netdev - Re: [RFC PATCH 1/6] hv_sock: Check hv_pkt_iter_first

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Date:   Fri, 15 Apr 2022 08:41:33 +0200
From:   Andrea Parri <parri.andrea@...il.com>
To:     "Michael Kelley (LINUX)" <mikelley@...rosoft.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 1/6] hv_sock: Check hv_pkt_iter_first_raw()'s return
 value

On Fri, Apr 15, 2022 at 03:33:23AM +0000, Michael Kelley (LINUX) wrote:
> From: Andrea Parri (Microsoft) <parri.andrea@...il.com> Sent: Wednesday, April 13, 2022 1:48 PM
> > 
> > The function returns NULL if the ring buffer has no enough space
> > available for a packet descriptor.  The ring buffer's write_index
> 
> The first sentence wording is a bit scrambled.  I think you mean the
> ring buffer doesn't contain enough readable bytes to constitute a
> packet descriptor.

Indeed, replaced with your working.


> > is in memory which is shared with the Hyper-V host, its value is
> > thus subject to being changed at any time.
> 
> This second sentence is true, but I'm not making the connection
> with the code change below.   Evidently, there is some previous
> check made to ensure that enough bytes are available to be
> received when hvs_stream_dequeue() is called, so we assumed that
> NULL could never be returned?  I looked but didn't find such a check, 
> so maybe I didn't look carefully enough.  But now we are assuming
> that Hyper-V might have invalidated that previous check by 
> subsequently changing the write_index in a bogus way?  So now, NULL
> could be returned when previously we assumed it couldn't.

I think you're looking for hvs_stream_has_data().  (Previous checks
apart, hvs_stream_dequeue() will "dereference" the pointer so...)

Thanks,
  Andrea