linux-kernel - Re: [PATCH bpf-next v3 3/5] bpf: add 'bpf_mptcp

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Date:   Mon, 21 Sep 2020 21:08:30 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Nicolas Rybowski <nicolas.rybowski@...sares.net>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Mat Martineau <mathew.j.martineau@...ux.intel.com>,
        Matthieu Baerts <matthieu.baerts@...sares.net>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org, mptcp@...ts.01.org
Subject: Re: [PATCH bpf-next v3 3/5] bpf: add 'bpf_mptcp_sock' structure and
 helper

On Fri, Sep 18, 2020 at 02:10:42PM +0200, Nicolas Rybowski wrote:
> +
> +BPF_CALL_1(bpf_mptcp_sock, struct sock *, sk)
> +{
> +	if (sk_fullsock(sk) && sk->sk_protocol == IPPROTO_TCP && sk_is_mptcp(sk)) {
> +		struct mptcp_subflow_context *mptcp_sfc = mptcp_subflow_ctx(sk);

Could you add !sk check here as well?
See commit 8c33dadc3e0e ("bpf: Bpf_skc_to_* casting helpers require a NULL check on sk")
It's not strictly necessary yet, but see below.

Also this new helper is not exercised from C test. Only from asm.
Could you update patch 4 with such additional logic?

> +
> +		return (unsigned long)mptcp_sfc->conn;

I think we shouldn't extend the verifier with PTR_TO_MPTCP_SOCK and similar concept anymore.
This approach doesn't scale and we have better way to handle such field access with BTF.

> +	}
> +	return (unsigned long)NULL;
> +}
> +
> +const struct bpf_func_proto bpf_mptcp_sock_proto = {
> +	.func           = bpf_mptcp_sock,
> +	.gpl_only       = false,
> +	.ret_type       = RET_PTR_TO_MPTCP_SOCK_OR_NULL,

In this particular case you can do:
+	.ret_type       = RET_PTR_TO_BTF_ID_OR_NULL,

Then bpf_mptcp_sock_convert_ctx_access() will no longer be necessary
and bpf prog will be able to access all mptcp_sock fields right away.
Will that work for your use case?