netdev - Re: [net-next v2 1/8] net-zerocopy: Copy straggler unaligned data for TCP Rx. zerocopy.

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Date:   Wed, 2 Dec 2020 16:15:27 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Arjun Roy <arjunroy.kdev@...il.com>
Cc:     davem@...emloft.net, netdev@...r.kernel.org, arjunroy@...gle.com,
        edumazet@...gle.com, soheil@...gle.com
Subject: Re: [net-next v2 1/8] net-zerocopy: Copy straggler unaligned data
 for TCP Rx. zerocopy.

On Wed,  2 Dec 2020 14:09:38 -0800
Arjun Roy <arjunroy.kdev@...il.com> wrote:

> diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
> index cfcb10b75483..62db78b9c1a0 100644
> --- a/include/uapi/linux/tcp.h
> +++ b/include/uapi/linux/tcp.h
> @@ -349,5 +349,7 @@ struct tcp_zerocopy_receive {
>  	__u32 recv_skip_hint;	/* out: amount of bytes to skip */
>  	__u32 inq; /* out: amount of bytes in read queue */
>  	__s32 err; /* out: socket error */
> +	__u64 copybuf_address;	/* in: copybuf address (small reads) */
> +	__s32 copybuf_len; /* in/out: copybuf bytes avail/used or error */
>  };
>  #endif /* _UAPI_LINUX_TCP_H */

You can't safely grow the size of a userspace API without handling the
case of older applications.  Logic in setsockopt() would have to handle
both old and new sizes of the structure.