ftrace: fix trace entry size computation when header len is 0

In a trace.dat files, trace entries have a 32 bit header that contains
the payload len (in 32 bit words) in the lower 5 bits.

A len of 0 is a special case: the actual entry length is in the next
32 bit word, and that length includes the length field itself, so:

- the event data starts after the length fields
- the event data is len_in_next_word - 4

For documentation see:
https://elixir.bootlin.com/linux/latest/source/include/linux/ring_buffer.h#L51

Change-Id: I91a459deb11c63f606406b6a1dac9b2b4fcf9130
Signed-off-by: Fabrizio Iannetti <fabrizio.iannetti@gmail.com>
Reviewed-on: https://git.eclipse.org/r/c/tracecompass.incubator/org.eclipse.tracecompass.incubator/+/197834
Tested-by: Trace Compass Bot <tracecompass-bot@eclipse.org>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Reviewed-by: Hoang Thuan Pham <hoangpham.eclipse@gmail.com>
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>

tracetypes/org.eclipse.tracecompass.incubator.ftrace.core/src/org/eclipse/tracecompass/incubator/internal/ftrace/core/binary/iterator/BinaryFTraceCPUPageIterator.java

@@ -178,10 +178,14 @@ private void readNextEventTimeStamp() throws IOException {
             // Now we have a data event, we store the event type length, we save
             // the event type length and delta time
             // to load the event lazily later
-            fEventDef = new BinaryFTraceEventDefinition(fCurrentOffset, getCurrentEventPayloadSize());
+            int payloadSize = getCurrentEventPayloadSize();
+
+            // fCurrentOffset may have been modified by getCurrentEventPayloadSize(), pass it
+            // after calling the function above
+            fEventDef = new BinaryFTraceEventDefinition(fCurrentOffset, payloadSize);
 
             // Move the pointer to the next event
-            skip(fEventDef.getPayloadSize());
+            skip(payloadSize);
 
             // We peek the next event, to see if the event is an time extended
             // event
@@ -297,7 +301,8 @@ private int getCurrentEventPayloadSize() throws IOException {
             // length
             BinaryFTraceByteBuffer buffer = fBuffer;
             if (buffer != null) {
-                payloadSize = buffer.getNextInt();
+                // the size includes the size field itself, subtract it to get the actual event length
+                payloadSize = buffer.getNextInt() - 4;
                 this.fCurrentOffset += 4;
             }
         } else if (fCurrentTypeLen <= fFileHeader.getHeaderEventInfo().getDataMaxTypeLen()) {