Reject unsafe Core Foundation object hooks

CHANGELOG.md

@@ -14,6 +14,7 @@
 * Preserve object-hook super trampolines in optimized Release builds, fixing https://github.com/steipete/InterposeKit/issues/29. Thanks to [@Thomvis](https://github.com/Thomvis).
 * Preserve floating-point arguments when object hooks invoke original methods on arm64. Thanks to [@ishutinvv](https://github.com/ishutinvv).
 * Run class-availability hooks after Objective-C loads a new image, fixing https://github.com/steipete/InterposeKit/issues/26.
+* Reject Core Foundation-backed object hooks before dynamic subclassing, preventing crashes such as https://github.com/steipete/InterposeKit/issues/23.
 
 ## 0.01
 

README.md

@@ -85,7 +85,7 @@ Hi there 👋 and Interpose
 
 InterposeKit can hook classes and object. Class hooking is similar to swizzling, but object-based hooking offers a variety of new ways to set hooks. This is achieved via creating a dynamic subclass at runtime. 
 
-Caveat: Hooking will fail with an error if the object uses KVO. The KVO machinery is fragile and it's to easy to cause a crash. Using KVO after a hook was created is supported and will not cause issues.
+Caveat: Hooking will fail with an error if the object uses KVO or is backed by Core Foundation, such as `NSURL`. These objects rely on runtime behavior that is incompatible with InterposeKit's dynamic subclass. Using KVO after a hook was created is supported and will not cause issues.
 
 ## Various ways to define the signature
 

Sources/InterposeKit/InterposeError.swift

@@ -23,6 +23,9 @@ public enum InterposeError: LocalizedError {
     /// Adding a hook eventually crashes the KVO management code so we reject hooking altogether in this case.
     case keyValueObservationDetected(AnyObject)
 
+    /// Core Foundation-backed objects do not support isa-swizzling from Swift.
+    case coreFoundationObjectDetected(AnyObject)
+
     /// Object is lying about it's actual class metadata.
     /// This usually happens when other swizzling libraries (like Aspects) also interfere with a class.
     /// While this might just work, it's not worth risking a crash, so similar to KVO this case is rejected.
@@ -61,6 +64,8 @@ extension InterposeError: Equatable {
             return "Unable to add method: -[\(klass) \(selector)]"
         case .keyValueObservationDetected(let obj):
             return "Unable to hook object that uses Key Value Observing: \(obj)"
+        case .coreFoundationObjectDetected(let obj):
+            return "Unable to hook Core Foundation-backed object: \(obj)"
         case .objectPosingAsDifferentClass(let obj, let actualClass):
             return "Unable to hook \(type(of: obj)) posing as \(NSStringFromClass(actualClass))/"
         case .invalidState(let expectedState):

Sources/InterposeKit/InterposeKit.swift

@@ -1,3 +1,6 @@
+#if !os(Linux)
+import CoreFoundation
+#endif
 import Foundation
 
 extension NSObject {
@@ -41,7 +44,7 @@ final public class Interpose {
 
     // Checks if a object is posing as a different class
     // via implementing 'class' and returning something else.
-    private func checkObjectPosingAsDifferentClass(_ object: AnyObject) -> AnyClass? {
+    private static func checkObjectPosingAsDifferentClass(_ object: AnyObject) -> AnyClass? {
         let perceivedClass: AnyClass = type(of: object)
         let actualClass: AnyClass = object_getClass(object)!
         if actualClass != perceivedClass {
@@ -51,10 +54,35 @@ final public class Interpose {
     }
 
     // This is based on observation, there is no documented way
-    private func isKVORuntimeGeneratedClass(_ klass: AnyClass) -> Bool {
+    private static func isKVORuntimeGeneratedClass(_ klass: AnyClass) -> Bool {
         String(cString: class_getName(klass)).contains("NSKVONotifying_")
     }
 
