[clang] [clang][dataflow] Fix crash when `operator=` result type is not destination type. (PR #90898)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-analysis

Author: None (martinboehme)

<details>
<summary>Changes</summary>

The existing code was full of comments about how we assume this is always the
case, but it's not mandated by the standard, and there is code out there that
returns a different type. So check that the result type is in fact the same as
the destination type before attempting to copy to the result.

To make sure that we don't bail out in more cases than intended, I've extended
existing tests to verify that in the common case, we do return the destination
object (by reference or value, as the case may be).


---
Full diff: https://github.com/llvm/llvm-project/pull/90898.diff


2 Files Affected:

- (modified) clang/lib/Analysis/FlowSensitive/Transfer.cpp (+15-6) 
- (modified) clang/unittests/Analysis/FlowSensitive/TransferTest.cpp (+71-2) 


``````````diff
diff --git a/clang/lib/Analysis/FlowSensitive/Transfer.cpp b/clang/lib/Analysis/FlowSensitive/Transfer.cpp
index fd224aeb79b151..5a57f11b000d8a 100644
--- a/clang/lib/Analysis/FlowSensitive/Transfer.cpp
+++ b/clang/lib/Analysis/FlowSensitive/Transfer.cpp
@@ -556,14 +556,23 @@ class TransferVisitor : public ConstStmtVisitor<TransferVisitor> {
 
       copyRecord(*LocSrc, *LocDst, Env);
 
-      // If the expr is a glvalue, we can reasonably assume the operator is
-      // returning T& and thus we can assign it `LocDst`.
-      if (S->isGLValue()) {
+      // The assignment operator can have an arbitrary return type. We model the
+      // return value only if the return type is the same as or a base class of
+      // the destination type.
+      if (S->getType().getCanonicalType().getUnqualifiedType() !=
+          LocDst->getType().getCanonicalType().getUnqualifiedType()) {
+        auto ReturnDecl = S->getType()->getAsCXXRecordDecl();
+        auto DstDecl = LocDst->getType()->getAsCXXRecordDecl();
+        if (ReturnDecl == nullptr || DstDecl == nullptr)
+          return;
+        if (!DstDecl->isDerivedFrom(ReturnDecl))
+          return;
+      }
+
+      if (S->isGLValue())
         Env.setStorageLocation(*S, *LocDst);
-      } else if (S->getType()->isRecordType()) {
-        // Assume that the assignment returns the assigned value.
+      else
         copyRecord(*LocDst, Env.getResultObjectLocation(*S), Env);
-      }
 
       return;
     }
diff --git a/clang/unittests/Analysis/FlowSensitive/TransferTest.cpp b/clang/unittests/Analysis/FlowSensitive/TransferTest.cpp
index 301bec32c0cf1d..0b441faa6c76da 100644
--- a/clang/unittests/Analysis/FlowSensitive/TransferTest.cpp
+++ b/clang/unittests/Analysis/FlowSensitive/TransferTest.cpp
@@ -2228,7 +2228,7 @@ TEST(TransferTest, AssignmentOperator) {
       A Foo = { 1 };
       A Bar = { 2 };
       // [[p1]]
-      Foo = Bar;
+      A &Rval = (Foo = Bar);
       // [[p2]]
       Foo.Baz = 3;
       // [[p3]]
@@ -2274,6 +2274,7 @@ TEST(TransferTest, AssignmentOperator) {
               cast<RecordStorageLocation>(Env2.getStorageLocation(*BarDecl));
 
           EXPECT_TRUE(recordsEqual(*FooLoc2, *BarLoc2, Env2));
+          EXPECT_EQ(&getLocForDecl(ASTCtx, Env2, "Rval"), FooLoc2);
 
           const auto *FooBazVal2 =
               cast<IntegerValue>(getFieldValue(FooLoc2, *BazDecl, Env2));
@@ -2441,7 +2442,75 @@ TEST(TransferTest, AssignmentOperatorReturnsByValue) {
   // This is a crash repro.
   std::string Code = R"(
     struct S {
-      S operator=(S&& other);
+      S operator=(const S&);
+      int i;
+    };
+    void target() {
+      S S1 = { 1 };
+      S S2 = { 2 };
+      S S3 = { 3 };
+      // [[before]]
+      // Test that the returned value is modeled by assigning to another value.
+      S1 = (S2 = S3);
+      (void)0;
+      // [[after]]
+    }
+  )";
+  runDataflow(
+      Code,
+      [](const llvm::StringMap<DataflowAnalysisState<NoopLattice>> &Results,
+         ASTContext &ASTCtx) {
+        const ValueDecl *S1Decl = findValueDecl(ASTCtx, "S1");
+        const ValueDecl *S2Decl = findValueDecl(ASTCtx, "S2");
+        const ValueDecl *S3Decl = findValueDecl(ASTCtx, "S3");
+
+        const Environment &EnvBefore =
+            getEnvironmentAtAnnotation(Results, "before");
+
+        EXPECT_FALSE(recordsEqual(
+            *EnvBefore.get<RecordStorageLocation>(*S1Decl),
+            *EnvBefore.get<RecordStorageLocation>(*S2Decl), EnvBefore));
+        EXPECT_FALSE(recordsEqual(
+            *EnvBefore.get<RecordStorageLocation>(*S2Decl),
+            *EnvBefore.get<RecordStorageLocation>(*S3Decl), EnvBefore));
+
+        const Environment &EnvAfter =
+            getEnvironmentAtAnnotation(Results, "after");
+
+        EXPECT_TRUE(recordsEqual(*EnvAfter.get<RecordStorageLocation>(*S1Decl),
+                                 *EnvAfter.get<RecordStorageLocation>(*S2Decl),
+                                 EnvAfter));
+        EXPECT_TRUE(recordsEqual(*EnvAfter.get<RecordStorageLocation>(*S2Decl),
+                                 *EnvAfter.get<RecordStorageLocation>(*S3Decl),
+                                 EnvAfter));
+      });
+}
+
+TEST(TransferTest, AssignmentOperatorReturnsDifferentTypeByRef) {
+  // This is a crash repro.
+  std::string Code = R"(
+    struct DifferentType {};
+    struct S {
+      DifferentType& operator=(const S&);
+    };
+    void target() {
+      S s;
+      s = S();
+      // [[p]]
+    }
+  )";
+  runDataflow(
+      Code,
+      [](const llvm::StringMap<DataflowAnalysisState<NoopLattice>> &Results,
+         ASTContext &ASTCtx) {});
+}
+
+TEST(TransferTest, AssignmentOperatorReturnsDifferentTypeByValue) {
+  // This is a crash repro.
+  std::string Code = R"(
+    struct DifferentType {};
+    struct S {
+      DifferentType operator=(const S&);
     };
     void target() {
       S s;

``````````

</details>


https://github.com/llvm/llvm-project/pull/90898