tracing: Have filter check for balanced ops

When the following filter is used it causes a warning to trigger:

 # cd /sys/kernel/debug/tracing
 # echo "((dev==1)blocks==2)" > events/ext4/ext4_truncate_exit/filter
-bash: echo: write error: Invalid argument
 # cat events/ext4/ext4_truncate_exit/filter
((dev==1)blocks==2)
^
parse_error: No error

 ------------[ cut here ]------------
 WARNING: CPU: 2 PID: 1223 at kernel/trace/trace_events_filter.c:1640 replace_preds+0x3c5/0x990()
 Modules linked in: bnep lockd grace bluetooth  ...
 CPU: 3 PID: 1223 Comm: bash Tainted: G        W       4.1.0-rc3-test+ #450
 Hardware name: Hewlett-Packard HP Compaq Pro 6300 SFF/339A, BIOS K01 v02.05 05/07/2012
  0000000000000668 ffff8800c106bc98 ffffffff816ed4f9 ffff88011ead0cf0
  0000000000000000 ffff8800c106bcd8 ffffffff8107fb07 ffffffff8136b46c
  ffff8800c7d81d48 ffff8800d4c2bc00 ffff8800d4d4f920 00000000ffffffea
 Call Trace:
  [<ffffffff816ed4f9>] dump_stack+0x4c/0x6e
  [<ffffffff8107fb07>] warn_slowpath_common+0x97/0xe0
  [<ffffffff8136b46c>] ? _kstrtoull+0x2c/0x80
  [<ffffffff8107fb6a>] warn_slowpath_null+0x1a/0x20
  [<ffffffff81159065>] replace_preds+0x3c5/0x990
  [<ffffffff811596b2>] create_filter+0x82/0xb0
  [<ffffffff81159944>] apply_event_filter+0xd4/0x180
  [<ffffffff81152bbf>] event_filter_write+0x8f/0x120
  [<ffffffff811db2a8>] __vfs_write+0x28/0xe0
  [<ffffffff811dda43>] ? __sb_start_write+0x53/0xf0
  [<ffffffff812e51e0>] ? security_file_permission+0x30/0xc0
  [<ffffffff811dc408>] vfs_write+0xb8/0x1b0
  [<ffffffff811dc72f>] SyS_write+0x4f/0xb0
  [<ffffffff816f5217>] system_call_fastpath+0x12/0x6a
 ---[ end trace e11028bd95818dcd ]---

Worse yet, reading the error message (the filter again) it says that
there was no error, when there clearly was. The issue is that the
code that checks the input does not check for balanced ops. That is,
having an op between a closed parenthesis and the next token.

This would only cause a warning, and fail out before doing any real
harm, but it should still not caues a warning, and the error reported
should work:

 # cd /sys/kernel/debug/tracing
 # echo "((dev==1)blocks==2)" > events/ext4/ext4_truncate_exit/filter
-bash: echo: write error: Invalid argument
 # cat events/ext4/ext4_truncate_exit/filter
((dev==1)blocks==2)
^
parse_error: Meaningless filter expression

And give no kernel warning.

Link: http://lkml.kernel.org/r/20150615175025.7e809215@gandalf.local.home

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: stable@vger.kernel.org # 2.6.31+
Reported-by: Vince Weaver <vincent.weaver@maine.edu>
Tested-by: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace_events_filter.c

@@ -1369,19 +1369,26 @@ static int check_preds(struct filter_parse_state *ps)
 {
 	int n_normal_preds = 0, n_logical_preds = 0;
 	struct postfix_elt *elt;
+	int cnt = 0;
 
 	list_for_each_entry(elt, &ps->postfix, list) {
-		if (elt->op == OP_NONE)
+		if (elt->op == OP_NONE) {
+			cnt++;
 			continue;
+		}
 
 		if (elt->op == OP_AND || elt->op == OP_OR) {
 			n_logical_preds++;
+			cnt--;
 			continue;
 		}
+		if (elt->op != OP_NOT)
+			cnt--;
 		n_normal_preds++;
+		WARN_ON_ONCE(cnt < 0);
 	}
 
-	if (!n_normal_preds || n_logical_preds >= n_normal_preds) {
+	if (cnt != 1 || !n_normal_preds || n_logical_preds >= n_normal_preds) {
 		parse_error(ps, FILT_ERR_INVALID_FILTER, 0);
 		return -EINVAL;
 	}