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<p class="MsoNormal"><span lang="EN-US">HI </span>Chuanqi<span lang="EN-US">,<br>
<br>
></span><span lang="EN-US"> </span><span lang="EN-US">I met this problem before. I agree that this behavior would make symmetric transfer much harder.<br>
<br>
Thanks for taking a look at this - I really appreciate it!<br>
<br>
> </span>the coroutine **isn't suspended** in the await_suspend. So it is an undefined behavior if we call `coroutine_handle::destroy()` in await_suspend.<o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US"><br>
I think section 7.6.2.4, subpoint 5.1 is the relevant part of the standard here:<br>
```<br>
The await-expression evaluates the (possibly-converted) o expression and the await-ready expression, then:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">(5.1) — If the result of await-ready is false, the coroutine is considered suspended. Then:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">(5.1.1) — If the type of await-suspend is std::coroutine_handle<Z>, await-suspend.resume() is evaluated.<br>
```<br>
<br>
My understanding of this sections, if mapped to my example, is:<br>
`await_ready` was called and returned false -> the coroutine is suspended.<br>
Now, await_suspend gets called. The coroutine should still be suspended and it should be valid to destroy it.<br>
<br>
Cheers,<br>
Adrian<br>
<br>
<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><b><span style="font-size:12.0pt;color:black">From:
</span></b><span style="font-size:12.0pt;color:black">chuanqi.xcq <yedeng.yd@linux.alibaba.com><br>
<b>Date: </b>Thursday, 4. February 2021 at 03:07<br>
<b>To: </b>cfe-dev@lists.llvm.org <cfe-dev@lists.llvm.org>, Adrian Vogelsgesang <avogelsgesang@tableau.com><br>
<b>Subject: </b>Re: [cfe-dev] Miscompilation: heap-use-after-free in C++ coroutines</span><span style="font-size:12.0pt;color:black;mso-fareast-language:EN-GB"><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Tahoma",sans-serif;color:black">Hi Adrian,</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Tahoma",sans-serif;color:black">I met this problem before. I agree that this behavior would make symmetric transfer much harder.</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Tahoma",sans-serif;color:black">From my point of view, this is an undefined behavior. From
<a href="http://17.12.4.6">
17.12.4.6</a> of N4878, we can find:</span><o:p></o:p></p>
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<p class="MsoNormal">```<o:p></o:p></p>
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<p style="margin:0cm;font-variant-caps:normal;font-stretch:normal"><span style="font-size:12.0pt;font-family:Helvetica">void destroy() const;</span><o:p></o:p></p>
<p style="margin:0cm;font-variant-caps:normal;font-stretch:normal"><span style="font-size:12.0pt;font-family:Helvetica">4 Preconditions: *this refers to a suspended coroutine.</span><o:p></o:p></p>
<p style="margin:0cm;font-variant-caps:normal;font-stretch:normal"><span style="font-size:12.0pt;font-family:Helvetica">5 Effects: Destroys the coroutine</span><o:p></o:p></p>
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<p class="MsoNormal">```<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I would look into the details and try to fix it.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Thanks,<o:p></o:p></p>
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<p class="MsoNormal">Chuanqi<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Tahoma",sans-serif;color:black">------------------------------------------------------------------</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Tahoma",sans-serif;color:black">From:Adrian Vogelsgesang via cfe-dev <cfe-dev@lists.llvm.org></span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Tahoma",sans-serif;color:black">Send Time:2021</span><span lang="EN-US" style="font-size:10.5pt;font-family:"MS Gothic";color:black">年</span><span style="font-size:10.5pt;font-family:"Tahoma",sans-serif;color:black">2</span><span lang="EN-US" style="font-size:10.5pt;font-family:"MS Gothic";color:black">月</span><span style="font-size:10.5pt;font-family:"Tahoma",sans-serif;color:black">4</span><span lang="EN-US" style="font-size:10.5pt;font-family:"MS Gothic";color:black">日</span><span style="font-size:10.5pt;font-family:"Tahoma",sans-serif;color:black">(</span><span lang="EN-US" style="font-size:10.5pt;font-family:"MS Gothic";color:black">星期四</span><span style="font-size:10.5pt;font-family:"Tahoma",sans-serif;color:black">)
04:21</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Tahoma",sans-serif;color:black">To:cfe-dev@lists.llvm.org <cfe-dev@lists.llvm.org></span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Tahoma",sans-serif;color:black">Subject:[cfe-dev] Miscompilation: heap-use-after-free in C++ coroutines</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="color:black">Dear Clang community,<br>
<br>
I think I stumbled across a front-end bug in clang’s coroutine implementation.<br>
<br>
Having an awaitable with<br>
<br>
```<br>
template<typename PROMISE> std::experimental::coroutine_handle<> await_suspend(std::experimental::coroutine_handle<PROMISE> coro) noexcept {</span><o:p></o:p></p>
<p class="MsoNormal" style="caret-color:#000000;font-variant-caps:normal;orphans:auto;text-align:start;widows:auto">
<span style="color:black"> coro.destroy();</span><o:p></o:p></p>
<p class="MsoNormal" style="caret-color:#000000;font-variant-caps:normal;orphans:auto;text-align:start;widows:auto">
<span style="color:black"> return std::experimental::noop_coroutine();</span><o:p></o:p></p>
<p class="MsoNormal" style="caret-color:#000000;font-variant-caps:normal;orphans:auto;text-align:start;widows:auto">
<span style="color:black">}<br>
```<br>
<br>
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
<a href="https://godbolt.org/z/eq6eoc" target="_blank">
https://godbolt.org/z/eq6eoc</a> )<br>
<br>
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.<br>
<br>
You can find a minimal repro in <a href="https://godbolt.org/z/eq6eoc" target="_blank">https://godbolt.org/z/eq6eoc</a> and a more complex end-to-end version in <a href="https://godbolt.org/z/8Yadv1" target="_blank">https://godbolt.org/z/8Yadv1</a> .
See <a href="https://stackoverflow.com/questions/65991264/c-coroutines-is-it-valid-to-call-handle-destroy-from-the-final-suspend-poin" target="_blank">https://stackoverflow.com/questions/65991264/c-coroutines-is-it-valid-to-call-handle-destroy-from-the-final-suspend-poin</a> (in particular the
comments to David Haim’s reply) for more context.<br>
<br>
Cheers,<br>
Adrian</span><o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
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