netdev - RE: [RFC PATCH 4/6] hv_sock: Initialize send_buf in hvs_stream

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Date:   Fri, 15 Apr 2022 14:30:29 +0000
From:   "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
To:     Andrea Parri <parri.andrea@...il.com>
CC:     KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        Stefano Garzarella <sgarzare@...hat.com>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [RFC PATCH 4/6] hv_sock: Initialize send_buf in
 hvs_stream_enqueue()

From: Andrea Parri <parri.andrea@...il.com> Sent: Thursday, April 14, 2022 11:51 PM
> 
> > > @@ -655,7 +655,7 @@ static ssize_t hvs_stream_enqueue(struct vsock_sock *vsk,
> > > struct msghdr *msg,
> > >
> > >  	BUILD_BUG_ON(sizeof(*send_buf) != HV_HYP_PAGE_SIZE);
> > >
> > > -	send_buf = kmalloc(sizeof(*send_buf), GFP_KERNEL);
> > > +	send_buf = kzalloc(sizeof(*send_buf), GFP_KERNEL);
> >
> > Is this change really needed?
> 
> The idea was...
> 
> 
> > All fields are explicitly initialized, and in the data
> > array, only the populated bytes are copied to the ring buffer.  There should not
> > be any uninitialized values sent to the host.   Zeroing the memory ahead of
> > time certainly provides an extra protection (particularly against padding bytes,
> > but there can't be any since the layout of the data is part of the protocol with
> > Hyper-V).
> 
> Rather than keeping checking that...

The extra protection might be obtained by just zero'ing the header (i.e., the
bytes up to the 16 Kbyte data array).   I don't have a strong preference either
way, so up to you.

Michael

> 
> 
> > It is expensive protection to zero out 16K+ bytes every time we send
> > out a small message.
> 
> Do this.  ;-)
> 
> Will drop the patch.
> 
> Thanks,
>   Andrea