Read fault when running example network/nfsd-trace.stp stap

[Yun-Chih Chen]

I am running the example NFS stap file network/nfsd-trace.stp[1] on a
NFS server.
The stap file fails with the following message when a new NFS read
request comes in:

# sudo stap -v nfsd-trace.stp
Pass 1: parsed user script and 467 library scripts using
138276virt/44988res/7568shr/37488data kb, in 130usr/20sys/154real ms.
Pass 2: analyzed script: 22 probes, 66 functions, 9 embeds, 1 global
using 342304virt/246608res/9036shr/241516data kb, in
6020usr/730sys/6779real ms.
Pass 3: translated to C into
"/tmp/stapoyPncz/stap_b12e64c3fc1fc31297eb3b8c93c2dacf_107403_src.c"
using 342304virt/246856res/9228shr/241516data kb, in
200usr/70sys/265real ms.
Pass 4: compiled C into
"stap_b12e64c3fc1fc31297eb3b8c93c2dacf_107403.ko" in
4230usr/880sys/5113real ms.
Pass 5: starting run.
ERROR: read fault [man error::fault] at 0x0000000000000040 (addr) near
operator '->' at nfsd-trace.stp:18:31

It says "fh->fh_export->ex_path->mnt" is probably a reference on null
pointer ....

The test is conducted on both Linux 4.10 on Fedora 25 and Linux 3.10
on CentOS 7.  Both fails at the same place.
Does anyone know what's going on here?

Thanks,


Yunchih

[1] https://sourceware.org/systemtap/examples/network/nfsd-trace.stp