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report from Philip A. Prindeville <philipp@zembu.com>:
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:
250
251 if (m->m_pkthdr.len < sizeof(struct ip))
252 goto tooshort;
253
254 #ifdef DIAGNOSTIC
255 if (m->m_len < sizeof(struct ip))
256 panic("ipintr mbuf too short");
257 #endif
258
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.
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