tracing: Have trace_stack nr_entries compare not be so subtle

mainline inclusion
from mainline-5.0
commit ca16b0fbb052
category: bugfix
bugzilla: 5788
CVE: NA

-------------------------------------------------

Dan Carpenter reviewed the trace_stack.c code and figured he found an off by
one bug.

 "From reviewing the code, it seems possible for
  stack_trace_max.nr_entries to be set to .max_entries and in that case we
  would be reading one element beyond the end of the stack_dump_trace[]
  array.  If it's not set to .max_entries then the bug doesn't affect
  runtime."

Although it looks to be the case, it is not. Because we have:

 static unsigned long stack_dump_trace[STACK_TRACE_ENTRIES+1] =
	 { [0 ... (STACK_TRACE_ENTRIES)] = ULONG_MAX };

 struct stack_trace stack_trace_max = {
	.max_entries		= STACK_TRACE_ENTRIES - 1,
	.entries		= &stack_dump_trace[0],
 };

And:

	stack_trace_max.nr_entries = x;
	for (; x < i; x++)
		stack_dump_trace[x] = ULONG_MAX;

Even if nr_entries equals max_entries, indexing with it into the
stack_dump_trace[] array will not overflow the array. But if it is the case,
the second part of the conditional that tests stack_dump_trace[nr_entries]
to ULONG_MAX will always be true.

By applying Dan's patch, it removes the subtle aspect of it and makes the if
conditional slightly more efficient.

Link: http://lkml.kernel.org/r/20180620110758.crunhd5bfep7zuiz@kili.mountainSigned-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NSteven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: NCheng Jian <cj.chengjian@huawei.com>
Reviewed-by: NXie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

kernel/trace/trace_stack.c

@@ -286,7 +286,7 @@ __next(struct seq_file *m, loff_t *pos)
 {
 	long n = *pos - 1;
 
-	if (n > stack_trace_max.nr_entries || stack_dump_trace[n] == ULONG_MAX)
+	if (n >= stack_trace_max.nr_entries || stack_dump_trace[n] == ULONG_MAX)
 		return NULL;
 
 	m->private = (void *)n;