[PATCH] kernel-shark: Add check for return of ksmodel_get_cpu_front()

[Steven Rostedt <rostedt@goodmis.org>]

From: Steven Rostedt (VMware) <rostedt@goodmis.org>

When I loaded two trace.dat files (host and guest), but the mapping of
which host thread is associated to which guest vCPU was missing from the
file, it caused a SEGFAULT. That's because in fillTaskGraph(), the
lamGetPidCPU() calls ksmodule_get_cpu_front() which returns a negative
number and does not set index. But the next line checks data[index] where
index is some random number, and the application crashes.

By checking the return of ksmodule_get_cpu_front(), and if it is negative
do not reference data[index] and just let eFront be nullptr, the
application shows no mapping, but at least it does not crash.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
You can reproduce this with:

   http://rostedt.org/private/trace-no-map.tar.bz2

diff --git a/src/KsPlotTools.cpp b/src/KsPlotTools.cpp
index 225dc34..abef5f8 100644
--- a/src/KsPlotTools.cpp
+++ b/src/KsPlotTools.cpp
@@ -1280,7 +1280,7 @@ void Graph::fillTaskGraph(int sd, int pid)
 						 false,
 						 _collectionPtr,
 						 &index);
-		if (index >= 0)
+		if (cpuFront >= 0 && index >= 0)
 			eFront = _histoPtr->data[index];
 
 		cpuBack = ksmodel_get_cpu_back(_histoPtr, bin,