netdev - Re: [PATCH net-next v5 1/3] net: flow_dissector: extend bpf flow dissector support with vnet hdr

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Message-ID: <20210612011342.2aywi36zfe6a5qh5@kafai-mbp.dhcp.thefacebook.com>
Date:   Fri, 11 Jun 2021 18:13:42 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Tanner Love <tannerlove.kernel@...il.com>
CC:     <netdev@...r.kernel.org>, "David S . Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Petar Penkov <ppenkov@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Tanner Love <tannerlove@...gle.com>,
        Stanislav Fomichev <sdf@...gle.com>
Subject: Re: [PATCH net-next v5 1/3] net: flow_dissector: extend bpf flow
 dissector support with vnet hdr

On Fri, Jun 11, 2021 at 02:07:10PM -0700, Tanner Love wrote:
> > A nit. It is a good chance to move the new BTF_ID_LIST_SINGLE
> > and most of the check_flow_keys_access() to filter.c.
> > Take a look at check_sock_access().
> >
> 
> It's not clear to me why it's preferable to move most of the
> check_flow_keys_access() to filter.c. In particular, the part of
> your comment that I don't understand is the "most of" part. Why
> would we want to separate the flow-keys-access-checking logic
> into two separate functions? Thanks
Right, actually, the whole function can be moved.
I found it easier to follow from flow_dissector_is_valid_access()
to flow_keys's access check without jumping around between two
different files.

> 
> Additionally, it seems that we don't actually need the
> BTF_ID_LIST_SINGLE. Can we not, instead, set
> regs[value_regno].btf_id to
> btf_find_by_name_kind(btf_vmlinux, "virtio_net_hdr", BTF_KIND_STRUCT)
> ? (And we'll check that that value is not <= 0.)
BTF_ID_LIST_SINGLE is resolved during compilation.
btf_find_by_name_kind() will be repeatedly finding the btf_id during runtime.
It is not like a killer but still unnecessary.