netdev - Re: [PATCH bpf-next 1/2] bpf: Make dst_port field in struct bpf

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Date:   Fri, 28 Jan 2022 10:17:19 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jakub Sitnicki <jakub@...udflare.com>
Cc:     bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Menglong Dong <imagedong@...cent.com>,
        Martin KaFai Lau <kafai@...com>
Subject: Re: [PATCH bpf-next 1/2] bpf: Make dst_port field in struct bpf_sock
 16-bit wide

On Thu, Jan 27, 2022 at 9:24 AM Jakub Sitnicki <jakub@...udflare.com> wrote:
>
> Menglong Dong reports that the documentation for the dst_port field in
> struct bpf_sock is inaccurate and confusing. From the BPF program PoV, the
> field is a zero-padded 16-bit integer in network byte order. The value
> appears to the BPF user as if laid out in memory as so:
>
>   offsetof(struct bpf_sock, dst_port) + 0  <port MSB>
>                                       + 8  <port LSB>
>                                       +16  0x00
>                                       +24  0x00
>
> 32-, 16-, and 8-bit wide loads from the field are all allowed, but only if
> the offset into the field is 0.
>
> 32-bit wide loads from dst_port are especially confusing. The loaded value,
> after converting to host byte order with bpf_ntohl(dst_port), contains the
> port number in the upper 16-bits.
>
> Remove the confusion by splitting the field into two 16-bit fields. For
> backward compatibility, allow 32-bit wide loads from offsetof(struct
> bpf_sock, dst_port).
>
> While at it, allow loads 8-bit loads at offset [0] and [1] from dst_port.
>
> Reported-by: Menglong Dong <imagedong@...cent.com>
> Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
> ---
>  include/uapi/linux/bpf.h | 3 ++-
>  net/core/filter.c        | 9 ++++++++-
>  2 files changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 4a2f7041ebae..027e84b18b51 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -5574,7 +5574,8 @@ struct bpf_sock {
>         __u32 src_ip4;
>         __u32 src_ip6[4];
>         __u32 src_port;         /* host byte order */
> -       __u32 dst_port;         /* network byte order */
> +       __be16 dst_port;        /* network byte order */
> +       __u16 zero_padding;

I was wondering can we do '__u16 :16' here ?

Should we do the same for bpf_sk_lookup->remote_port as well
for consistency?

Thanks for the idea and the patches!