Swift: Add String.Index.init as a source as as well.

Author: geoffw0

swift/ql/src/queries/Security/CWE-135/StringLengthConflation.ql

@@ -102,6 +102,10 @@ class StringLengthConflationConfiguration extends DataFlow::Configuration {
         // `prefix`, `suffix`
         funcName = ["prefix(_:)", "suffix(_:)"] and
         paramName = "maxLength"
+        or
+        // `String.Index.init`
+        funcName = "init(encodedOffset:)" and
+        paramName = "offset"
       ) and
       call.getFunction().(ApplyExpr).getStaticTarget().getName() = funcName and
       call.getFunction()

swift/ql/test/query-tests/Security/CWE-135/StringLengthConflation.expected

@@ -13,6 +13,7 @@ edges
 | StringLengthConflation.swift:135:36:135:38 | .count :  | StringLengthConflation.swift:135:36:135:46 | ... call to -(_:_:) ... |
 | StringLengthConflation.swift:141:28:141:30 | .count :  | StringLengthConflation.swift:141:28:141:38 | ... call to -(_:_:) ... |
 nodes
+| StringLengthConflation.swift:53:43:53:46 | .length | semmle.label | .length |
 | StringLengthConflation.swift:72:33:72:35 | .count | semmle.label | .count |
 | StringLengthConflation.swift:78:47:78:49 | .count | semmle.label | .count |
 | StringLengthConflation.swift:93:28:93:31 | .length :  | semmle.label | .length :  |
@@ -43,6 +44,7 @@ nodes
 | StringLengthConflation.swift:141:28:141:38 | ... call to -(_:_:) ... | semmle.label | ... call to -(_:_:) ... |
 subpaths
 #select
+| StringLengthConflation.swift:53:43:53:46 | .length | StringLengthConflation.swift:53:43:53:46 | .length | StringLengthConflation.swift:53:43:53:46 | .length | This NSString length is used in a String, but it may not be equivalent. |
 | StringLengthConflation.swift:72:33:72:35 | .count | StringLengthConflation.swift:72:33:72:35 | .count | StringLengthConflation.swift:72:33:72:35 | .count | This String length is used in an NSString, but it may not be equivalent. |
 | StringLengthConflation.swift:78:47:78:49 | .count | StringLengthConflation.swift:78:47:78:49 | .count | StringLengthConflation.swift:78:47:78:49 | .count | This String length is used in an NSString, but it may not be equivalent. |
 | StringLengthConflation.swift:93:28:93:40 | ... call to -(_:_:) ... | StringLengthConflation.swift:93:28:93:31 | .length :  | StringLengthConflation.swift:93:28:93:40 | ... call to -(_:_:) ... | This NSString length is used in a String, but it may not be equivalent. |

swift/ql/test/query-tests/Security/CWE-135/StringLengthConflation.swift

@@ -50,7 +50,7 @@ func test(s: String) {
     // --- constructing a String.Index from integer ---
 
     let ix1 = String.Index(encodedOffset: s.count) // GOOD
-    let ix2 = String.Index(encodedOffset: ns.length) // BAD: NSString length used in String.Index [NOT DETECTED]
+    let ix2 = String.Index(encodedOffset: ns.length) // BAD: NSString length used in String.Index
     let ix3 = String.Index(encodedOffset: s.utf8.count) // BAD: String.utf8 length used in String.Index [NOT DETECTED]
     let ix4 = String.Index(encodedOffset: s.utf16.count) // BAD: String.utf16 length used in String.Index [NOT DETECTED]
     let ix5 = String.Index(encodedOffset: s.unicodeScalars.count) // BAD: string.unicodeScalars length used in String.Index [NOT DETECTED]