Exclude bounds-check arithmetic from tainted-arithmetic sinks

The java/tainted-arithmetic query now recognizes when an arithmetic
expression appears directly as an operand of a comparison (e.g.,
`if (off + len > array.length)`). Such expressions are bounds checks,
not vulnerable computations, and are excluded via the existing
overflowIrrelevant predicate.

Add test cases for bounds-checking patterns that should not be flagged.

Author: MarkLee131

java/ql/lib/change-notes/2026-03-28-tainted-arithmetic-bounds-check.md

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* The `java/tainted-arithmetic` query no longer flags arithmetic expressions that are used directly as an operand of a comparison in bounds-checking patterns. For example, `if (off + len > array.length)` is now recognized as a bounds check rather than a potentially vulnerable computation, reducing false positives.

java/ql/lib/semmle/code/java/security/ArithmeticCommon.qll

@@ -132,7 +132,21 @@ private predicate inBitwiseAnd(Expr exp) {
 /** Holds if overflow/underflow is irrelevant for this expression. */
 predicate overflowIrrelevant(Expr exp) {
   inBitwiseAnd(exp) or
-  exp.getEnclosingCallable() instanceof HashCodeMethod
+  exp.getEnclosingCallable() instanceof HashCodeMethod or
+  arithmeticUsedInBoundsCheck(exp)
+}
+
+/**
+ * Holds if `exp` is an arithmetic expression used directly as an operand of a
+ * comparison, indicating it is part of a bounds check rather than a vulnerable
+ * computation. For example, in `if (off + len > array.length)`, the addition
+ * is the bounds check itself.
+ */
+private predicate arithmeticUsedInBoundsCheck(ArithExpr exp) {
+  exists(ComparisonExpr comp |
+    comp.getAnOperand() = exp and
+    comp.getEnclosingStmt() instanceof IfStmt
+  )
 }
 
 /**

java/ql/test/query-tests/security/CWE-190/semmle/tests/ArithmeticTainted.java

@@ -138,4 +138,18 @@ public static void test4(int data) {
 		// BAD: may underflow if input data is very small
 		--data;
 	}
+
+	public static void boundsCheckGood(byte[] bs, int off, int len) {
+		// GOOD: arithmetic used directly in a bounds check, not as a computation
+		if (off + len > bs.length) {
+			throw new IndexOutOfBoundsException();
+		}
+	}
+
+	public static void boundsCheckGood2(int[] arr, int offset, int count) {
+		// GOOD: subtraction used directly in a bounds check
+		if (offset - count < 0) {
+			throw new IndexOutOfBoundsException();
+		}
+	}
 }