netdev - Re: [net-next v2 2/2] tcp: Add receive timestamp support for receive zerocopy.

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Date:   Fri, 22 Jan 2021 22:55:45 -0700
From:   David Ahern <dsahern@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        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 2/2] tcp: Add receive timestamp support for receive
 zerocopy.

On 1/22/21 9:07 PM, Jakub Kicinski wrote:
> On Wed, 20 Jan 2021 16:41:48 -0800 Arjun Roy wrote:
>> diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
>> index 768e93bd5b51..b216270105af 100644
>> --- a/include/uapi/linux/tcp.h
>> +++ b/include/uapi/linux/tcp.h
>> @@ -353,5 +353,9 @@ struct tcp_zerocopy_receive {
>>  	__u64 copybuf_address;	/* in: copybuf address (small reads) */
>>  	__s32 copybuf_len; /* in/out: copybuf bytes avail/used or error */
>>  	__u32 flags; /* in: flags */
>> +	__u64 msg_control; /* ancillary data */
>> +	__u64 msg_controllen;
>> +	__u32 msg_flags;
>> +	/* __u32 hole;  Next we must add >1 u32 otherwise length checks fail. */
> 
> Well, let's hope nobody steps on this landmine.. :)
> 

Past suggestions were made to use anonymous declarations - e.g., __u32
:32; - as a way of reserving the space for future use. That or declare
'__u32 resvd', check that it must be 0 and makes it available for later
(either directly or with a union).