bz6436 backtraces from uprobes

This implements proper unwinding from uprobes in the presence of
uretprobe trampolines.

* runtime/stack.c (_stp_stack_print): Rework for uprobe context case
  and refactor a bit.
* runtime/uprobes2/uprobes.h (GET_PC_URETPROBE_NONE): new constant
* runtime/uprobes2/uprobes.c (uprobe_get_pc): Support translating the
  trampoline function from uprobe context in addition to uretprobe
  context.
* runtime/uprobes/uprobes.h (GET_PC_URETPROBE_NONE): ditto
* runtime/uprobes/uprobes.c (uprobe_get_pc): ditto
* tapsets.cxx (uprobe_derived_probe_group::emit_module_decls):
  Initialize ri in context to GET_PC_URETPROBE_NONE in generated
  enter_uprobe_probe.
* testsuite/systemtap.context/fib.stp: Add an option to do a backtrace
  on function entry.
* testsuite/systemtap.context/fib.exp: Test backtrace in function
  entry (uprobe) probes.

2 files changed, 17 insertions, 0 deletions

diff --git a/testsuite/systemtap.context/fib.exp b/testsuite/systemtap.context/fib.exp
index cc4d75a1..5026e5d0 100644
--- a/testsuite/systemtap.context/fib.exp
+++ b/testsuite/systemtap.context/fib.exp
@@ -35,3 +35,17 @@ expect {
 }
 wait
 if {$fibcalls == 18 && $maincalls == 2} { pass "$test ($fibcalls $maincalls)" } { fail "$test ($fibcalls $maincalls)" }
+
+spawn stap -c "$testexe 10" -- $teststp --entry
+set fibcalls 0
+set maincalls 0
+expect {
+    -timeout 120
+    -re {^fib[^\r\n]*[\r\n]} { incr fibcalls; exp_continue }
+    -re {^main[^\r\n]*[\r\n]} { incr maincalls; exp_continue }
+    -re {^[^\r\n]*[\r\n]} {exp_continue}
+    timeout { fail "$test (timeout)" }
+    eof { }
+}
+wait
+if {$fibcalls == 55 && $maincalls == 10} { pass "$test ($fibcalls $maincalls)" } { fail "$test ($fibcalls $maincalls)" }
diff --git a/testsuite/systemtap.context/fib.stp b/testsuite/systemtap.context/fib.stp
index 85c2fc1d..fe2415f5 100644
--- a/testsuite/systemtap.context/fib.stp
+++ b/testsuite/systemtap.context/fib.stp
@@ -5,13 +5,16 @@ probe process("fib").function("fib").call {
     depth++
     if (depth > max_depth) {
             max_depth = depth
+            %( $# > 0 %? print_ubacktrace_brief(); printf("\n") %)
     }
 }
 
 probe process("fib").function("fib").return {
+    %( $# == 0 %?
     if (depth == max_depth) {
             print_ubacktrace_brief()
             printf("\n")
     }
+    %)
     depth--
 }