref_obj_id check is not enough, user may also pass
PTR_TO_MEM | MEM_ALLOC which can be refcounted.

It would be easy to forget such subtle details later.

What we want to actually force here is 'please give me refcounted
PTR_TO_BTF_ID to foo'.
So maybe KF_TRUSTED_ARGS should change meaning to what you described,
and then a __btf tag should force PTR_TO_BTF_ID.
KF_TRUSTED_ARGS then just forces reg->ref_obj_id for PTR_TO_BTF_ID.