[llvm-bugs] [Bug 43363] New: Segfault/Assertion failure in ___kmp_fast_free

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43363

            Bug ID: 43363
           Summary: Segfault/Assertion failure in ___kmp_fast_free
           Product: OpenMP
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Runtime Library
          Assignee: unassignedbugs at nondot.org
          Reporter: lennart.braun at posteo.de
                CC: llvm-bugs at lists.llvm.org

Created attachment 22530
  --> https://bugs.llvm.org/attachment.cgi?id=22530&action=edit
reproducer and asan/gdb traces

Hi everyone,

I ran into an issue which causes crashes of our application when compiled with
Clang and libomp (8.0.1 each).  I was able to reproduce it with version
9.0.0rc6 of libomp compiled with debug symbols and ASan.

Most of the time, ASan reports a SEGV caused by a READ operation in
___kmp_fast_free (according to GDB at kmp_alloc.cpp:1956, according to ASan in
kmp_alloc.cpp:1944).

Sometimes one of the following assertions fails:

https://github.com/llvm/llvm-project/blob/12f174e98c3514755b21db2f040544c0b2ed5f67/openmp/runtime/src/kmp_alloc.cpp#L1970-L1976
https://github.com/llvm/llvm-project/blob/12f174e98c3514755b21db2f040544c0b2ed5f67/openmp/runtime/src/kmp_alloc.cpp#L1980


To reproduce the crash I have attached some example code (crash_omp.cpp) which
is based on one of our test cases.  I tried to strip all unnecessary code and
make the example as small as possible.  Hence, the code itself probably does
not make much sense anymore.  On my machine the crash appears usually in
iteration 4 of the loop in the main function.  Compile it as:

$ clang++ -o crash_omp -std=c++17 -fopenmp=libomp -g -fsanitize=address
crash_omp.cpp


I run Arch Linux on x86_64 and used the following versions of libomp and Clang:

libomp 9.0.0-rc6 (from
https://github.com/llvm/llvm-project/archive/llvmorg-9.0.0-rc6.tar.gz)
compiled with:

  cmake .. -G Ninja \
    -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ \
    -DCMAKE_C_FLAGS=-fsanitize=address \
    -DCMAKE_BUILD_TYPE=Debug \
    -DCMAKE_INSTALL_PREFIX=/usr

$ clang++ --version
clang version 8.0.1 (tags/RELEASE_801/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin


Attachments:
* crash_omp.cpp
* asan_output.txt
* gdb_stacktrace.txt


Please let me know if you need more information.

Best

Lennart


PS:

While debugging I noticed that

#pragma omp taskloop num_tasks(n) default(shared)

with n == 0 causes libomp to perform a division by 0 which results in a crash
with SIGFPE.  I don't know if this behavior is intended or an error message is
preferred here.  (The spec requires n to be positive
https://www.openmp.org/spec-html/5.0/openmpsu47.html)

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