netdev - Re: [net-next v2 1/8] net-zerocopy: Copy straggler unaligned data for TCP Rx. zerocopy.

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Date:   Fri, 4 Dec 2020 14:37:29 -0800
From:   Arjun Roy <arjunroy@...gle.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        Arjun Roy <arjunroy.kdev@...il.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "soheil@...gle.com" <soheil@...gle.com>
Subject: Re: [net-next v2 1/8] net-zerocopy: Copy straggler unaligned data for
 TCP Rx. zerocopy.

On Fri, Dec 4, 2020 at 1:03 AM David Laight <David.Laight@...lab.com> wrote:
>
> From: Arjun Roy
> > Sent: 03 December 2020 23:25
> ...
> > > > You also have to allow for old (working) applications being recompiled
> > > > with the new headers.
> > > > So you cannot rely on the fields being zero even if you are passed
> > > > the size of the structure.
> > > >
> > >
> > > I think this should already be taken care of in the current code; the
> > > full-sized struct with new fields is being zero-initialized, then
> > > we're getting the user-provided optlen, then copying from userspace
> > > only that much data. So the newer fields would be zero in that case,
> > > so this should handle the case of new kernels but old applications.
> > > Does this address the concern, or am I misunderstanding?
> > >
> >
> > Actually, on closer read, perhaps the following is what you have in
> > mind for the old application?
> >
> > struct zerocopy_args args;
> > args.address = ...;
> > args.length = ...;
> > args.recv_skip_hint = ...;
> > args.inq = ...;
> > args.err = ...;
> > getsockopt(fd, IPPROTO_TCP, TCP_ZEROCOPY_RECEIVE, &args, sizeof(args));
> > // sizeof(args) is now bigger when recompiled with new headers, but we
> > did not explicitly set the new fields to 0, therefore issues
>
> That's the one...
>

I'll defer in this case to Eric's previous response in this thread
(paraphrased: that there is precedent for this in tcp_mmap.c).

-Arjun

>         David
>
> -
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