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libcxx: racy use-after-free on condition_variable #23667

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KernelAddress mannequin opened this issue Apr 20, 2015 · 9 comments
Closed

libcxx: racy use-after-free on condition_variable #23667

KernelAddress mannequin opened this issue Apr 20, 2015 · 9 comments
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bugzilla Issues migrated from bugzilla libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi.

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@KernelAddress
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Mannequin

KernelAddress mannequin commented Apr 20, 2015

Bugzilla Link 23293
Resolution FIXED
Resolved on Jun 12, 2015 09:18
Version unspecified
OS Linux
CC @KernelAddress,@eugenis,@ramosian-glider,@kcc,@mclow

Extended Description

clang version 3.7.0 (trunk 235293)
Target: x86_64-unknown-linux-gnu

The program is:

#include
#include
#include
#include

std::atomic _counter;

int main()
{
_counter = 1;

auto f = std::async(std::launch::async,
                    []{
                        _counter += 2;
                        return true;
                    });
std::cout << "Result: " << std::boolalpha << f.get() << "\n";
return 0;

}

ThreadSanitizer says:

==================
WARNING: ThreadSanitizer: data race (pid=19479)
Write of size 8 at 0x7d200000bfc0 by main thread:
#​0 pthread_cond_destroy /ssd/src/llvm/build/../projects/compiler-rt/lib/tsan/rtl/tsan_interceptors.cc:1153 (a.out+0x00000044aa8c)
#​1 std::__1::condition_variable::condition_variable() build/../projects/libcxx/src/condition_variable.cpp:23:5 (libc++.so.1+0x00000008cb69)
#​2 std::__1::__assoc_sub_state::__assoc_sub_state() build/bin/../include/c++/v1/future:515:24 (a.out+0x00000049999a)
#​3 std::__1::__async_assoc_state<bool, std::__1::__async_funcmain::$_0 >::~__async_assoc_state() build/bin/../include/c++/v1/future:940:7 (a.out+0x0000004992d5)
#​4 std::__1::__assoc_state::__on_zero_shared() build/bin/../include/c++/v1/future:635:5 (a.out+0x000000499a63)
#​5 std::__1::__async_assoc_state<bool, std::__1::__async_funcmain::$_0 >::__on_zero_shared() build/bin/../include/c++/v1/future:990:5 (a.out+0x00000049930d)
#​6 std::__1::__shared_count::__release_shared() build/../projects/libcxx/src/memory.cpp:63:9 (libc++.so.1+0x00000008eb20)
#​7 std::__1::__release_shared_count::operator()(std::__1::__shared_count*) build/bin/../include/c++/v1/future:1155:41 (a.out+0x00000049a155)
#​8 std::__1::unique_ptr<std::__1::__shared_count, std::__1::__release_shared_count>::reset(std::__1::__shared_count*) build/bin/../include/c++/v1/memory:2669:13 (a.out+0x0000004996bd)
#​9 std::__1::unique_ptr<std::__1::__shared_count, std::__1::__release_shared_count>::~unique_ptr() build/bin/../include/c++/v1/memory:2637 (a.out+0x0000004996bd)
#​10 std::__1::future::get() build/bin/../include/c++/v1/future:1173 (a.out+0x0000004996bd)
#​11 main /tmp/shared_ptr.cc:17:50 (a.out+0x000000498f62)

