linux-kernel - Re: [PATCH bpf-next] bpf: Add document for 'dst_port' of 'struct bpf

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Date:   Thu, 20 Jan 2022 11:02:27 +0800
From:   Menglong Dong <menglong8.dong@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     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>,
        KP Singh <kpsingh@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Mengen Sun <mengensun@...cent.com>, flyingpeng@...cent.com,
        mungerjiang@...cent.com, Menglong Dong <imagedong@...cent.com>
Subject: Re: [PATCH bpf-next] bpf: Add document for 'dst_port' of 'struct bpf_sock'

Hello!

On Thu, Jan 20, 2022 at 6:03 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
[...]
>
> Looks like
>  __sk_buff->remote_port
>  bpf_sock_ops->remote_port
>  sk_msg_md->remote_port
> are doing the right thing,
> but bpf_sock->dst_port is not correct?
>
> I think it's better to fix it,
> but probably need to consolidate it with
> convert_ctx_accesses() that deals with narrow access.
> I suspect reading u8 from three flavors of 'remote_port'
> won't be correct.

What's the meaning of 'narrow access'? Do you mean to
make 'remote_port' u16? Or 'remote_port' should be made
accessible with u8? In fact, '*((u16 *)&skops->remote_port + 1)'
won't work, as it only is accessible with u32.

I can simply make 'dst_port' endianness right with what
'remote_port' do.

Thanks!
Menglong Dong


> 'dst_port' works with a narrow load, but gets endianness wrong.