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

On Tue, 2017-02-28 at 14:25 -0800, Andy Lutomirski wrote:

On Tue, Feb 28, 2017 at 1:47 PM, Eric Dumazet [off-list ref] wrote:

quoted

On Tue, 2017-02-28 at 13:09 -0800, Andy Lutomirski wrote:

quoted

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.

quoted

, 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.

On Tue, Feb 28, 2017 at 2:40 PM, Eric Dumazet [off-list ref] wrote:

On Tue, 2017-02-28 at 14:25 -0800, Andy Lutomirski wrote:

quoted

On Tue, Feb 28, 2017 at 1:47 PM, Eric Dumazet [off-list ref] wrote:

quoted

On Tue, 2017-02-28 at 13:09 -0800, Andy Lutomirski wrote:

quoted

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.

quoted

, 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.

On Tue, 2017-02-28 at 14:52 -0800, Andy Lutomirski wrote:

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.

TCP works just fine in this case.

TX checksum will be computed by the NIC after/while data is copied.

If really the application changes the data, that will not cause any
problems, other than user side consistency.

This is why we require a copy (for all buffers that came from zero-copy)
if network stack hits a device that can not offload TX checksum.

Even pwrite() does not guarantee consistency if multiple threads are
using it on overlapping regions.