Re: [dpdk-dev] [PATCH] net: fix rte_vlan_insert with shared mbuf

[Stephen Hemminger <stephen@networkplumber.org>]

On Tue, 26 Mar 2019 18:38:57 -0400
Chas Williams <3chas3@gmail.com> wrote:

> On 3/26/19 3:15 PM, Stephen Hemminger wrote:
> > If mbuf refcnt was > 1 then rte_vlan_insert() would incorrectly
> > modify the original copy. Original code was expecting clone to make
> > a copy (it doesn't). Better to let the caller deal with making
> > a copy or setting up mbuf chain to allow for header to be added.
> > 
> > Also fix docbook comment about parameters (function takes
> > pointer to mbuf).
> > 
> > Fixes: c974021a5949 ("ether: add soft vlan encap/decap")
> > Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
> > ---
> >   lib/librte_net/rte_ether.h | 15 ++++-----------
> >   1 file changed, 4 insertions(+), 11 deletions(-)
> > 
> > diff --git a/lib/librte_net/rte_ether.h b/lib/librte_net/rte_ether.h
> > index c2c5e249ffe9..bab2b198fa79 100644
> > --- a/lib/librte_net/rte_ether.h
> > +++ b/lib/librte_net/rte_ether.h
> > @@ -374,7 +374,7 @@ static inline int rte_vlan_strip(struct rte_mbuf *m)
> >    * Software version of VLAN unstripping
> >    *
> >    * @param m
> > - *   The packet mbuf.
> > + *   Pointer to the packet mbuf.
> >    * @return
> >    *   - 0: On success
> >    *   -EPERM: mbuf is is shared overwriting would be unsafe
> > @@ -385,16 +385,9 @@ static inline int rte_vlan_insert(struct rte_mbuf **m)
> >   	struct ether_hdr *oh, *nh;
> >   	struct vlan_hdr *vh;
> >   
> > -	/* Can't insert header if mbuf is shared */
> > -	if (rte_mbuf_refcnt_read(*m) > 1) {
> > -		struct rte_mbuf *copy;
> > -
> > -		copy = rte_pktmbuf_clone(*m, (*m)->pool);
> > -		if (unlikely(copy == NULL))
> > -			return -ENOMEM;
> > -		rte_pktmbuf_free(*m);
> > -		*m = copy;
> > -	}
> > +	/* Can't directly insert header if mbuf is shared */
> > +	if (rte_mbuf_refcnt_read(*m) > 1)  
> 
> This check probably isn't sufficient. You need something more like:
> 
>          if (rte_mbuf_refcnt_read(mbuf) > 1 ||
>              (RTE_MBUF_INDIRECT(mbuf) &&
>               rte_mbuf_refcnt_read(rte_mbuf_from_indirect(mbuf)) > 1)) {
> 
> If this is a cloned packet, the refcnt will be 1. So you need to examine 
> the parent mbuf to see if other clones exist.
> 

The problem is that indirect buffers probably can't be overwritten
because of lack of headroom.

Actually, why not push the problem into the pktmbuf_headroom logic?