C++: Account for the signedness of the lesser operand in cpp/comparison-with-wider-type

cpp/ql/src/Security/CWE/CWE-190/ComparisonWithWiderType.ql

@@ -45,13 +45,20 @@ Element friendlyLoc(Expr e) {
   not e instanceof Access and not e instanceof Call and result = e
 }
 
+int getComparisonSizeAdjustment(Expr e) {
+  if e.getType().(IntegralType).isSigned() then result = 1 else result = 0
+}
+
 from Loop l, RelationalOperation rel, VariableAccess small, Expr large
 where
   small = rel.getLesserOperand() and
   large = rel.getGreaterOperand() and
   rel = l.getCondition().getAChild*() and
   forall(Expr conv | conv = large.getConversion*() |
-    upperBound(conv).log2() > getComparisonSize(small) * 8
+    // We adjust the comparison size in the case of a signed integer type.
+    // This is to exclude the sign bit from the comparison that determines if the small type's size is sufficient to hold
+    // the value of the larger type determined with range analysis.
+    upperBound(conv).log2() > (getComparisonSize(small) * 8 - getComparisonSizeAdjustment(small))
   ) and
   // Ignore cases where the smaller type is int or larger
   // These are still bugs, but you should need a very large string or array to

cpp/ql/src/change-notes/2023-06-29-improve-comparison-wider-type.md

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* The `cpp/comparison-with-wider-type` query now correctly handles relational operations on signed operators. As a result the query may find more results.

cpp/ql/test/query-tests/Security/CWE/CWE-190/semmle/ComparisonWithWiderType/ComparisonWithWiderType.expected

@@ -1,6 +1,7 @@
 | test.c:4:14:4:18 | ... < ... | Comparison between $@ of type char and $@ of wider type int. | test.c:3:7:3:7 | c | c | test.c:2:17:2:17 | x | x |
 | test.c:9:14:9:18 | ... > ... | Comparison between $@ of type char and $@ of wider type int. | test.c:8:7:8:7 | c | c | test.c:7:17:7:17 | x | x |
 | test.c:14:14:14:18 | ... < ... | Comparison between $@ of type short and $@ of wider type int. | test.c:13:8:13:8 | s | s | test.c:12:17:12:17 | x | x |
+| test.c:42:15:42:29 | ... < ... | Comparison between $@ of type short and $@ of wider type int. | test.c:41:9:41:10 | s1 | s1 | test.c:42:20:42:29 | 65535 | 65535 |
 | test.c:65:14:65:18 | ... < ... | Comparison between $@ of type short and $@ of wider type int. | test.c:64:8:64:8 | s | s | test.c:63:17:63:17 | x | x |
 | test.c:87:14:87:18 | ... < ... | Comparison between $@ of type unsigned char and $@ of wider type unsigned int. | test.c:83:16:83:16 | c | c | test.c:84:15:84:15 | x | x |
 | test.c:91:14:91:23 | ... < ... | Comparison between $@ of type unsigned char and $@ of wider type int. | test.c:83:16:83:16 | c | c | test.c:91:18:91:23 | 65280 | 65280 |
@@ -13,3 +14,4 @@
 | test.c:107:14:107:26 | ... < ... | Comparison between $@ of type unsigned char and $@ of wider type unsigned int. | test.c:83:16:83:16 | c | c | test.c:107:19:107:25 | ... >> ... | ... >> ... |
 | test.c:128:15:128:21 | ... < ... | Comparison between $@ of type unsigned char and $@ of wider type unsigned int. | test.c:121:16:121:17 | uc | uc | test.c:123:19:123:20 | sz | sz |
 | test.c:139:15:139:21 | ... < ... | Comparison between $@ of type unsigned char and $@ of wider type unsigned int. | test.c:121:16:121:17 | uc | uc | test.c:123:19:123:20 | sz | sz |
+| test.c:156:9:156:14 | ... < ... | Comparison between $@ of type short and $@ of wider type int. | test.c:150:8:150:8 | s | s | test.c:151:6:151:7 | sx | sx |

cpp/ql/test/query-tests/Security/CWE/CWE-190/semmle/ComparisonWithWiderType/test.c

@@ -39,7 +39,7 @@ void test5 () {
 
 void test6() {
 	short  s1;
-	for (s1 = 0; s1 < 0x0000ffff; s1++) {}
+	for (s1 = 0; s1 < 0x0000ffff; s1++) {} // BAD
 }
 
 void test7(long long l) {
@@ -145,3 +145,22 @@ void test13() {
 	sz = (unsigned)sx & (unsigned)sy;
 	for (uc = 0; uc < sz; uc++) {} // GOOD
 }
+
+void test14() {
+	short s = 0;
+	int sx = 0x7FFF + 1;
+
+	// BAD: 's' is compared with a value of a wider type.
+	// 's' overflows before  reaching 'sx',
+	// causing an infinite loop
+	while (s < sx) {
+		s += 1;
+	}
+
+	unsigned int ux = 0;
+
+	// GOOD: 'ux' has a type  at least as wide as 'max_get'
+	while (ux < sx) {
+		ux += 1;
+	}
+}