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

On Tue, 22 Sep 2020 at 08:04, Martin KaFai Lau [off-list ref] 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?

  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.

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
life easier for sockmap update from iter, as mentioned in the other
email.

--
Lorenz Bauer  |  Systems Engineer
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