[StaticAnalyzer] Fix state update in VisitObjCForCollectionStmt (#124477)

In `VisitObjCForCollectionStmt`, the function does `evalLocation` for
the current element at the original source state `Pred`. The evaluation
may result in a new state, say `PredNew`. I.e., there is a transition:
`Pred -> PredNew`, though it is a very rare case that `Pred` is NOT
identical to `PredNew`. (This explains why the bug exists for many years
but no one noticed until recently a crash observed downstream.) Later,
the original code does NOT use `PredNew` as the new source state in
`StmtNodeBuilder` for next transitions. In cases `Pred != PredNew`, the
program ill behaves.

(rdar://143280254)

Author: ziqingluo-90

clang/lib/StaticAnalyzer/Core/ExprEngineObjC.cpp

@@ -124,24 +124,26 @@ void ExprEngine::VisitObjCForCollectionStmt(const ObjCForCollectionStmt *S,
 
   bool isContainerNull = state->isNull(collectionV).isConstrainedTrue();
 
-  ExplodedNodeSet dstLocation;
-  evalLocation(dstLocation, S, elem, Pred, state, elementV, false);
+  ExplodedNodeSet DstLocation; // states in `DstLocation` may differ from `Pred`
+  evalLocation(DstLocation, S, elem, Pred, state, elementV, false);
 
-  ExplodedNodeSet Tmp;
-  StmtNodeBuilder Bldr(Pred, Tmp, *currBldrCtx);
+  for (ExplodedNode *dstLocation : DstLocation) {
+    ExplodedNodeSet DstLocationSingleton{dstLocation}, Tmp;
+    StmtNodeBuilder Bldr(dstLocation, Tmp, *currBldrCtx);
 
-  if (!isContainerNull)
-    populateObjCForDestinationSet(dstLocation, svalBuilder, S, elem, elementV,
-                                  SymMgr, currBldrCtx, Bldr,
-                                  /*hasElements=*/true);
+    if (!isContainerNull)
+      populateObjCForDestinationSet(DstLocationSingleton, svalBuilder, S, elem,
+                                    elementV, SymMgr, currBldrCtx, Bldr,
+                                    /*hasElements=*/true);
 
-  populateObjCForDestinationSet(dstLocation, svalBuilder, S, elem, elementV,
-                                SymMgr, currBldrCtx, Bldr,
-                                /*hasElements=*/false);
+    populateObjCForDestinationSet(DstLocationSingleton, svalBuilder, S, elem,
+                                  elementV, SymMgr, currBldrCtx, Bldr,
+                                  /*hasElements=*/false);
 
-  // Finally, run any custom checkers.
-  // FIXME: Eventually all pre- and post-checks should live in VisitStmt.
-  getCheckerManager().runCheckersForPostStmt(Dst, Tmp, S, *this);
+    // Finally, run any custom checkers.
+    // FIXME: Eventually all pre- and post-checks should live in VisitStmt.
+    getCheckerManager().runCheckersForPostStmt(Dst, Tmp, S, *this);
+  }
 }
 
 void ExprEngine::VisitObjCMessage(const ObjCMessageExpr *ME,

clang/test/Analysis/bugfix-124477.m

@@ -0,0 +1,39 @@
+// RUN: %clang_analyze_cc1 -analyzer-checker=core,apiModeling,nullability.NullableDereferenced,nullability.NullabilityBase -x objective-c %s
+/*
+  This test is reduced from a static analyzer crash. The bug causing
+  the crash is explained in #124477.  It can only be triggered in some
+  rare cases so please do not modify this reproducer.
+*/
+
+#pragma clang assume_nonnull begin
+# 15 "some-sys-header.h" 1 3
+@class NSArray, NSObject;
+
+@interface Base
+@property (readonly, copy) NSArray *array;
+@end
+
+#pragma clang assume_nonnull end
+# 8 "this-file.m" 2
+
+
+@interface Test : Base
+
+@property (readwrite, copy, nullable) NSObject *label;
+@property (readwrite, strong, nullable) Test * field;
+
+- (void)f;
+
+@end
+
+@implementation Test
+- (void)f
+{
+  NSObject * X;
+
+  for (NSObject *ele in self.field.array) {}
+  self.label = X;  
+}
+@end
+
+

clang/unittests/StaticAnalyzer/CMakeLists.txt

@@ -15,6 +15,7 @@ add_clang_unittest(StaticAnalysisTests
   IsCLibraryFunctionTest.cpp
   MemRegionDescriptiveNameTest.cpp
   NoStateChangeFuncVisitorTest.cpp
+  ObjcBug-124477.cpp
   ParamRegionTest.cpp
   RangeSetTest.cpp
   RegisterCustomCheckersTest.cpp

clang/unittests/StaticAnalyzer/ObjcBug-124477.cpp

@@ -0,0 +1,63 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "CheckerRegistration.h"
+#include "clang/StaticAnalyzer/Core/Checker.h"
+#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
+#include "clang/StaticAnalyzer/Core/PathSensitive/ProgramStateTrait.h"
+#include "clang/StaticAnalyzer/Frontend/AnalysisConsumer.h"
+#include "clang/StaticAnalyzer/Frontend/CheckerRegistry.h"
+#include "gtest/gtest.h"
+
+using namespace clang;
+using namespace ento;
+
+// Some dummy trait that we can mutate back and forth to force a new State.
+REGISTER_TRAIT_WITH_PROGRAMSTATE(Flag, bool)
+
+namespace {
+class FlipFlagOnCheckLocation : public Checker<check::Location> {
+public:
+  // We make sure we alter the State every time we model a checkLocation event.
+  void checkLocation(SVal l, bool isLoad, const Stmt *S,
+                     CheckerContext &C) const {
+    ProgramStateRef State = C.getState();
+    State = State->set<Flag>(!State->get<Flag>());
+    C.addTransition(State);
+  }
+};
+
+void addFlagFlipperChecker(AnalysisASTConsumer &AnalysisConsumer,
+                           AnalyzerOptions &AnOpts) {
+  AnOpts.CheckersAndPackages = {{"test.FlipFlagOnCheckLocation", true}};
+  AnalysisConsumer.AddCheckerRegistrationFn([](CheckerRegistry &Registry) {
+    Registry.addChecker<FlipFlagOnCheckLocation>("test.FlipFlagOnCheckLocation",
+                                                 "Description", "");
+  });
+}
+
+TEST(ObjCTest, CheckLocationEventsShouldMaterializeInObjCForCollectionStmts) {
+  // Previously, the `ExprEngine::hasMoreIteration` may fired an assertion
+  // because we forgot to handle correctly the resulting nodes of the
+  // check::Location callback for the ObjCForCollectionStmts.
+  // This caused inconsistencies in the graph and triggering the assertion.
+  // See #124477 for more details.
+  std::string Diags;
+  EXPECT_TRUE(runCheckerOnCodeWithArgs<addFlagFlipperChecker>(
+      R"(
+    @class NSArray, NSDictionary, NSString;
+    extern void NSLog(NSString *format, ...) __attribute__((format(__NSString__, 1, 2)));
+    void entrypoint(NSArray *bowl) {
+      for (NSString *fruit in bowl) { // no-crash
+        NSLog(@"Fruit: %@", fruit);
+      }
+    })",
+      {"-x", "objective-c"}, Diags));
+}
+
+} // namespace