tracing/histograms: Fix parsing of "sym-offset" modifier

With the addition of simple mathematical operations (plus and minus), the
parsing of the "sym-offset" modifier broke, as it took the '-' part of the
"sym-offset" as a minus, and tried to break it up into a mathematical
operation of "field.sym - offset", in which case it failed to parse
(unless the event had a field called "offset").

Both .sym and .sym-offset modifiers should not be entered into
mathematical calculations anyway. If ".sym-offset" is found in the
modifier, then simply make it not an operation that can be calculated on.

Link: https://lkml.kernel.org/r/[email protected]

Cc: Ingo Molnar <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Daniel Bristot de Oliveira <[email protected]>
Cc: [email protected]
Fixes: 100719d ("tracing: Add simple expression support to hist triggers")
Reviewed-by: Tom Zanussi <[email protected]>
Signed-off-by: Steven Rostedt (VMware) <[email protected]>

kernel/trace/trace_events_hist.c

@@ -1555,6 +1555,13 @@ static int contains_operator(char *str)
 
 	switch (*op) {
 	case '-':
+		/*
+		 * Unfortunately, the modifier ".sym-offset"
+		 * can confuse things.
+		 */
+		if (op - str >= 4 && !strncmp(op - 4, ".sym-offset", 11))
+			return FIELD_OP_NONE;
+
 		if (*str == '-')
 			field_op = FIELD_OP_UNARY_MINUS;
 		else