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

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Date:   Thu, 3 Dec 2020 15:19:53 -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 Thu, Dec 3, 2020 at 3:01 PM David Laight <David.Laight@...lab.com> wrote:
>
> From: Stephen Hemminger
> > Sent: 03 December 2020 00:15
> >
> > On Wed,  2 Dec 2020 14:09:38 -0800
> > Arjun Roy <arjunroy.kdev@...il.com> wrote:
> >
> > > diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
> > > index cfcb10b75483..62db78b9c1a0 100644
> > > --- a/include/uapi/linux/tcp.h
> > > +++ b/include/uapi/linux/tcp.h
> > > @@ -349,5 +349,7 @@ struct tcp_zerocopy_receive {
> > >     __u32 recv_skip_hint;   /* out: amount of bytes to skip */
> > >     __u32 inq; /* out: amount of bytes in read queue */
> > >     __s32 err; /* out: socket error */
> > > +   __u64 copybuf_address;  /* in: copybuf address (small reads) */
> > > +   __s32 copybuf_len; /* in/out: copybuf bytes avail/used or error */
>
> You need to swap the order of the above fields to avoid padding
> and differing alignments for 32bit and 64bit apps.
>

Just to double check, are you referring to the order of
copybuf_address and copybuf_len?
If so, it seems that the current ordering is not creating any
alignment holes, but flipping it would: https://godbolt.org/z/bdxP6b


> > >  };
> > >  #endif /* _UAPI_LINUX_TCP_H */
> >
> > You can't safely grow the size of a userspace API without handling the
> > case of older applications.  Logic in setsockopt() would have to handle
> > both old and new sizes of the structure.
>
> 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?

Thanks,
-Arjun

>         David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
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