[PATCH 1/2] perf hists browser: Add NULL pointer check to prevent crash

[Wang Nan <wangnan0@huawei.com>]

Before this patch we can trigger a segfault by following steps:

 Step 1: perf report

 Step 2: Use UP/DOWN to select an entry, don't press 'ENTER'

 Step 3: Use '/' to filter symbols, use a filter which returns
         empty result

 Step 4: Press 'ENTER' (notice here that the old selection is still
		        there. This is another problem)

 Step 5: Press 'ENTER' to annotate that symbol

 Step 6: Press 'LEFT' to go out.

 Result: segfault:

 perf: Segmentation fault
 -------- backtrace --------
 /home/wangnan/perf[0x53e568]
 /lib64/libc.so.6(+0x3545f)[0x7fba75d3245f]
 /home/wangnan/perf[0x537516]
 /home/wangnan/perf[0x533fef]
 /home/wangnan/perf[0x53b347]
 /home/wangnan/perf(perf_evlist__tui_browse_hists+0x96)[0x53d206]
 /home/wangnan/perf(cmd_report+0x1b9f)[0x442c7f]
 /home/wangnan/perf[0x47efa2]
 /home/wangnan/perf(main+0x5f5)[0x432fa5]
 /lib64/libc.so.6(__libc_start_main+0xf4)[0x7fba75d1ebd4]
 /home/wangnan/perf[0x4330d4]

This is because in this case 'nd' could be NULL in
ui_browser__hists_seek(), but that function never check it.

This patch adds checker for potential NULL pointer in that function.
After this patch the above steps won't segfault again.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
---
 tools/perf/ui/browsers/hists.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/tools/perf/ui/browsers/hists.c b/tools/perf/ui/browsers/hists.c
index 33da341..9458da8 100644
--- a/tools/perf/ui/browsers/hists.c
+++ b/tools/perf/ui/browsers/hists.c
@@ -1280,8 +1280,10 @@ static void ui_browser__hists_seek(struct ui_browser *browser,
 	 * Moves not relative to the first visible entry invalidates its
 	 * row_offset:
 	 */
-	h = rb_entry(browser->top, struct hist_entry, rb_node);
-	h->row_offset = 0;
+	if (browser->top) {
+		h = rb_entry(browser->top, struct hist_entry, rb_node);
+		h->row_offset = 0;
+	}
 
 	/*
 	 * Here we have to check if nd is expanded (+), if it is we can't go
@@ -1297,6 +1299,8 @@ static void ui_browser__hists_seek(struct ui_browser *browser,
 	 * and stop when we printed enough lines to fill the screen.
 	 */
 do_offset:
+	if (!nd)
+		return;
 	if (offset > 0) {
 		do {
 			h = rb_entry(nd, struct hist_entry, rb_node);
-- 
1.8.3.4