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

[Song Liu]

On Wed, Jan 12, 2022 at 11:03 PM <menglong8.dong@xxxxxxxxx> wrote:
>
> From: Menglong Dong <imagedong@xxxxxxxxxxx>
>
> The description of 'dst_port' in 'struct bpf_sock' is not accurated.
> In fact, 'dst_port' is not in network byte order, it is 'partly' in
> network byte order.
>
> We can see it in bpf_sock_convert_ctx_access():
>
> > case offsetof(struct bpf_sock, dst_port):
> >       *insn++ = BPF_LDX_MEM(
> >               BPF_FIELD_SIZEOF(struct sock_common, skc_dport),
> >               si->dst_reg, si->src_reg,
> >               bpf_target_off(struct sock_common, skc_dport,
> >                              sizeof_field(struct sock_common,
> >                                           skc_dport),
> >                              target_size));
>
> It simply passes 'sock_common->skc_dport' to 'bpf_sock->dst_port',
> which makes that the low 16-bits of 'dst_port' is equal to 'skc_port'
> and is in network byte order, but the high 16-bites of 'dst_port' is
> 0. And the actual port is 'bpf_ntohs((__u16)dst_port)', and
> 'bpf_ntohl(dst_port)' is totally not the right port.
>
> This is different form 'remote_port' in 'struct bpf_sock_ops' or
> 'struct __sk_buff':
>
> > case offsetof(struct __sk_buff, remote_port):
> >       BUILD_BUG_ON(sizeof_field(struct sock_common, skc_dport) != 2);
> >
> >       *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, sk),
> >                             si->dst_reg, si->src_reg,
> >                                     offsetof(struct sk_buff, sk));
> >       *insn++ = BPF_LDX_MEM(BPF_H, si->dst_reg, si->dst_reg,
> >                             bpf_target_off(struct sock_common,
> >                                            skc_dport,
> >                                            2, target_size));
> > #ifndef __BIG_ENDIAN_BITFIELD
> >       *insn++ = BPF_ALU32_IMM(BPF_LSH, si->dst_reg, 16);
> > #endif
>
> We can see that it will left move 16-bits in little endian, which makes
> the whole 'remote_port' is in network byte order, and the actual port
> is bpf_ntohl(remote_port).
>
> Note this in the document of 'dst_port'. ( Maybe this should be unified
> in the code? )
>
> Signed-off-by: Menglong Dong <imagedong@xxxxxxxxxxx>

Acked-by: Song Liu <songliubraving@xxxxxx>

> ---
>  include/uapi/linux/bpf.h | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index b0383d371b9a..891a182a749a 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -5500,7 +5500,11 @@ struct bpf_sock {
>         __u32 src_ip4;
>         __u32 src_ip6[4];
>         __u32 src_port;         /* host byte order */
> -       __u32 dst_port;         /* network byte order */
> +       __u32 dst_port;         /* low 16-bits are in network byte order,
> +                                * and high 16-bits are filled by 0.
> +                                * So the real port in host byte order is
> +                                * bpf_ntohs((__u16)dst_port).
> +                                */
>         __u32 dst_ip4;
>         __u32 dst_ip6[4];
>         __u32 state;
> --
> 2.34.1
>