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

[Alexei Starovoitov]

On Thu, Jan 20, 2022 at 6:18 AM Menglong Dong <menglong8.dong@xxxxxxxxx> wrote:
>
> On Thu, Jan 20, 2022 at 12:17 PM Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
> >
> > 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@xxxxxxxxx> 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.
>
> Yeah, I understand narrower load in convert_ctx_accesses()
> now. Seems u8 access to dst_port can't pass the verifier too,
> which can be seen form bpf_sock_is_valid_access():
>
> $    switch (off) {
> $    case offsetof(struct bpf_sock, state):
> $    case offsetof(struct bpf_sock, family):
> $    case offsetof(struct bpf_sock, type):
> $    case offsetof(struct bpf_sock, protocol):
> $    case offsetof(struct bpf_sock, dst_port):  // u8 access is not allowed
> $    case offsetof(struct bpf_sock, src_port):
> $    case offsetof(struct bpf_sock, rx_queue_mapping):
> $    case bpf_ctx_range(struct bpf_sock, src_ip4):
> $    case bpf_ctx_range_till(struct bpf_sock, src_ip6[0], src_ip6[3]):
> $    case bpf_ctx_range(struct bpf_sock, dst_ip4):
> $    case bpf_ctx_range_till(struct bpf_sock, dst_ip6[0], dst_ip6[3]):
> $        bpf_ctx_record_field_size(info, size_default);
> $        return bpf_ctx_narrow_access_ok(off, size, size_default);
> $    }
>
> I'm still not sure what should we do now. Should we make all
> remote_port and dst_port narrower accessable and endianness
> right? For example the remote_port in struct bpf_sock_ops:
>
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -8414,6 +8414,7 @@ static bool sock_ops_is_valid_access(int off, int size,
>                                 return false;
>                         info->reg_type = PTR_TO_PACKET_END;
>                         break;
> +               case bpf_ctx_range(struct bpf_sock_ops, remote_port):

Ahh. bpf_sock_ops don't have it.
But bpf_sk_lookup and sk_msg_md have it.

bpf_sk_lookup->remote_port
supports narrow access.

When it accesses sport from bpf_sk_lookup_kern.

and we have tests that do u8 access from remote_port.
See verifier/ctx_sk_lookup.c

>                 case offsetof(struct bpf_sock_ops, skb_tcp_flags):
>                         bpf_ctx_record_field_size(info, size_default);
>                         return bpf_ctx_narrow_access_ok(off, size,
>
> If remote_port/dst_port are made narrower accessable, the
> result will be right. Therefore, *((u16*)&sk->remote_port) will
> be the port with network byte order. And the port in host byte
> order can be get with:
> bpf_ntohs(*((u16*)&sk->remote_port))
> or
> bpf_htonl(sk->remote_port)

So u8, u16, u32 will work if we make them narrow-accessible, right?

The summary if I understood it:
. only bpf_sk_lookup->remote_port is doing it correctly for u8,u16,u32 ?
. bpf_sock->dst_port is not correct for u32,
  since it's missing bpf_ctx_range() ?
. __sk_buff->remote_port
 bpf_sock_ops->remote_port
 sk_msg_md->remote_port
 correct for u32 access only. They don't support narrow access.

but wait
we have a test for bpf_sock->dst_port in progs/test_sock_fields.c.
How does it work then?

I think we need more eyes on the problem.
cc-ing more experts.