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 5:15 PM
> 
> 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)

No, the buffer only needs to be the size of the accessed data. I use it like this:

char buffer[sizeof(uint32_t)];

for (;; pc++) {
    switch (pc->code) {
        case BPF_LD_ABS_32:
            p = rte_pktmbuf_read(m, pc->k, sizeof(uint32_t), buffer);
            if (unlikely(p == NULL)) return 0; /* Attempting to read beyond packet. Bail out. */
            a = rte_be_to_cpu_32(*(const uint32_t *)p);
            continue;
        case BPF_LD_ABS_16:
            p = rte_pktmbuf_read(m, pc->k, sizeof(uint16_t), buffer);
            if (unlikely(p == NULL)) return 0; /* Attempting to read beyond packet. Bail out. */
            a = rte_be_to_cpu_16(*(const uint16_t *)p);
            continue;