dfilter: Allow comparison relation to commute

Comparison relations should be allowed to commute but they can not
because we need type information to resolve literals to fvalues. For
that reason an expression like "1 == some.field"  is invalid. Solve
that by commuting the relation if the first try did not succeed in
assigning a type to the LHS.

After the second try give up, that means we have a relation with
constants on both sides and that is not semantically valid.

Other relations like "matches" and "contains" are not symmetric and
should not commute anyway.

Before:

    Filter: _ws.ftypes.int32 == 10

    Syntax tree:
     0 TEST_ANY_EQ:
       1 FIELD(_ws.ftypes.int32 <FT_INT32>)
       1 FVALUE(10 <FT_INT32>)

    Instructions:
    00000 READ_TREE		_ws.ftypes.int32 <FT_INT32> -> reg#0
    00001 IF_FALSE_GOTO	3
    00002 ANY_EQ		reg#0 == 10 <FT_INT32>
    00003 RETURN

    Filter: 10 == _ws.ftypes.int32
    dftest: Left side of "==" expression must be a field or function, not 10.
    	10 == _ws.ftypes.int32
    	^~

After:

    Filter: _ws.ftypes.int32 == 10

    Syntax tree:
     0 TEST_ANY_EQ:
       1 FIELD(_ws.ftypes.int32 <FT_INT32>)
       1 FVALUE(10 <FT_INT32>)

    Instructions:
    00000 READ_TREE		_ws.ftypes.int32 <FT_INT32> -> reg#0
    00001 IF_FALSE_GOTO	3
    00002 ANY_EQ		reg#0 == 10 <FT_INT32>
    00003 RETURN

    Filter: 10 == _ws.ftypes.int32

    Syntax tree:
     0 TEST_ANY_EQ:
       1 FVALUE(10 <FT_INT32>)
       1 FIELD(_ws.ftypes.int32 <FT_INT32>)

    Instructions:
    00000 READ_TREE		_ws.ftypes.int32 <FT_INT32> -> reg#0
    00001 IF_FALSE_GOTO	3
    00002 ANY_EQ		10 <FT_INT32> == reg#0
    00003 RETURN

1 files changed, 22 insertions, 5 deletions

diff --git a/epan/dfilter/semcheck.c b/epan/dfilter/semcheck.c
index 508d844fa7..96ea7e5a4f 100644
--- a/epan/dfilter/semcheck.c
+++ b/epan/dfilter/semcheck.c
@@ -920,10 +920,17 @@ check_relation_LHS_ARITHMETIC(dfwork_t *dfw, stnode_op_t st_op _U_,
 static void
 check_relation(dfwork_t *dfw, stnode_op_t st_op,
 		FtypeCanFunc can_func, gboolean allow_partial_value,
-		stnode_t *st_node, stnode_t *st_arg1, stnode_t *st_arg2)
+		stnode_t *st_node, stnode_t *st_arg1, stnode_t *st_arg2,
+		int commute)
 {
 	LOG_NODE(st_node);
 
+	if (commute < 0) {
+		/* We have already commuted the LHS with the RHS and still
+		   cannot assign a field type to any side of the relation. */
+		FAIL(dfw, st_node, "Constant expression is invalid.");
+	}
+
 	switch (stnode_type_id(st_arg1)) {
 		case STTYPE_FIELD:
 		case STTYPE_REFERENCE:
@@ -942,9 +949,19 @@ check_relation(dfwork_t *dfw, stnode_op_t st_op,
 			check_relation_LHS_ARITHMETIC(dfw, st_op, can_func,
 					allow_partial_value, st_node, st_arg1, st_arg2);
 			break;
+		case STTYPE_LITERAL:
+		case STTYPE_STRING:
+		case STTYPE_CHARCONST:
+			/* We cannot semantically check a relation with literals on the LHS because we
+			   don't have a type to assign them. Commute the LHS with the RHS and retry
+			   the relation semantic check. */
+			check_relation(dfw, st_op, can_func,
+					allow_partial_value,st_node, st_arg2, st_arg1, --commute);
+			break;
 		default:
-			FAIL(dfw, st_arg1, "Left side of \"%s\" expression must be a field or function, not %s.",
-					stnode_todisplay(st_node), stnode_todisplay(st_arg1));
+			/* Should not happen. */
+			FAIL(dfw, st_arg1, "(FIXME) Syntax node type \"%s\" is invalid for relation \"%s\".",
+					stnode_type_name(st_arg1), stnode_todisplay(st_node));
 	}
 }
 
@@ -1096,13 +1113,13 @@ check_test(dfwork_t *dfw, stnode_t *st_node)
 		case STNODE_OP_ANY_EQ:
 		case STNODE_OP_ALL_NE:
 		case STNODE_OP_ANY_NE:
-			check_relation(dfw, st_op, ftype_can_eq, FALSE, st_node, st_arg1, st_arg2);
+			check_relation(dfw, st_op, ftype_can_eq, FALSE, st_node, st_arg1, st_arg2, 1);
 			break;
 		case STNODE_OP_GT:
 		case STNODE_OP_GE:
 		case STNODE_OP_LT:
 		case STNODE_OP_LE:
-			check_relation(dfw, st_op, ftype_can_cmp, FALSE, st_node, st_arg1, st_arg2);
+			check_relation(dfw, st_op, ftype_can_cmp, FALSE, st_node, st_arg1, st_arg2, 1);
 			break;
 		case STNODE_OP_CONTAINS:
 			check_relation_contains(dfw, st_node, st_arg1, st_arg2);