Re: [dpdk-dev] [PATCH] mbuf: extend pktmbuf pool private structure

[Stephen Hemminger <stephen@networkplumber.org>]

On Tue, 19 Nov 2019 23:30:15 +0100
Thomas Monjalon <thomas@monjalon.net> wrote:

> 19/11/2019 17:25, Stephen Hemminger:
> > On Tue, 19 Nov 2019 15:23:50 +0000
> > Shahaf Shuler <shahafs@mellanox.com> wrote:
> >   
> > > Tuesday, November 19, 2019 11:33 AM, Thomas Monjalon:  
> > > > Subject: Re: [PATCH] mbuf: extend pktmbuf pool private structure
> > > > 
> > > > 18/11/2019 11:02, Shahaf Shuler:    
> > > > >  struct rte_pktmbuf_pool_private {
> > > > >  	uint16_t mbuf_data_room_size; /**< Size of data space in each    
> > > > mbuf. */    
> > > > >  	uint16_t mbuf_priv_size;      /**< Size of private area in each mbuf.    
> > > > */    
> > > > > +	uint32_t reserved; /**< reserved for future use. */    
> > > > 
> > > > Maybe simpler to give the future name "flags" and keep the comment
> > > > "reserved for future use".    
> > > 
> > > I'm am OK w/ changing to flags.
> > > If Olivier accepts maybe you can change while applying?   
> > 
> > After the Linux openat experience if you want to add flags.
> > Then all usage of API needs to validate that flags is 0.  
> 
> Sorry Stephen, I don't understand what you mean.
> Please could you explain?
> 
> 

Any time a new field is added that maybe used later you can
not guarantee that existing code correctly initializes the value to
zero. What happened with openat() was that there was a flag value
that was originally unused, but since kernel did not enforce that
it was zero; it could not later be used for extensions.

You need to make sure that all reserved fields are initialized.
That means when a private pool is created it is zeroed. And if
a flag is new argument to an API, check for zero at create time.

An example of how DPDK failed at this is the filter field in
rte_pdump. Since it is not checked for NULL, it can't safely
be used now (and still claim API/ABI compatiablity).