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

From: David Laight <[email-masked]>
Date: Thu, 3 Dec 2020 23:01:11 +0000

From: Stephen Hemminger
> Sent: 03 December 2020 00:15
> 
> On Wed,  2 Dec 2020 14:09:38 -0800
> Arjun Roy <[email-masked]> 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.

> >  };
> >  #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.

	David

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