[PATCH perf] perf annotate: fix annotate crash for fused instructions

[Yao]

From: yao <dragonlinux@xxxxxxxxx>

Function is_fused tries to find the previous fused instruction with
the current conditional jump instruction in the annotate_line list.
disasm_line is the container of annotate_line.
The annotate_line list uses list_head and the head is in
annotated_source: notes->src->source which is not an annotate_line,
so it couldn't walk without checking the list_head.
If so it will read annoted_source neighbour memory mistakenly and most
likely cause crash.

Fix the case where current instruction is a conditional jump and it is
the first instruction in the disassembled list, the original code will
walk to the head and treats it as disasm_line which is not.

Signed-off-by: yao <dragonlinux@xxxxxxxxx>
---
 tools/perf/ui/browsers/annotate.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/tools/perf/ui/browsers/annotate.c b/tools/perf/ui/browsers/annotate.c
index cf36b5347f72..07449944fa2e 100644
--- a/tools/perf/ui/browsers/annotate.c
+++ b/tools/perf/ui/browsers/annotate.c
@@ -155,17 +155,26 @@ static void annotate_browser__write(struct ui_browser *browser, void *entry, int
 
 static int is_fused(struct annotate_browser *ab, struct disasm_line *cursor)
 {
-	struct disasm_line *pos = list_prev_entry(cursor, al.node);
+	struct annotation *notes = browser__annotation(&ab->b);
+	struct list_head *node = cursor->al.node.prev;
+	struct disasm_line *pos;
 	const char *name;
 	int diff = 1;
 
-	while (pos && pos->al.offset == -1) {
-		pos = list_prev_entry(pos, al.node);
+	/*
+	 * backwalk to the nearest neighbour instruction till the list head
+	 * instructions/non-insn are ordered from head to tail
+	 * head is not a annotate_line/disasm_line
+	 */
+	for (; node != &notes->src->source; node = node->prev) {
+		pos = list_entry(node, struct disasm_line, al.node);
+		if (pos->al.offset != -1)
+			break;
 		if (!annotate_opts.hide_src_code)
 			diff++;
 	}
 
-	if (!pos)
+	if (node == &notes->src->source)
 		return 0;
 
 	if (ins__is_lock(&pos->ins))
-- 
2.56.0