[dpdk-dev] [PATCH v2] net/af_packet: fix vlan_insert corruption

[Stephen Hemminger]

On Tue, 16 Apr 2019 10:42:13 +0100
Bruce Richardson <bruce.richardson at intel.com> wrote:

> On Tue, Apr 16, 2019 at 10:37:07AM +0100, Ferruh Yigit wrote:
> > On 4/12/2019 11:08 PM, Stephen Hemminger wrote:  
> > > On Fri, 12 Apr 2019 17:28:17 +0100
> > > Ferruh Yigit <ferruh.yigit at intel.com> wrote:
> > >   
> > >> On 4/8/2019 5:41 PM, Stephen Hemminger wrote:  
> > >>> If the af_packet transmit is sending a VLAN packet,
> > >>> and the transmit path to the kernel os full, then it would
> > >>> mismanage the outgoing mbuf. The original mbuf would end up
> > >>> being freed twice, once by AF_PACKET PMD and once by caller.    
> > >>
> > >> This comment is valid with your new patch [1] that updates 'rte_vlan_insert()'
> > >> to duplicate the mbuf, right?
> > >>
> > >> That patch looks like won't make the release, so I suggest this one wait that
> > >> patch, although this is harmless on its own, commit log is misleading.
> > >>
> > >> [1]
> > >> https://patches.dpdk.org/patch/51870/  
> > > 
> > > It was always true, even with existing vlan_insert.
> > > Existing vlan_insert has issues if it ever creates a clone packet.
> > > Existing vlan_insert can duplicate mbuf through clone
> > >   
> > 
> > Right, existing vlan_insert has same issue on af_packet.
> > 
> > But, should vlan_insert try to duplicate the mbuf when it is shared, does it
> > worth the complexity it brings? And when that support removed this patch won't
> > be needed.  
> 
> I don't think vlan insert or other mbuf manipulation APIs should be
> checking for shared state or not - that's the job of the app. We could have
> cases where the user does want to modify a shared mbuf.
> 
> Regards,
> /Bruce

The vlan_insert code is called on transmit, and there are lots of
cases where a transmit mbuf might be shared (like TCP stack). And in that
case inserting the vlan must be non-destructive to the original mbuf.

Whether you want to push the problem to the driver or do it in the
library, it is still a problem.