Date:   Sat, 6 Nov 2021 11:13:28 -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 Sat, Nov 06, 2021 at 02:43:12AM +0530, Kumar Kartikeya Dwivedi wrote:
> 
> Right now only PTR_TO_BTF_ID and PTR_TO_SOCK and scalars are supported, as you
> noted, for kfunc arguments.
> 
> So in 3/6 I move the PTR_TO_CTX block before btf_is_kernel check, that means if
> reg type is PTR_TO_CTX and it matches the argument for the program, it will use
> that, otherwise it moves to btf_is_kernel(btf) block, which checks if reg->type
> is PTR_TO_BTF_ID or one of PTR_TO_SOCK* and does struct match for those. Next, I
> punt to ptr_to_mem for the rest of the cases, which I think is problematic,
> since now you may pass PTR_TO_MEM where some kfunc wants a PTR_TO_BTF_ID.
> 
> But without bpf_func_proto, I am not sure we can decide what is expected in the
> kfunc. For something like bpf_sock_tuple, we'd want a PTR_TO_MEM, but taking in
> a PTR_TO_BTF_ID also isn't problematic since it is just data, but for a struct
> embedding pointers or other cases, it may be a problem.
> 
> For PTR_TO_CTX in kfunc case, based on my reading and testing, it will reject
> any attempts to pass anything other than PTR_TO_CTX due to btf_get_prog_ctx_type
> for that argument. So that works fine.
> 
> To me it seems like extending with some limited argument checking is necessary,
> either using tagging as you mentioned or bpf_func_proto, or some other hardcoded
> checking for now since the number of helpers needing this support is low.

Got it. The patch 3 commit log was too terse for me to comprehend.
Even with detailed explanation above it took me awhile to understand the
consequences of the patch... and 'goto ptr_to_mem' I misunderstood completely.
I think now we're on the same page :)

Agree that allowing PTR_TO_CTX into kfunc is safe to do in all cases.
Converting PTR_TO_MEM to PTR_TO_BTF_ID is also safe when kernel side 'struct foo'
contains only scalars. The patches don't have this check yet (as far as I can see).
That's the only missing piece.
With that in place 'struct bpf_sock_tuple' can be defined on the kernel side.
The bpf prog can do include "vmlinux.h" to use it to pass as PTR_TO_MEM
into kfunc. The patch 5 kernel function bpf_skb_ct_lookup can stay as-is.
So no tagging or extensions to bpf_func_proto are necessary.

The piece I'm still missing is why you need two additional *btf_struct_access.
Why do you want to restrict read access?
The bpf-tcp infra has bpf_tcp_ca_btf_struct_access() to allow-list
few safe fields for writing.
Is there a use case to write into 'struct nf_conn' from bpf prog? Probably not yet.
Then let's keep the default btf_struct_access() behavior for now.
The patch 5 will be defining bpf_xdp_ct_lookup_tcp/bpf_skb_ct_lookup_tcp
and no callbacks at all.
acquire/release are probably cleaner as explicit btf_id_list-s.
Similar to btf_id_list for PTR_TO_BTF_ID_OR_NULL vs PTR_TO_BTF_ID return type.

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