Re: [PATCH RFC v2 00/12] socket sendmsg MSG_ZEROCOPY

[Andy Lutomirski <luto@amacapital.net>]

On Tue, Feb 28, 2017 at 2:40 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> On Tue, 2017-02-28 at 14:25 -0800, Andy Lutomirski wrote:
>> On Tue, Feb 28, 2017 at 1:47 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
>> > On Tue, 2017-02-28 at 13:09 -0800, Andy Lutomirski wrote:
>> >
>> >> Does this mean that a user program that does a zerocopy send can cause
>> >> a retransmitted segment to contain different data than the original
>> >> segment?  If so, is that okay?
>> >
>> > Same remark applies to sendfile() already
>>
>> True.
>>
>> >, or other zero copy modes
>> > (vmsplice() + splice() )
>>
>> I hate vmsplice().  I thought I remembered it being essentially
>> disabled at some point due to security problems.
>
> Right, zero copy is hard ;)
>
> vmsplice() is not disabled in current kernels, unless I missed
> something.
>

I think you're right.  That being said, from the man page:

The user pages are a gift to the kernel.  The application  may  not
modify this memory ever, otherwise the page cache and on-disk data may
differ.

This is just not okay IMO.