netdev - Re: [PATCH v2] tcp: Add TCP_INFO counter for packets received out-of-order

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Date:   Tue, 10 Sep 2019 22:38:31 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Thomas Higdon <tph@...com>
Cc:     netdev <netdev@...r.kernel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Dave Jones <dsj@...com>
Subject: Re: [PATCH v2] tcp: Add TCP_INFO counter for packets received out-of-order

On Tue, Sep 10, 2019 at 10:11 PM Thomas Higdon <tph@...com> wrote:
>
>
...
> Because an additional 32-bit member in struct tcp_info would cause
> a hole on 64-bit systems, we reserve a struct member '_reserved'.
...
> diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
> index b3564f85a762..990a5bae3ac1 100644
> --- a/include/uapi/linux/tcp.h
> +++ b/include/uapi/linux/tcp.h
> @@ -270,6 +270,9 @@ struct tcp_info {
>         __u64   tcpi_bytes_retrans;  /* RFC4898 tcpEStatsPerfOctetsRetrans */
>         __u32   tcpi_dsack_dups;     /* RFC4898 tcpEStatsStackDSACKDups */
>         __u32   tcpi_reord_seen;     /* reordering events seen */
> +
> +       __u32   _reserved;           /* Reserved for future 32-bit member. */
> +       __u32   tcpi_rcv_ooopack;    /* Out-of-order packets received */
>  };
>

Unfortunately we won't be able to use this hole, because the way the
TCP_INFO works,

The kernel will report the same size after the reserved field is
renamed to something else.

User space code is able to detect which fields are there or not based
on what the kernel
returns for the size of the structure.