[coroutines][coro_lifetimebound] Detect lifetime issues with lambda c…

…aptures (#77066)

### Problem

```cpp
co_task<int> coro() {
    int a = 1;
    auto lamb = [a]() -> co_task<int> {
        co_return a; // 'a' in the lambda object dies after the iniital_suspend in the lambda coroutine.
    }();
    co_return co_await lamb;
}
```
[use-after-free](https://godbolt.org/z/GWPEovWWc)

Lambda captures (even by value) are prone to use-after-free once the
lambda object dies. In the above example, the lambda object appears only
as a temporary in the call expression. It dies after the first
suspension (`initial_suspend`) in the lambda.
On resumption in `co_await lamb`, the lambda accesses `a` which is part
of the already-dead lambda object.

---

### Solution

This problem can be formulated by saying that the `this` parameter of
the lambda call operator is a lifetimebound parameter. The lambda object
argument should therefore live atleast as long as the return object.
That said, this requirement does not hold if the lambda does not have a
capture list. In principle, the coroutine frame still has a reference to
a dead lambda object, but it is easy to see that the object would not be
used in the lambda-coroutine body due to no capture list.

It is safe to use this pattern inside a`co_await` expression due to the
lifetime extension of temporaries. Example:

```cpp
co_task<int> coro() {
    int a = 1;
    int res = co_await [a]() -> co_task<int> { co_return a; }();
    co_return res;
}
```
---
### Background

This came up in the discussion with seastar folks on
[RFC](https://discourse.llvm.org/t/rfc-lifetime-bound-check-for-parameters-of-coroutines/74253/19?u=usx95).
This is a fairly common pattern in continuation-style-passing (CSP)
async programming involving futures and continuations. Document ["Lambda
coroutine
fiasco"](https://github.com/scylladb/seastar/blob/master/doc/lambda-coroutine-fiasco.md)
by Seastar captures the problem.
This pattern makes the migration from CSP-style async programming to
coroutines very bugprone.


Fixes #76995

---------

Co-authored-by: Chuanqi Xu <yedeng.yd@linux.alibaba.com>

clang/include/clang/Sema/Sema.h

@@ -11249,6 +11249,11 @@ class Sema final {
   VarDecl *buildCoroutinePromise(SourceLocation Loc);
   void CheckCompletedCoroutineBody(FunctionDecl *FD, Stmt *&Body);
 
+  // Heuristically tells if the function is `get_return_object` member of a
+  // coroutine promise_type by matching the function name.
+  static bool CanBeGetReturnObject(const FunctionDecl *FD);
+  static bool CanBeGetReturnTypeOnAllocFailure(const FunctionDecl *FD);
+
   // As a clang extension, enforces that a non-coroutine function must be marked
   // with [[clang::coro_wrapper]] if it returns a type marked with
   // [[clang::coro_return_type]].

clang/lib/Sema/SemaCoroutine.cpp

@@ -16,6 +16,7 @@
 #include "CoroutineStmtBuilder.h"
 #include "clang/AST/ASTLambda.h"
 #include "clang/AST/Decl.h"
+#include "clang/AST/Expr.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/StmtCXX.h"
 #include "clang/Basic/Builtins.h"

clang/lib/Sema/SemaDecl.cpp

@@ -15912,13 +15912,26 @@ static void diagnoseImplicitlyRetainedSelf(Sema &S) {
           << FixItHint::CreateInsertion(P.first, "self->");
 }
 
+static bool methodHasName(const FunctionDecl *FD, StringRef Name) {
+  return isa<CXXMethodDecl>(FD) && FD->param_empty() &&
+         FD->getDeclName().isIdentifier() && FD->getName().equals(Name);
+}
+
+bool Sema::CanBeGetReturnObject(const FunctionDecl *FD) {
+  return methodHasName(FD, "get_return_object");
+}
+
+bool Sema::CanBeGetReturnTypeOnAllocFailure(const FunctionDecl *FD) {
+  return FD->isStatic() &&
+         methodHasName(FD, "get_return_object_on_allocation_failure");
+}
+
 void Sema::CheckCoroutineWrapper(FunctionDecl *FD) {
   RecordDecl *RD = FD->getReturnType()->getAsRecordDecl();
   if (!RD || !RD->getUnderlyingDecl()->hasAttr<CoroReturnTypeAttr>())
     return;
-  // Allow `get_return_object()`.
-  if (FD->getDeclName().isIdentifier() &&
-      FD->getName().equals("get_return_object") && FD->param_empty())
+  // Allow some_promise_type::get_return_object().
+  if (CanBeGetReturnObject(FD) || CanBeGetReturnTypeOnAllocFailure(FD))
     return;
   if (!FD->hasAttr<CoroWrapperAttr>())
     Diag(FD->getLocation(), diag::err_coroutine_return_type) << RD;

clang/lib/Sema/SemaInit.cpp

@@ -12,6 +12,7 @@
 
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/DeclObjC.h"
+#include "clang/AST/Expr.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/ExprObjC.h"
 #include "clang/AST/ExprOpenMP.h"
@@ -7583,15 +7584,27 @@ static void visitLifetimeBoundArguments(IndirectLocalPath &Path, Expr *Call,
     Path.pop_back();
   };
 
