tracing/filters: support for operator reserved characters in strings

When we set a filter for an event, such as:

echo "name == my_lock_name" > \
	/debug/tracing/events/lockdep/lock_acquired/filter

then the following order of token type is parsed:

- space
- operator
- parentheses
- operand

Because the operators and parentheses have a higher precedence
than the operand characters, which is normal, then we can't
use any string containing such special characters:

()=<>!&|

To get this support and also avoid ambiguous intepretation from
the parser or the human, we can do it using double quotes so that
we keep the usual languages habits.

Then after this patch you can still declare string condition like
before:

echo name == myname

But if you want to compare against a string containing an operator
character, you can use double quotes:

echo 'name == "&myname"'

Don't forget to include the whole expression into single quotes or
the double ones will be eaten by echo.

[ Impact: support strings with special characters for tracing filters ]

Cc: Tom Zanussi <tzanussi@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Zhaolei <zhaolei@cn.fujitsu.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>

kernel/trace/trace_events_filter.c

@@ -851,10 +851,19 @@ static void postfix_clear(struct filter_parse_state *ps)
 
 static int filter_parse(struct filter_parse_state *ps)
 {
+	int in_string = 0;
 	int op, top_op;
 	char ch;
 
 	while ((ch = infix_next(ps))) {
+		if (ch == '"') {
+			in_string ^= 1;
+			continue;
+		}
+
+		if (in_string)
+			goto parse_operand;
+
 		if (isspace(ch))
 			continue;
 
@@ -908,6 +917,7 @@ static int filter_parse(struct filter_parse_state *ps)
 			}
 			continue;
 		}
+parse_operand:
 		if (append_operand_char(ps, ch)) {
 			parse_error(ps, FILT_ERR_OPERAND_TOO_LONG, 0);
 			return -EINVAL;