tracing: Clean up and document pred_funcs_##type creation and use

The pred_funcs_##type arrays consist of five functions that are assigned
based on the ops. The array must be in the same order of the ops each
function represents. The PRED_FUNC_START macro denotes the op enum that
starts the op that maps to the pred_funcs_##type arrays. This is all very
subtle and prone to bugs if the code is changed.

Add comments describing how PRED_FUNC_START and pred_funcs_##type array is
used, and also a PRED_FUNC_MAX that is the maximum number of functions in
the arrays.

Clean up select_comparison_fn() that assigns the predicates to the
pred_funcs_##type array function as well as add protection in case an op is
passed in that does not map correctly to the array.

Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_events_filter.c

@@ -65,6 +65,13 @@ struct filter_op {
 
 static struct filter_op filter_ops[] = { OPS };
 
+/*
+ * pred functions are OP_LT, OP_LE, OP_GT, OP_GE, and OP_BAND
+ * pred_funcs_##type below must match the order of them above.
+ */
+#define PRED_FUNC_START			OP_LT
+#define PRED_FUNC_MAX			(OP_BAND - PRED_FUNC_START)
+
 #define ERRORS								\
 	C( NONE,	 	"No error"),				\
 	C( INVALID_OP,		"Invalid operator"),			\
@@ -172,8 +179,6 @@ static const filter_pred_fn_t pred_funcs_##type[] = {			\
 	filter_pred_BAND_##type,					\
 };
 
-#define PRED_FUNC_START			OP_LT
-
 #define DEFINE_EQUALITY_PRED(size)					\
 static int filter_pred_##size(struct filter_pred *pred, void *event)	\
 {									\
@@ -946,39 +951,52 @@ static filter_pred_fn_t select_comparison_fn(enum filter_op_ids op,
 					    int field_size, int field_is_signed)
 {
 	filter_pred_fn_t fn = NULL;
+	int pred_func_index = -1;
+
+	switch (op) {
+	case OP_EQ:
+	case OP_NE:
+		break;
+	default:
+		if (WARN_ON_ONCE(op < PRED_FUNC_START))
+			return NULL;
+		pred_func_index = op - PRED_FUNC_START;
+		if (WARN_ON_ONCE(pred_func_index > PRED_FUNC_MAX))
+			return NULL;
+	}
 
 	switch (field_size) {
 	case 8:
-		if (op == OP_EQ || op == OP_NE)
+		if (pred_func_index < 0)
 			fn = filter_pred_64;
 		else if (field_is_signed)
-			fn = pred_funcs_s64[op - PRED_FUNC_START];
+			fn = pred_funcs_s64[pred_func_index];
 		else
-			fn = pred_funcs_u64[op - PRED_FUNC_START];
+			fn = pred_funcs_u64[pred_func_index];
 		break;
 	case 4:
-		if (op == OP_EQ || op == OP_NE)
+		if (pred_func_index < 0)
 			fn = filter_pred_32;
 		else if (field_is_signed)
-			fn = pred_funcs_s32[op - PRED_FUNC_START];
+			fn = pred_funcs_s32[pred_func_index];
 		else
-			fn = pred_funcs_u32[op - PRED_FUNC_START];
+			fn = pred_funcs_u32[pred_func_index];
 		break;
 	case 2:
-		if (op == OP_EQ || op == OP_NE)
+		if (pred_func_index < 0)
 			fn = filter_pred_16;
 		else if (field_is_signed)
-			fn = pred_funcs_s16[op - PRED_FUNC_START];
+			fn = pred_funcs_s16[pred_func_index];
 		else
-			fn = pred_funcs_u16[op - PRED_FUNC_START];
+			fn = pred_funcs_u16[pred_func_index];
 		break;
 	case 1:
-		if (op == OP_EQ || op == OP_NE)
+		if (pred_func_index < 0)
 			fn = filter_pred_8;
 		else if (field_is_signed)
-			fn = pred_funcs_s8[op - PRED_FUNC_START];
+			fn = pred_funcs_s8[pred_func_index];
 		else
-			fn = pred_funcs_u8[op - PRED_FUNC_START];
+			fn = pred_funcs_u8[pred_func_index];
 		break;
 	}