tracing/histograms: String compares should not care about signed values

When comparing two strings for the "onmatch" histogram trigger, fields
that are strings use string comparisons, which do not care about being
signed or not.

Do not fail to match two string fields if one is unsigned char array and
the other is a signed char array.

Link: https://lore.kernel.org/all/[email protected]/

Cc: [email protected]
Cc: Tom Zanussi <[email protected]>
Cc: Yafang Shao <[email protected]>
Fixes: b05e89a ("tracing: Accept different type for synthetic event fields")
Reviewed-by: Masami Hiramatsu <[email protected]>
Reported-by: Sven Schnelle <[email protected]>
Signed-off-by: Steven Rostedt (VMware) <[email protected]>

kernel/trace/trace_events_hist.c

@@ -3757,7 +3757,7 @@ static int check_synth_field(struct synth_event *event,
 
 	if (strcmp(field->type, hist_field->type) != 0) {
 		if (field->size != hist_field->size ||
-		    field->is_signed != hist_field->is_signed)
+		    (!field->is_string && field->is_signed != hist_field->is_signed))
 			return -EINVAL;
 	}