Re: [dpdk-dev] packet data access bug in bpf and pdump libs

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 9 Oct 2019 17:06:24 +0200
Morten Brørup <mb@smartsharesystems.com> wrote:

> > -----Original Message-----
> > From: Stephen Hemminger [mailto:stephen@networkplumber.org]
> > Sent: Wednesday, October 9, 2019 5:02 PM
> > 
> > On Wed, 9 Oct 2019 11:11:46 +0000
> > "Ananyev, Konstantin" <konstantin.ananyev@intel.com> wrote:
> >   
> > > Hi Morten,
> > >  
> > > >
> > > > Hi Konstantin and Stephen,
> > > >
> > > > I just noticed the same bug in your bpf and pcap libraries:
> > > >
> > > > You are using rte_pktmbuf_mtod(), but should be using  
> > rte_pktmbuf_read(). Otherwise you cannot read data across multiple
> > segments.  
> > >
> > > In plain data buffer mode expected input for BPF program is start of  
> > first segment packet data.  
> > > Other segments are simply not available to BPF program in that mode.
> > > AFAIK, cBPF uses the same model.
> > >  
> > > >
> > > >
> > > > Med venlig hilsen / kind regards
> > > > - Morten Brørup  
> > >  
> > 
> > For packet capture, the BPF program is only allowed to look at first
> > segment.
> > pktmbuf_read is expensive and can cause a copy.  
> 
> It is only expensive if going beyond the first segment:
> 
> static inline const void *rte_pktmbuf_read(const struct rte_mbuf *m,
> 	uint32_t off, uint32_t len, void *buf)
> {
> 	if (likely(off + len <= rte_pktmbuf_data_len(m)))
> 		return rte_pktmbuf_mtod_offset(m, char *, off);
> 	else
> 		return __rte_pktmbuf_read(m, off, len, buf);
> }

But it would mean potentially big buffer on the stack (in case)