-  if (ObjectArg && implicitObjectParamIsLifetimeBound(Callee))
-    VisitLifetimeBoundArg(Callee, ObjectArg);
-
   bool CheckCoroCall = false;
   if (const auto *RD = Callee->getReturnType()->getAsRecordDecl()) {
     CheckCoroCall = RD->hasAttr<CoroLifetimeBoundAttr>() &&
                     RD->hasAttr<CoroReturnTypeAttr>() &&
                     !Callee->hasAttr<CoroDisableLifetimeBoundAttr>();
   }
+
+  if (ObjectArg) {
+    bool CheckCoroObjArg = CheckCoroCall;
+    // Coroutine lambda objects with empty capture list are not lifetimebound.
+    if (auto *LE = dyn_cast<LambdaExpr>(ObjectArg->IgnoreImplicit());
+        LE && LE->captures().empty())
+      CheckCoroObjArg = false;
+    // Allow `get_return_object()` as the object param (__promise) is not
+    // lifetimebound.
+    if (Sema::CanBeGetReturnObject(Callee))
+      CheckCoroObjArg = false;
+    if (implicitObjectParamIsLifetimeBound(Callee) || CheckCoroObjArg)
+      VisitLifetimeBoundArg(Callee, ObjectArg);
+  }
+
   for (unsigned I = 0,
                 N = std::min<unsigned>(Callee->getNumParams(), Args.size());
        I != N; ++I) {

clang/test/SemaCXX/coro-lifetimebound.cpp

@@ -64,6 +64,10 @@ Co<int> bar_coro(const int &b, int c) {
       : bar_coro(0, 1); // expected-warning {{returning address of local temporary object}}
 }
 
+// =============================================================================
+// Lambdas
+// =============================================================================
+namespace lambdas {
 void lambdas() {
   auto unsafe_lambda = [] [[clang::coro_wrapper]] (int b) {
     return foo_coro(b); // expected-warning {{address of stack memory associated with parameter}}
@@ -84,14 +88,59 @@ void lambdas() {
     co_return x + co_await foo_coro(b);
   };
 }
+
+Co<int> lambda_captures() {
+  int a = 1;
+  // Temporary lambda object dies.
+  auto lamb = [a](int x, const int& y) -> Co<int> { // expected-warning {{temporary whose address is used as value of local variable 'lamb'}}
+    co_return x + y + a;
+  }(1, a);
+  // Object dies but it has no capture.
+  auto no_capture = []() -> Co<int> { co_return 1; }();
+  auto bad_no_capture = [](const int& a) -> Co<int> { co_return a; }(1); // expected-warning {{temporary}}
+  // Temporary lambda object with lifetime extension under co_await.
+  int res = co_await [a](int x, const int& y) -> Co<int> {
+    co_return x + y + a;
+  }(1, a);
+  // Lambda object on stack should be fine.
+  auto lamb2 = [a]() -> Co<int> { co_return a; };
+  auto on_stack = lamb2();
+  auto res2 = co_await on_stack;
+  co_return 1;
+}
+} // namespace lambdas
+
+// =============================================================================
+// Member coroutines
+// =============================================================================
+namespace member_coroutines{
+struct S {
+  Co<int> member(const int& a) { co_return a; }
+};
+
+Co<int> use() {
+  S s;
+  int a = 1;
+  auto test1 = s.member(1);  // expected-warning {{temporary whose address is used as value of local variable}}
+  auto test2 = s.member(a);
+  auto test3 = S{}.member(a);  // expected-warning {{temporary whose address is used as value of local variable}}
+  co_return 1;
+}
+
+[[clang::coro_wrapper]] Co<int> wrapper(const int& a) {
+  S s;
+  return s.member(a); // expected-warning {{address of stack memory}}
+}
+} // member_coroutines
+
 // =============================================================================
 // Safe usage when parameters are value
 // =============================================================================
 namespace by_value {
 Co<int> value_coro(int b) { co_return co_await foo_coro(b); }
 [[clang::coro_wrapper]] Co<int> wrapper1(int b) { return value_coro(b); }
 [[clang::coro_wrapper]] Co<int> wrapper2(const int& b) { return value_coro(b); }
-}
+} // namespace by_value
 
 // =============================================================================
 // Lifetime bound but not a Coroutine Return Type: No analysis.
@@ -122,11 +171,29 @@ CoNoCRT<int> bar(int a) {
 namespace disable_lifetimebound {
 Co<int> foo(int x) {  co_return x; }
 
-[[clang::coro_wrapper, clang::coro_disable_lifetimebound]] 
+[[clang::coro_wrapper, clang::coro_disable_lifetimebound]]
 Co<int> foo_wrapper(const int& x) { return foo(x); }
 
 [[clang::coro_wrapper]] Co<int> caller() {
   // The call to foo_wrapper is wrapper is safe.
   return foo_wrapper(1);
 }
+
+struct S{
+[[clang::coro_wrapper, clang::coro_disable_lifetimebound]]
+Co<int> member(const int& x) { return foo(x); }
+};
+
+Co<int> use() {
+  S s;
+  int a = 1;
+  auto test1 = s.member(1); // param is not flagged.
+  auto test2 = S{}.member(a); // 'this' is not flagged.
+  co_return 1;
+}
+
+[[clang::coro_wrapper]] Co<int> return_stack_addr(const int& a) {
+  S s;
+  return s.member(a); // return of stack addr is not flagged.
+}
 } // namespace disable_lifetimebound

clang/test/SemaCXX/coro-return-type-and-wrapper.cpp

@@ -5,11 +5,23 @@ using std::suspend_always;
 using std::suspend_never;
 
 
+namespace std {
+  struct nothrow_t {};
+  constexpr nothrow_t nothrow = {};
+}
+
+using SizeT = decltype(sizeof(int));
+
+void* operator new(SizeT __sz, const std::nothrow_t&) noexcept;
+
 template <typename T> struct [[clang::coro_return_type]] Gen {
   struct promise_type {
     Gen<T> get_return_object() {
       return {};
     }
+    static Gen<T> get_return_object_on_allocation_failure() {
+      return {};
+    }
     suspend_always initial_suspend();
     suspend_always final_suspend() noexcept;
     void unhandled_exception();