Previous read of size 8 at 0x7d200000bfc0 by thread T1:
#​0 pthread_cond_broadcast /ssd/src/llvm/build/../projects/compiler-rt/lib/tsan/rtl/tsan_interceptors.cc:1146 (a.out+0x000000447e40)
#​1 std::__1::condition_variable::notify_all() build/../projects/libcxx/src/condition_variable.cpp:35:5 (libc++.so.1+0x00000008cbc9)
#​2 void std::__1::__assoc_state::set_value(bool&&) build/bin/../include/c++/v1/future:655:5 (a.out+0x000000499c01)
#​3 std::__1::__async_assoc_state<bool, std::__1::__async_funcmain::$_0 >::__execute() build/bin/../include/c++/v1/future:975:9 (a.out+0x00000049934e)
#​4 _ZNSt3__18__invokeIMNS_19__async_assoc_stateIbNS_12__async_funcIZ4mainE3$0JEEEEEFvvEPS5_JEvEEDTcldsdeclsr3std3__1E7forwardIT0_Efp0_Efp_spclsr3std3__1E7forwardIT1_Efp1_EEEOT_OS9_DpOSA build/bin/../include/c++/v1/__functional_base:382:12 (a.out+0x0000004994a1)
#​5 void std::__1::__thread_execute<void (std::__1::__async_assoc_state<bool, std::__1::__async_funcmain::$_0 >::)(), std::__1::__async_assoc_state<bool, std::__1::__async_funcmain::$_0 >, 1ul>(std::__1::tuple<void (std::__1::__async_assoc_state<bool, std::__1::__async_funcmain::$_0 >::)(), std::__1::__async_assoc_state<bool, std::__1::__async_funcmain::$_0 >>&, std::__1::__tuple_indices<1ul>) build/bin/../include/c++/v1/thread:337 (a.out+0x0000004994a1)
#​6 void* std::__1::__thread_proxy<std::__1::tuple<void (std::__1::__async_assoc_state<bool, std::__1::__async_funcmain::$_0 >::)(), std::__1::__async_assoc_state<bool, std::__1::__async_funcmain::$_0 >> >(void*) build/bin/../include/c++/v1/thread:347 (a.out+0x0000004994a1)

Location is heap block of size 120 at 0x7d200000bf80 allocated by main thread:
#​0 operator new(unsigned long) /ssd/src/llvm/build/../projects/compiler-rt/lib/tsan/rtl/tsan_interceptors.cc:615 (a.out+0x00000043e08f)
#​1 std::__1::future std::__1::__make_async_assoc_state<bool, std::__1::__async_funcmain::$_0 >(std::__1::__async_funcmain::$_0&&) build/bin/../include/c++/v1/future:2312:13 (a.out+0x000000499035)
#​2 std::__1::future<std::__1::__invoke_of<std::__1::decaymain::$_0::type>::type> std::__1::asyncmain::$_0(std::__1::launch, main::$_0&&) build/bin/../include/c++/v1/future:2361:16 (a.out+0x000000498fce)
#​3 main /tmp/shared_ptr.cc:12:14 (a.out+0x000000498f0d)

Thread T1 (tid=19481, finished) created by main thread at:
#​0 pthread_create /ssd/src/llvm/build/../projects/compiler-rt/lib/tsan/rtl/tsan_interceptors.cc:951 (a.out+0x000000447123)
#​1 std::__1::thread::thread<void (std::__1::__async_assoc_state<bool, std::__1::__async_funcmain::$_0 >::)(), std::__1::__async_assoc_state<bool, std::__1::__async_funcmain::$_0 >, void>(void (std::__1::__async_assoc_state<bool, std::__1::__async_funcmain::$_0 >::&&)(), std::__1::__async_assoc_state<bool, std::__1::__async_funcmain::$_0 >&&) build/bin/../include/c++/v1/thread:359:16 (a.out+0x000000499282)
#​2 std::__1::future std::__1::__make_async_assoc_state<bool, std::__1::__async_funcmain::$_0 >(std::__1::__async_funcmain::$_0&&) build/bin/../include/c++/v1/future:2313:5 (a.out+0x0000004990a4)
#​3 std::__1::future<std::__1::__invoke_of<std::__1::decaymain::$_0::type>::type> std::__1::asyncmain::$_0(std::__1::launch, main::$_0&&) build/bin/../include/c++/v1/future:2361:16 (a.out+0x000000498fce)
#​4 main /tmp/shared_ptr.cc:12:14 (a.out+0x000000498f0d)

SUMMARY: ThreadSanitizer: data race build/../projects/libcxx/src/condition_variable.cpp:23:5 in std::__1::condition_variable::~condition_variable()

The program was built as:

clang++ test.cc -fsanitize=thread -std=c++14 -stdlib=libc++ -lc++abi -Wall -g -O1

with tsan-instrumented libcxx (see instructions here https://code.google.com/p/memory-sanitizer/wiki/LibcxxHowTo).

