[cfe-dev] Miscompilation: heap-use-after-free in C++ coroutines

Mon Feb 8 08:49:46 PST 2021

Hi Chuanqi,

Amazing! Already fixed bugs are the nicest type of bugs :)
(I guess I should have checked trunk, too)

FYI: It seems this was fixed by https://reviews.llvm.org/D90990

Cheers,
Adrian

From: chuanqi.xcq <yedeng.yd at linux.alibaba.com>
Date: Monday, 8. February 2021 at 10:46
To: cfe-dev at lists.llvm.org <cfe-dev at lists.llvm.org>, Adrian Vogelsgesang <avogelsgesang at tableau.com>
Subject: Re: [cfe-dev] Miscompilation: heap-use-after-free in C++ coroutines
Hi Adrian,

Sorry for disturbing. When I am trying to look into this, I find that this bug is fixed in llvm trunk. (https://godbolt.org/z/ehTa78<https://godbolt.org/z/ehTa78>).

------------------------------------------------------------------
From:chuanqi.xcq via cfe-dev <cfe-dev at lists.llvm.org>
Send Time:2021年2月8日(星期一) 16:27
To:cfe-dev at lists.llvm.org <cfe-dev at lists.llvm.org>; Adrian Vogelsgesang <avogelsgesang at tableau.com>
Subject:Re: [cfe-dev] Miscompilation: heap-use-after-free in C++ coroutines

Hi Adrian,

Oh yes, the corourtine should be considered as suspended in await_suspend from the wording. I'm trying to fix this.

------------------------------------------------------------------
From:Adrian Vogelsgesang <avogelsgesang at tableau.com>
Send Time:2021年2月6日(星期六) 04:33
To:chuanqi.xcq <yedeng.yd at linux.alibaba.com>; cfe-dev at lists.llvm.org <cfe-dev at lists.llvm.org>
Subject:Re: [cfe-dev] Miscompilation: heap-use-after-free in C++ coroutines

HI Chuanqi,

> I met this problem before. I agree that this behavior would make symmetric transfer much harder.

Thanks for taking a look at this - I really appreciate it!

> the coroutine **isn't suspended** in the await_suspend. So it is an undefined behavior if we call `coroutine_handle::destroy()` in await_suspend.

I think section 7.6.2.4<http://7.6.2.4>, subpoint 5.1 is the relevant part of the standard here:
```
The await-expression evaluates the (possibly-converted) o expression and the await-ready expression, then:
(5.1) — If the result of await-ready is false, the coroutine is considered suspended. Then:
(5.1.1) — If the type of await-suspend is std::coroutine_handle<Z>, await-suspend.resume() is evaluated.
```

My understanding of this sections, if mapped to my example, is:
`await_ready` was called and returned false -> the coroutine is suspended.
Now, await_suspend gets called. The coroutine should still be suspended and it should be valid to destroy it.

Cheers,
Adrian

From: chuanqi.xcq <yedeng.yd at linux.alibaba.com>
Date: Thursday, 4. February 2021 at 03:07
To: cfe-dev at lists.llvm.org <cfe-dev at lists.llvm.org>, Adrian Vogelsgesang <avogelsgesang at tableau.com>
Subject: Re: [cfe-dev] Miscompilation: heap-use-after-free in C++ coroutines
Hi Adrian,

I met this problem before. I agree that this behavior would make symmetric transfer much harder.

From my point of view, this is an undefined behavior. From 17.12.4.6<http://17.12.4.6> of N4878, we can find:
```

void destroy() const;

4 Preconditions: *this refers to a suspended coroutine.

5 Effects: Destroys the coroutine
```

To my mind, the coroutine **isn't suspended** in the await_suspend. So it is an undefined behavior if we call `coroutine_handle::destroy()` in await_suspend.

But I strongly agree with you it is really suffering that if we want symmetric transfer and we can't destroy the coroutine in final await_suspend directly.

I would look into the details and try to fix it.

Thanks,
Chuanqi

------------------------------------------------------------------
From:Adrian Vogelsgesang via cfe-dev <cfe-dev at lists.llvm.org>
Send Time:2021年2月4日(星期四) 04:21
To:cfe-dev at lists.llvm.org <cfe-dev at lists.llvm.org>
Subject:[cfe-dev] Miscompilation: heap-use-after-free in C++ coroutines

Dear Clang community,

I think I stumbled across a front-end bug in clang’s coroutine implementation.

Having an awaitable with

```
template<typename PROMISE> std::experimental::coroutine_handle<> await_suspend(std::experimental::coroutine_handle<PROMISE> coro) noexcept {
      coro.destroy();
     return std::experimental::noop_coroutine();
}
```

destroys the function’s own coroutine frame. This is valid, as long as no-one afterwards resumes the coroutine anymore. However, address-sanitizer reports a heap-use-after-free error (see https://godbolt.org/z/eq6eoc<https://godbolt.org/z/eq6eoc> )

Afaict, this is because clang stores the return value of `await_suspend` in the coroutine frame. Instead, clang should probably store this return value on the stack. Storing on the stack should be valid, as it is guaranteed that this return value will never live across a suspension point.

You can find a minimal repro in https://godbolt.org/z/eq6eoc<https://godbolt.org/z/eq6eoc> and a more complex end-to-end version in https://godbolt.org/z/8Yadv1<https://godbolt.org/z/8Yadv1> . See https://stackoverflow.com/questions/65991264/c-coroutines-is-it-valid-to-call-handle-destroy-from-the-final-suspend-poin<https://stackoverflow.com/questions/65991264/c-coroutines-is-it-valid-to-call-handle-destroy-from-the-final-suspend-poin> (in particular the comments to David Haim’s reply) for more context.

Cheers,
Adrian

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