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Question about -DDIAGNOSTICS and netinet/ip_input.c


    I was working on a device driver for a certain ethernet chipset that

occassionally wraps in its buffer, and causes a resulting mbuf chain
with only a few dozen bytes in the first mbuf of the chain.

    I wouldn't have thought this would be a problem, until I ran some
stress tests that flooded the ethernet receiver with packets and
started to get panics here:

   251          if (m->m_pkthdr.len < sizeof(struct ip))
   252                  goto tooshort;
   254  #ifdef  DIAGNOSTIC
   255          if (m->m_len < sizeof(struct ip))
   256                  panic("ipintr mbuf too short");
   257  #endif
   259          if (m->m_len < sizeof (struct ip) &&
   260              (m = m_pullup(m, sizeof (struct ip))) == 0) {
   261                  ipstat.ips_toosmall++;
   262                  return;
   263          }
   264          ip = mtod(m, struct ip *);

and the panic was at line 256.  But if I #undef'd DIAGNOSTICS,
then the m_pullup() at line 260 does the right thing and the packet
ends up being processed just fine.

So I started wondering, (a) why was the test checking for
something that apparently wasn't a fatal condition but rather
one that is subsequently recovered from a couple of lines later
and (b) why panic as a diagnostic "aid" from a recoverable
condition rather than just (say) log a message to the console?

All of this seems overly severe for no reason that is readily
apparent to me.  Am I missing something?