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

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Date:   Wed, 19 Jan 2022 20:17:54 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Menglong Dong <menglong8.dong@...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'

On Thu, Jan 20, 2022 at 11:02:27AM +0800, Menglong Dong wrote:
> 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.

u8 access to remote_port won't pass the verifier,
but u8 access to dst_port will.
Though it will return incorrect data.
See how convert_ctx_accesses() handles narrow loads.
I think we need to generalize it for different endian fields.