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

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Message-ID: <871r0psaei.fsf@cloudflare.com>
Date:   Sun, 30 Jan 2022 12:58:13 +0100
From:   Jakub Sitnicki <jakub@...udflare.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.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 Fri, Jan 28, 2022 at 07:17 PM CET, Alexei Starovoitov wrote:
> 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 ?

Great idea. Now done in v2:

https://lore.kernel.org/bpf/20220130115518.213259-1-jakub@cloudflare.com/

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

I can tend to that this upcoming week.