linux-kernel - Re: Checking to see if a bit is _not

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Date:	Mon, 1 Dec 2014 23:51:23 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Checking to see if a bit is _not_ set in a ftrace event filter

On Mon, 1 Dec 2014 22:57:25 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Mon, 1 Dec 2014 19:52:11 -0800
> Alexei Starovoitov <ast@...mgrid.com> wrote:
> 
> > yeah. teaching tree walk to do (state & 2048) == 0 is not trivial,
> > since it doesn't have a concept of value of expression.
> > Doing !(state & 2048) is probably a bit easier, since there
> > are hacks for 'not' already.
> 
> That's exactly what I was looking at doing. If I can get something
> working without spending more than an hour or two, I may even push it
> for this release cycle.

Actually, this was rather trivial. It took me longer because I was
doing this half asleep, but it seems to work.

Note, this is a major beta patch. It probably doesn't work with ANDS
and ORS, that is !(x == 1 && y == 2) probably doesn't work, but this
should: !(x == 1), and so should !(state & 2048).

I'll clean this up and get it ready for 3.19.

Cheers!

-- Steve

diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
index 7a8c1528e141..aee7b743c9df 100644
--- a/kernel/trace/trace_events_filter.c
+++ b/kernel/trace/trace_events_filter.c
@@ -37,6 +37,7 @@ enum filter_op_ids
 {
 	OP_OR,
 	OP_AND,
+	OP_NOT,
 	OP_GLOB,
 	OP_NE,
 	OP_EQ,
@@ -59,6 +60,7 @@ struct filter_op {
 static struct filter_op filter_ops[] = {
 	{ OP_OR,	"||",		1 },
 	{ OP_AND,	"&&",		2 },
+	{ OP_NOT,	"!",		3 },
 	{ OP_GLOB,	"~",		4 },
 	{ OP_NE,	"!=",		4 },
 	{ OP_EQ,	"==",		4 },
@@ -166,7 +168,7 @@ static int filter_pred_##type(struct filter_pred *pred, void *event)	\
 		break;							\
 	}								\
 									\
-	return match;							\
+	return match == !pred->not;					\
 }
 
 #define DEFINE_EQUALITY_PRED(size)					\
@@ -565,7 +567,7 @@ int filter_match_preds(struct event_filter *filter, void *rec)
 	data.preds = preds = rcu_dereference_sched(filter->preds);
 	ret = walk_pred_tree(preds, root, filter_match_preds_cb, &data);
 	WARN_ON(ret);
-	return data.match;
+	return !!data.match;
 }
 EXPORT_SYMBOL_GPL(filter_match_preds);
 
@@ -1028,7 +1030,7 @@ static int init_pred(struct filter_parse_state *ps,
 	}
 
 	if (pred->op == OP_NE)
-		pred->not = 1;
+		pred->not ^= 1;
 
 	pred->fn = fn;
 	return 0;
@@ -1590,6 +1592,16 @@ static int replace_preds(struct ftrace_event_call *call,
 			continue;
 		}
 
+		if (elt->op == OP_NOT) {
+			if (!n_preds) {
+				err = -EINVAL;
+				goto fail;
+			}
+			if (!dry_run)
+				filter->preds[n_preds - 1].not ^= 1;
+			continue;
+		}
+
 		if (WARN_ON(n_preds++ == MAX_FILTER_PRED)) {
 			parse_error(ps, FILT_ERR_TOO_MANY_PREDS, 0);
 			err = -ENOSPC;
--
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