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

["Morten Brørup" <mb@smartsharesystems.com>]

> -----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);
}