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

[Alexei Starovoitov]

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? )

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