The problem is here:

__assoc_state<_Rp>::set_value(_Arg&& __arg)
{
unique_lock __lk(this->_mut);
#ifndef _LIBCPP_NO_EXCEPTIONS
if (this->__has_value())
throw future_error(make_error_code(future_errc::promise_already_satisfied));
#endif
::new(&_value) _Rp(_VSTD::forward<_Arg>(__arg));
this->_state |= base::__constructed | base::ready;
__lk.unlock();
_cv.notify_all();
}

The function first unlocks the mutex and after than signals the cond var. But once the mutex is unlocked the object can be destroyed as it is in ready state.
@llvmbot
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llvmbot commented Apr 20, 2015

Howard is no longer active in libc++. I'll take a look at this. I am already somewhat familiar with the issue.

@llvmbot
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llvmbot commented May 13, 2015

I'm not 100% convinced that this is a real defect. TSAN is reporting the race condition on the pthread_condvar_t in std::condition_variable. I think that TSAN's semantics for condition_variable are incorrect is some cases and this may be one of those.

Could you provide a pseudo-example where you think this will cause an error? I just want to fully understand this before I make any changes.

@KernelAddress
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KernelAddress mannequin commented May 13, 2015

What's wrong with my reasoning? Condition variables are semantical no-ops, so there is nothing to model.

@llvmbot
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llvmbot commented May 13, 2015

What's wrong with my reasoning?

I just don't see the race condition in the example you gave. I'm not saying it is not there but it isn't clear to me yet. I don't see how the __assoc_state can get destroyed before it exits the call to get_value(...) even if it releases the lock.

However I don't think the TSAN diagnostics are caused by the race condition you described.

Condition variables are semantical no-ops, so there is nothing to model.

Sorry, I see you've done a bunch of work on how TSAN handles condition variables recently. I wasn't aware that a lot had changed in the past month. Why is it that TSAN's interceptor for pthread_cond_wait(Cv, Mut) releases the mutex before marking Cv as read?
(see: https://github.com/llvm-mirror/compiler-rt/blob/master/lib/tsan/rtl/tsan_interceptors.cc#L1005)

@llvmbot
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llvmbot commented May 20, 2015

Ping Dimitry, Kostya, Eugenis:

Why is it that TSAN's interceptor for pthread_cond_wait(Cv, Mut) releases the mutex before marking Cv as read?

@KernelAddress
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KernelAddress mannequin commented May 20, 2015

Eric,

Sorry for the delay, I was OOO.

I don't think that I did any work on condvar in the last months. cond_wait interceptor always released and reaqcuired the mutex. Why? Because that's what pthread_cond_wait itself does -- it releases the mutex before waiting, otherwise the mutex-protected predicate won't be able to change.

I don't see how the __assoc_state can get destroyed before it exits the call to get_value(...).

You misread the report and my explanation. It is not get_value that runs concurrently with destruction, it is set_value that runs concurrently with destruction.

The report says that:

  1. set_value changes object state to "ready" and releases the mutex.
  2. then get_value observes the "ready" state and destroys the object.
  3. then set_value signals on the condvar which is already destroyed.

@llvmbot
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llvmbot commented May 21, 2015

You misread the report and my explanation. It is not get_value that runs
concurrently with destruction, it is set_value that runs concurrently with
destruction.

The report says that:

  1. set_value changes object state to "ready" and releases the mutex.
  2. then get_value observes the "ready" state and destroys the object.
  3. then set_value signals on the condvar which is already destroyed.

Thanks for pointing out my mistake. I incorrectly assumed that the producer that called set_value had a reference to the shared state. I'll try and push a fix for this today.

@llvmbot
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llvmbot commented May 21, 2015

FYI, I filed a bug against compiler-rt that documents my concerns with how TSAN handles condition variables.

https://llvm.org/bugs/show_bug.cgi?id=23616

@llvmbot
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llvmbot commented Jun 12, 2015

Fixed in r239577. Thanks for the analysis and sorry for the delay.

@llvmbot llvmbot transferred this issue from llvm/llvm-bugzilla-archive Dec 10, 2021
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