Warning for buildok/context_test.stp on utrace kernel

[Wenji Huang]

Hi,

There will be warning if we run the following test on
RHEL5, Fedora kernel or utrace kernel.

$ stap -p4 testsuite/buildok/context_test.stp
WARNING: "uprobe_get_pc" 
[/tmp/staphrGioI/stap_0aef4ead3fec39f2dee30c60b1b684d2_8042.ko] undefined!
/home/wjhuang/.systemtap/cache/0a/stap_0aef4ead3fec39f2dee30c60b1b684d2_8042.ko

There reason is from print_ubacktrace() in probe.

In fact, if we run stap -ve 'probe begin(print_ubacktrace()){}' on
utrace kernel without loaded uprobes module.  We can see the error

Error inserting module 
'/tmp/stap0t5pyH/stap_5bd1a1d6b83ff84cfa8e7c67a402fa40_1153.ko': Unknown 
symbol in module

since no symbol uprobe_get_pc can be resolved.

Yes, the test case is a little picky, but maybe we can
process it in a more graceful way.

BTW: there is no manual for print_ubacktrace, ubacktrace(), etc.

Regards,
Wenji