netdev - Re: [bpf-next PATCH 05/16] bpf: create tcp_bpf_ulp allowing BPF to monitor socket TX/RX data

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Date:   Mon, 5 Mar 2018 22:22:21 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     ast@...nel.org, daniel@...earbox.net, netdev@...r.kernel.org,
        davejwatson@...com
Subject: Re: [bpf-next PATCH 05/16] bpf: create tcp_bpf_ulp allowing BPF to
 monitor socket TX/RX data

On 03/05/2018 09:42 PM, David Miller wrote:
> From: John Fastabend <john.fastabend@...il.com>
> Date: Mon, 5 Mar 2018 14:53:08 -0800
> 
>> I decided to make the default no-copy to mirror the existing
>> sendpage() semantics and then to add the flag later. The flag
>> support is not in this series simply because I wanted to get the
>> base support in first.
> 
> What existing sendpage semantics are you referring to?
> 

All I meant by this is if an application uses sendfile() call
there is no good way to know when/if the kernel side will copy or
xmit the  data. So a reliable user space application will need to
only modify the data if it "knows" there are no outstanding sends
in-flight. So if we assume applications follow this then it
is OK to avoid the copy. Of course this is not good enough for
security, but for monitoring/statistics (my use case 1 it works).

By keep existing sendpage semantics I just meant applications
should already follow the above.