From 54bc9710689c3c853e75c574948e0a99ce3430f4 Mon Sep 17 00:00:00 2001
From: Francis Deslauriers <francis.deslauriers@efficios.com>
Date: Thu, 28 Mar 2019 18:20:52 -0400
Subject: [PATCH] Fix: flt.lttng-utils.debug-info: Error in src line reporting

Issue
=====
A `debug-info` component does not print the `src` field of a tracepoint
properly when tracepoint is within a for loop.

  #include "tp.h"
  int main(int argc, char *argv[])
  {
      tracepoint(my_app, empty);
      for(int i = 0; i < 1; i++) {
          tracepoint(my_app, empty);
      }
      return 0;
  }

The problem derives from the DWARF information generated by the
compiler. The compiler may place the DWARF content of the for loop in
`DW_TAG_lexical_block` which the current `debug-info` design doesn't
expect.

Here is a summarized version of DWARF tree containing the first
tracepoint in the code above:
  DW_TAG_subprogram
    DW_TAG_inlined_subroutine
      /* tracepoint callsite info */

Here is a summarized version of the DWARF tree containing the tracepoint
in the for loop:
  DW_TAG_subprogram
    DW_TAG_lexical_block
      DW_TAG_inlined_subroutine
      	/* tracepoint callsite info */

The current implementation doesn't expect the presence of a
`DW_TAG_lexical_block` entry as child of the `DW_TAG_inlined_subroutine`
entry and won't look any further down that branch to find the source
information for the current tracepoint. This results in the component
not finding the line number for such callsite and thus leaving the `src`
field empty. Also, on some occasions, the `src` field would contain the
tracepoint definition location (e.g. "tp.h:12") rather then the actual
callsite of that tracepoint.

Solution
========
When iterating over the Debugging Information Entries, if the current
entry contains the address to resolve _but_ is not a of the type
`DW_TAG_inlined_subroutine` try to iterate over the children of that
entry.

Known drawbacks
===============
None.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
---
 plugins/lttng-utils/bin-info.c | 27 +++++++++++++++++++++------
 plugins/lttng-utils/dwarf.c    |  6 ++++++
 plugins/lttng-utils/dwarf.h    |  9 +++++++++
 3 files changed, 36 insertions(+), 6 deletions(-)

diff --git a/plugins/lttng-utils/bin-info.c b/plugins/lttng-utils/bin-info.c
index c810f8c6..c3e16cec 100644
--- a/plugins/lttng-utils/bin-info.c
+++ b/plugins/lttng-utils/bin-info.c
@@ -1218,22 +1218,37 @@ int bin_info_child_die_has_address(struct bt_dwarf_die *die, uint64_t addr, bool
 	}
 
 	do {
-		int tag;
-
-		ret = bt_dwarf_die_get_tag(die, &tag);
+		ret = bt_dwarf_die_contains_addr(die, addr, &_contains);
 		if (ret) {
 			goto error;
 		}
 
-		if (tag == DW_TAG_inlined_subroutine) {
-			ret = bt_dwarf_die_contains_addr(die, addr, &_contains);
+		if (_contains) {
+			/*
+			 * The address is within the range of the current DIE
+			 * or its children.
+			 */
+			int tag;
+
+			ret = bt_dwarf_die_get_tag(die, &tag);
 			if (ret) {
 				goto error;
 			}
 
-			if (_contains) {
+			if (tag == DW_TAG_inlined_subroutine) {
+				/* Found the tracepoint. */
 				goto end;
 			}
+
+			if (bt_dwarf_die_has_children(die)) {
+				/*
+				 * Look for the address in the children DIEs.
+				 */
+				ret = bt_dwarf_die_child(die);
+				if (ret) {
+					goto error;
+				}
+			}
 		}
 	} while (bt_dwarf_die_next(die) == 0);
 
diff --git a/plugins/lttng-utils/dwarf.c b/plugins/lttng-utils/dwarf.c
index 3c7388a5..534d2883 100644
--- a/plugins/lttng-utils/dwarf.c
+++ b/plugins/lttng-utils/dwarf.c
@@ -131,6 +131,12 @@ void bt_dwarf_die_destroy(struct bt_dwarf_die *die)
 	g_free(die);
 }
 
+BT_HIDDEN
+int bt_dwarf_die_has_children(struct bt_dwarf_die *die)
+{
+	return dwarf_haschildren(die->dwarf_die);
+}
+
 BT_HIDDEN
 int bt_dwarf_die_child(struct bt_dwarf_die *die)
 {
diff --git a/plugins/lttng-utils/dwarf.h b/plugins/lttng-utils/dwarf.h
index c3d56b78..707431f9 100644
--- a/plugins/lttng-utils/dwarf.h
+++ b/plugins/lttng-utils/dwarf.h
@@ -119,6 +119,15 @@ struct bt_dwarf_die *bt_dwarf_die_create(struct bt_dwarf_cu *cu);
 BT_HIDDEN
 void bt_dwarf_die_destroy(struct bt_dwarf_die *die);
 
+/**
+ * Indicates if the debug information entry `die` has children DIEs.
+ *
+ * @param die	bt_dwarf_die instance
+ * @returns	0 if the die no child, 1 otherwise
+ */
+BT_HIDDEN
+int bt_dwarf_die_has_children(struct bt_dwarf_die *die);
+
 /**
  * Advance the debug information entry `die` to its first child, if
  * any.
-- 
2.34.1