+    #if !os(Linux)
+    private static let objectiveCObjectTypeID = CFGetTypeID(NSObject())
+    #endif
+
+    private static func isCoreFoundationBackedObject(_ object: AnyObject) -> Bool {
+        #if os(Linux)
+        return false
+        #else
+        return CFGetTypeID(object as CFTypeRef) != objectiveCObjectTypeID
+        #endif
+    }
+
+    static func validateObjectForHooking(_ object: AnyObject) throws {
+        if let actualClass = checkObjectPosingAsDifferentClass(object) {
+            if isKVORuntimeGeneratedClass(actualClass) {
+                throw InterposeError.keyValueObservationDetected(object)
+            } else if !InterposeSubclass.isInterposeSubclass(actualClass) {
+                throw InterposeError.objectPosingAsDifferentClass(object, actualClass: actualClass)
+            }
+        }
+        if isCoreFoundationBackedObject(object) {
+            throw InterposeError.coreFoundationObjectDetected(object)
+        }
+    }
+
     /// Initializes an instance of Interpose for a specific class.
     /// If `builder` is present, `apply()` is automatically called.
     public init(_ `class`: AnyClass, builder: ((Interpose) throws -> Void)? = nil) throws {
@@ -72,13 +100,7 @@ final public class Interpose {
         self.object = object
         self.class = type(of: object)
 
-        if let actualClass = checkObjectPosingAsDifferentClass(object) {
-            if isKVORuntimeGeneratedClass(actualClass) {
-                throw InterposeError.keyValueObservationDetected(object)
-            } else {
-                throw InterposeError.objectPosingAsDifferentClass(object, actualClass: actualClass)
-            }
-        }
+        try Self.validateObjectForHooking(object)
 
         // Only apply if a builder is present
         if let builder = builder {

Sources/InterposeKit/InterposeSubclass.swift

@@ -69,12 +69,16 @@ class InterposeSubclass {
     /// We need to reuse a dynamic subclass if the object already has one.
     private func getExistingSubclass() -> AnyClass? {
         let actualClass: AnyClass = object_getClass(object)!
-        if NSStringFromClass(actualClass).hasPrefix(Constants.subclassSuffix) {
+        if Self.isInterposeSubclass(actualClass) {
             return actualClass
         }
         return nil
     }
 
+    static func isInterposeSubclass(_ klass: AnyClass) -> Bool {
+        NSStringFromClass(klass).hasPrefix(Constants.subclassSuffix)
+    }
+
     #if !os(Linux)
     private func replaceGetClass(in class: AnyClass, decoy perceivedClass: AnyClass) {
         // crashes on linux

Sources/InterposeKit/ObjectHook.swift

@@ -18,6 +18,7 @@ extension Interpose {
         /// Initialize a new hook to interpose an instance method.
         public init(object: AnyObject, selector: Selector,
                     implementation: (ObjectHook<MethodSignature, HookSignature>) -> HookSignature?) throws {
+            try Interpose.validateObjectForHooking(object)
             self.object = object
             try super.init(class: type(of: object), selector: selector)
             let block = implementation(self) as AnyObject

Tests/InterposeKitTests/ObjectInterposeTests.swift

@@ -4,6 +4,26 @@
 
 final class ObjectInterposeTests: InterposeKitTestCase {
 
+    func testRejectsCoreFoundationBackedObject() throws {
+        let url = try XCTUnwrap(NSURL(string: "https://www.google.com"))
+        let expectedError = InterposeError.coreFoundationObjectDetected(url)
+
+        XCTAssertThrowsError(try Interpose(url)) { error in
+            XCTAssertEqual(error as? InterposeError, expectedError)
+        }
+        XCTAssertThrowsError(try url.hook(
+            #selector(getter: NSURL.host),
+            methodSignature: (@convention(c) (AnyObject, Selector) -> String).self,
+            hookSignature: (@convention(block) (AnyObject) -> String).self
+        ) { _ in
+            { _ in "www.facebook.com" }
+        }) { error in
+            XCTAssertEqual(error as? InterposeError, expectedError)
+        }
+
+        XCTAssertEqual(url.host, "www.google.com")
+    }
+
     func testInterposeSingleObject() throws {
         let testObj = TestClass()
         let testObj2 = TestClass()