Re: [PATCH v3 bpf-next 02/11] bpf: Enable bpf_skc_to_* sock casting helper to networking prog type

[Lorenz Bauer <lmb@cloudflare.com>]

On Tue, 22 Sep 2020 at 19:26, Martin KaFai Lau <kafai@fb.com> wrote:
>
> On Tue, Sep 22, 2020 at 10:46:41AM +0100, Lorenz Bauer wrote:
> > On Tue, 22 Sep 2020 at 08:04, Martin KaFai Lau <kafai@fb.com> wrote:
> > >
> > > There is a constant need to add more fields into the bpf_tcp_sock
> > > for the bpf programs running at tc, sock_ops...etc.
> > >
> > > A current workaround could be to use bpf_probe_read_kernel().  However,
> > > other than making another helper call for reading each field and missing
> > > CO-RE, it is also not as intuitive to use as directly reading
> > > "tp->lsndtime" for example.  While already having perfmon cap to do
> > > bpf_probe_read_kernel(), it will be much easier if the bpf prog can
> > > directly read from the tcp_sock.
> > >
> > > This patch tries to do that by using the existing casting-helpers
> > > bpf_skc_to_*() whose func_proto returns a btf_id.  For example, the
> > > func_proto of bpf_skc_to_tcp_sock returns the btf_id of the
> > > kernel "struct tcp_sock".
> > >
> > > These helpers are also added to is_ptr_cast_function().
> > > It ensures the returning reg (BPF_REF_0) will also carries the ref_obj_id.
> > > That will keep the ref-tracking works properly.
> > >
> > > The bpf_skc_to_* helpers are made available to most of the bpf prog
> > > types in filter.c. They are limited by perfmon cap.
> > >
> > > This patch adds a ARG_PTR_TO_BTF_ID_SOCK_COMMON.  The helper accepting
> > > this arg can accept a btf-id-ptr (PTR_TO_BTF_ID + &btf_sock_ids[BTF_SOCK_TYPE_SOCK_COMMON])
> > > or a legacy-ctx-convert-skc-ptr (PTR_TO_SOCK_COMMON).  The bpf_skc_to_*()
> > > helpers are changed to take ARG_PTR_TO_BTF_ID_SOCK_COMMON such that
> > > they will accept pointer obtained from skb->sk.
> > >
> > > PTR_TO_*_OR_NULL is not accepted as an ARG_PTR_TO_BTF_ID_SOCK_COMMON
> > > at verification time.  All PTR_TO_*_OR_NULL reg has to do a NULL check
> > > first before passing into the helper or else the bpf prog will be
> > > rejected by the verifier.
> > >
> > > [ ARG_PTR_TO_SOCK_COMMON_OR_NULL was attempted earlier.  The _OR_NULL was
> > >   needed because the PTR_TO_BTF_ID could be NULL but note that a could be NULL
> > >   PTR_TO_BTF_ID is not a scalar NULL to the verifier.  "_OR_NULL" implicitly
> > >   gives an expectation that the helper can take a scalar NULL which does
> > >   not make sense in most (except one) helpers.  Passing scalar NULL
> > >   should be rejected at the verification time.
> >
> > What is the benefit of requiring a !sk check from the user if all of
> > the helpers know how to deal with a NULL pointer?
> I don't see a reason why the verifier should not reject an incorrect
> program at load time if it can.
>
> >
> > >
> > >   Thus, this patch uses ARG_PTR_TO_BTF_ID_SOCK_COMMON to specify that the
> > >   helper can take both the btf-id ptr or the legacy PTR_TO_SOCK_COMMON but
> > >   not scalar NULL.  It requires the func_proto to explicitly specify the
> > >   arg_btf_id such that there is a very clear expectation that the helper
> > >   can handle a NULL PTR_TO_BTF_ID. ]
> >
> > I think ARG_PTR_TO_BTF_ID_SOCK_COMMON is actually a misnomer, since
> > nothing enforces that arg_btf_id is actually an ID for sock common.
> > This is where ARG_PTR_TO_SOCK_COMMON_OR_NULL is much easier to
> > understand, even though it's more permissive than it has to be. It
> > communicates very clearly what values the argument can take.
> _OR_NULL is incorrect which implies a scalar NULL as mentioned in
> this commit message.  From verifier pov, _OR_NULL can take
> a scalar NULL.

Yes, I know. I'm saying that the distinction between scalar NULL and
runtime NULL only makes sense after you understand how BTF pointers
are implemented. It only clicked for me after I read the support code
in the JIT that Yonghong pointed out. Should everybody that writes a
helper need to read the JIT? In my opinion we shouldn't. I guess I
don't even care about the verifier rejecting scalar NULL or not, I'd
just like the types to have a name that conveys their NULLness.

>
> >
> > If you're set on ARG_PTR_TO_BTF_ID_SOCK_COMMON I'd suggest forcing the
> > btf_id in struct bpf_reg_types. This avoids the weird case where the
> > btf_id doesn't actually point at sock_common, and it also makes my
> I have considered the bpf_reg_types option.  I prefer all
> arg info (arg_type and arg_btf_id) stay in the same one
> place (i.e. func_proto) as much as possible for now
> instead of introducing another place to specify/override it
> which then depends on a particular arg_type that some arg_type may be
> in func_proto while some may be in other places.

In my opinion that ship sailed when we started aliasing arg_type to
multiple reg_type, but OK.

>
> The arg_btf_id can be checked in check_btf_id_ok() if it would be a
> big concern that it might slip through the review but I think the
> chance is pretty low.

Why increase the burden on human reviewers? Why add code to check an
invariant that we could get rid of in the first place?

-- 
Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

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