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

[Chas Williams <3chas3@gmail.com>]

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.

> +		return -EPERM;
>   
>   	oh = rte_pktmbuf_mtod(*m, struct ether_hdr *);
>   	nh = (struct ether_hdr *)
>