On Wed, 23 Sep 2020 at 18:06, Martin KaFai Lau <kafai@xxxxxx> wrote: > > On Wed, Sep 23, 2020 at 10:27:27AM +0100, Lorenz Bauer wrote: > > On Tue, 22 Sep 2020 at 19:26, Martin KaFai Lau <kafai@xxxxxx> 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@xxxxxx> 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. > It is not only about verifier and/or JIT, not sure why it is related to > JIT also. > > For some helpers, explicitly passing NULL may make sense. > e.g. bpf_sk_assign(ctx, NULL, 0) makes sense. > > For most helpers, the bpf prog is wrong for sure, for example > in sockmap, what does bpf_map_update_elem(sock_map, key, NULL, 0) > mean? I would expect a delete from the sock_map if the verifier > accepted it. > > > > > > > > > > > > > > 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? > Lets take the scalar NULL example that requires to read multiple > pieces of codes in different places (verifier, JIT...etc.). > As you also mentioned, yes, it may be easy for a few people. > However, for most others, having some obvious things in the same place is > easier to review. > > I think we have to agree we disagree on this one implementation details > which I think it has been over-thought (and time also). > > If you insist that should go into bpf_reg_types (i.e. compatible->btf_id), > I can do that in v4 and then add another check in another place to > ensure "!compatible->btf_id" as in v2. No, I don't insist. I was hoping I could convince you, but alas :) -- Lorenz Bauer | Systems Engineer 6th Floor, County Hall/The Riverside Building, SE1 7PB, UK www.cloudflare.com
