[PATCH v1] net/vhost: clear data of packet mbuf after sending pkts

[Stephen Hemminger]

On Tue, 1 Mar 2022 10:47:32 +0100
David Marchand <david.marchand at redhat.com> wrote:

> On Tue, Mar 1, 2022 at 10:02 AM Zhang, Yuying <yuying.zhang at intel.com> wrote:
> > > -----Original Message-----
> > > From: David Marchand <david.marchand at redhat.com>
> > > Sent: Tuesday, March 1, 2022 4:44 PM
> > > To: Zhang, Yuying <yuying.zhang at intel.com>
> > > Cc: dev <dev at dpdk.org>; Maxime Coquelin <maxime.coquelin at redhat.com>;
> > > Xia, Chenbo <chenbo.xia at intel.com>; dpdk stable <stable at dpdk.org>
> > > Subject: Re: [PATCH v1] net/vhost: clear data of packet mbuf after sending pkts
> > >
> > > On Tue, Mar 1, 2022 at 8:29 AM Yuying Zhang <yuying.zhang at intel.com> wrote:  
> > > >
> > > > The PMD frees a packet mbuf back into its original mempool after
> > > > sending a packet. However, old data is not cleaned up which causes
> > > > error in payload of new packets. This patch clear data of packet mbuf
> > > > before freeing mbuf.  
> > >
> > > This patch looks wrong to me.
> > > What is the actual issue you want to fix?  
> >
> > eth_vhost_tx() frees the packet mbuf back into its original mempool every time after a packet sent without clearing the data field.
> > Then packet transmit  function will get bulk directly without reset. New generated packet contains old data of previous packet. This is wrong.  
> 
> With the proposed patch, if the mbuf refcnt is != 0, you are shooting
> the data while some other part of the application might be needing it.
> 
> Plus, there should be no expectation about a mbuf data content when
> retrieving one from a mempool.
> The only bytes that are guaranteed to be initialised by the mbuf API
> are its metadata.
> 
> 
> If there is an issue somewhere in dpdk where the mbuf data content is
> expected to be 0 on allocation, please point at it.
> Or share the full test that failed.
> 
> 

Agree. There is no guarantee that mbuf you get was not just used by
some other driver or library. Only the fields in rte_pktmbuf_reset are
guaranteed to be set.