Re: [bpf-next PATCH 4/7] net: do_tcp_sendpages flag to avoid SKBTX_SHARED_FRAG

[Eric Dumazet <eric.dumazet@gmail.com>]

On Fri, 2018-01-12 at 10:10 -0800, John Fastabend wrote:
> When calling do_tcp_sendpages() from in kernel and we know the data
> has no references from user side we can omit SKBTX_SHARED_FRAG flag.
> This patch adds an internal flag, NO_SKBTX_SHARED_FRAG that can be used
> to omit setting SKBTX_SHARED_FRAG.
> 
> Signed-off-by: John Fastabend <john.fastabend@gmail.com>
> ---
>  include/linux/socket.h |    1 +
>  net/ipv4/tcp.c         |    4 +++-
>  2 files changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/socket.h b/include/linux/socket.h
> index 9286a5a..add9360 100644
> --- a/include/linux/socket.h
> +++ b/include/linux/socket.h
> @@ -287,6 +287,7 @@ struct ucred {
>  #define MSG_SENDPAGE_NOTLAST 0x20000 /* sendpage() internal : not the last page */
>  #define MSG_BATCH	0x40000 /* sendmmsg(): more messages coming */
>  #define MSG_EOF         MSG_FIN
> +#define MSG_NO_SHARED_FRAGS 0x80000 /* sendpage() internal : page frags are not shared */
>  
>  #define MSG_ZEROCOPY	0x4000000	/* Use user data in kernel path */
>  #define MSG_FASTOPEN	0x20000000	/* Send data in TCP SYN */
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 7ac583a..56c6f49 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -995,7 +995,9 @@ ssize_t do_tcp_sendpages(struct sock *sk, struct page *page, int offset,
>  			get_page(page);
>  			skb_fill_page_desc(skb, i, page, offset, copy);
>  		}
> -		skb_shinfo(skb)->tx_flags |= SKBTX_SHARED_FRAG;
> +
> +		if (!(flags & MSG_NO_SHARED_FRAGS))
> +			skb_shinfo(skb)->tx_flags |= SKBTX_SHARED_FRAG;
>  
>  		skb->len += copy;
>  		skb->data_len += copy;

What would prevent user space from using this flag ?