charts: fix ancestor siblings check segfault

This loop assumed ancestor->parent was !NULL, and that's not necessarily
always true.  Due to these circular linked lists from the kernel's
list.h, they're not simply NULL delimited and we need the pointer to the
actual head to detect the end of the list.  In libvmon, the head for the
siblings list is either the parent proc's children member, or the
processes member of the vmon struct.  It may be more elegant to switch
to always having a root proc in libvmon, even if it's just synthetic,
for simplifying this crap.  But for now, just determine which head is
relevant and check against it for loop termination.

Under some heavy parallel kernel compilations I was seeing occasional
vwm segfaults, and the addr2line of the ip in dmesg mapped to this
particular loop.  I'm assuming the ancestor walk landed on a top-level
process and then this sibling detection tried dereferencing the
top-level proc's NULL parent and boom, segfault.

1 files changed, 21 insertions, 9 deletions

diff --git a/src/charts.c b/src/charts.c
index b124084..6fb630e 100644
--- a/src/charts.c
+++ b/src/charts.c
@@ -535,6 +535,23 @@ static void print_argv(vwm_charts_t *charts, vwm_chart_t *chart, int x, int row,
 }
 
 
+/* determine if a given process has subsequent siblings in the heirarchy */
+static inline int proc_has_subsequent_siblings(vmon_t *vmon, vmon_proc_t *proc)
+{
+	struct list_head	*sib, *head = &vmon->processes;
+
+	if (proc->parent)
+		head = &proc->parent->children;
+
+	for (sib = proc->siblings.next; sib != head; sib = sib->next) {
+		if (!(list_entry(sib, vmon_proc_t, siblings)->is_stale))
+			return 1;
+	}
+
+	return 0;
+}
+
+
 /* draws proc in a row of the process heirarchy */
 static void draw_heirarchy_row(vwm_charts_t *charts, vwm_chart_t *chart, vmon_proc_t *proc, int depth, int row, int heirarchy_changed)
 {
@@ -591,7 +608,6 @@ static void draw_heirarchy_row(vwm_charts_t *charts, vwm_chart_t *chart, vmon_pr
 	/* only if this process isn't the root process @ the window shall we consider all relational drawing conditions */
 	if (proc != chart->monitor) {
 		vmon_proc_t		*ancestor, *sibling, *last_sibling = NULL;
-		struct list_head	*rem;
 		int			needs_tee = 0;
 		int			bar_x = 0, bar_y = (row + 1) * CHART_ROW_HEIGHT;
 		int			sub;
@@ -605,14 +621,10 @@ static void draw_heirarchy_row(vwm_charts_t *charts, vwm_chart_t *chart, vmon_pr
 			bar_x = (depth - sub) * (CHART_ROW_HEIGHT / 2) + 4;
 
 			/* determine if the ancestor has remaining siblings which are not stale, if so, draw a connecting bar at its depth */
-			for (rem = ancestor->siblings.next; rem != &ancestor->parent->children; rem = rem->next) {
-				if (!(list_entry(rem, vmon_proc_t, siblings)->is_stale)) {
-					XDrawLine(xserver->display, chart->text_pixmap, charts->text_gc,
-						  bar_x, bar_y - CHART_ROW_HEIGHT,	/* dst x1, y1 */
-						  bar_x, bar_y);			/* dst x2, y2 (vertical line) */
-					break; /* stop looking for more siblings at this ancestor when we find one that isn't stale */
-				}
-			}
+			if (proc_has_subsequent_siblings(&charts->vmon, ancestor))
+				XDrawLine(xserver->display, chart->text_pixmap, charts->text_gc,
+					  bar_x, bar_y - CHART_ROW_HEIGHT,	/* dst x1, y1 */
+					  bar_x, bar_y);			/* dst x2, y2 (vertical line) */
 		}
 
 		/* determine if _any_ of our siblings have children requiring us to draw a tee immediately before our comm string.