ftrace: Avoid potential division by zero in function profiler

commit e31f7939c1c27faa5d0e3f14519eaf7c89e8a69d upstream.

The ftrace_profile->counter is unsigned long and
do_div truncates it to 32 bits, which means it can test
non-zero and be truncated to zero for division.
Fix this issue by using div64_ul() instead.

Link: http://lkml.kernel.org/r/20200103030248.14516-1-wenyang@linux.alibaba.com
Fixes: e330b3bcd8319 ("tracing: Show sample std dev in function profiling")
Fixes: 34886c8bc590f ("tracing: add average time in function to function profiler")
Signed-off-by: Wen Yang <wenyang@linux.alibaba.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index c59d43d54d321b6aaf2fdd6f623dc090e19470bf..ff5281a9c5d7cc03be658548b71a933cf6b71444 100644 (file)
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -595,8 +595,7 @@ static int function_stat_show(struct seq_file *m, void *v)
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
        seq_printf(m, "    ");
-       avg = rec->time;
-       do_div(avg, rec->counter);
+       avg = div64_ul(rec->time, rec->counter);
 
        /* Sample standard deviation (s^2) */
        if (rec->counter <= 1)
@@ -613,7 +612,8 @@ static int function_stat_show(struct seq_file *m, void *v)
                 * Divide only 1000 for ns^2 -> us^2 conversion.
                 * trace_print_graph_duration will divide 1000 again.
                 */
-               do_div(stddev, rec->counter * (rec->counter - 1) * 1000);
+               stddev = div64_ul(stddev,
+                                 rec->counter * (rec->counter - 1) * 1000);
        }
 
        trace_seq_init(&s);