[llvm-dev] Writing a test for gcov style coverage crashing after dlclose

[Vedant Kumar via llvm-dev]

Hi Benoit,

> Now, I¹d like to include a test program to demonstrate the fix. AFAICT,
> there seems to be a single test for
> "-fprofile­arcs/-ftest-coverage/-femit-coverage-data", namely:
> tools/clang/test/CodeGen/code-coverage.c. (To be fair, there are some
> tests related to the testing of the command line options themselves.)
> There seems to be no tests that actually execute the instrumented
> executables to gather coverage data.
> 
> - Is that a fair assessment of the current situation ?

Yes, I think that's fair.


> - Should I create a new test directory such as
> llvm/projects/compiler-rt/test/gcov (or gcda) to put my test case ?

I think such a test could live in, e.g, compiler-rt/test/profile/gcda-$foo.

The integration tests for profiling instrumentation and code coverage also
live in this directory.

thanks
vedant


> 
> Guidance would be appreciated.
> Benoit
> 
> ‹‹‹‹‹‹‹‹‹‹‹‹‹‹‹‹‹‹‹‹
> 
> See: https://llvm.org/bugs/show_bug.cgi?id=27224
> 
> 
> Bug 27224 <https://llvm.org/bugs/show_bug.cgi?id=27224> - gcov /
> gcda-based profiling crashes when shared libraries are unloaded (dlclose())
> 
> 
> I believe that GCDA profiling functions should be declared as hidden, i.e.
> symbols that are not exported by shared libraries. This would allow the
> GCDA based profiling to work correctly with code using dlopen/dlclose to
> load and unload shared libraries.
> 
> Each shared library must have its own copy of the following GCDA functions:
> 
>     - __gcov_flush
>     - llvm_delete_flush_function_list
>     - llvm_delete_writeout_function_list
>     - llvm_gcda_emit_arcs
>     - llvm_gcda_emit_function
>     - llvm_gcda_end_file
>     - llvm_gcda_increment_indirect_counter
>     - llvm_gcda_start_file
>     - llvm_gcda_summary_info
>     - llvm_gcov_init
>     - llvm_register_flush_function
>     - llvm_register_writeout_function
>     - llvm_writeout_files
> 
> Defining these function as "hidden" prevents a shared library from
> exporting them in its interface. The end-result is that each profiled
> shared library will be guaranteed to get its own set of the GCDA functions
> extracted from libclang_rt.profile.a.
> 
> These GCDA functions are referencing the static writeout_fn and flush_fn
> lists. These lists are traversed when a shared library is unloaded or the
> program exits through an atexit() function. It is important that each
> shared library gets its own set of these lists so that the proper .gcda
> files are updated when a library is unloaded.
> 
> Furthermore, if a shared library doesn't get its own set of GCDA
> functions and associated lists, it will leave dangling pointers to its own
> "writeout" and "flush" functions in the static writeout_fn and flush_fn
> lists of another shared library. These dangling pointers lead to crashes
> when the other shared library is unloaded or when the program exits.
> 
> 
> 
> ‹‹‹‹‹‹‹‹‹‹‹‹‹‹
> 
> 
> Benoit Belley
> Sr Principal Developer
> M&E-Product Development Group
> Autodesk, Inc.
> 10 Duke Street
> Montreal, Quebec, Canada H3C 2L7
> www.autodesk.com <http://www.autodesk.com/>
> 
> 
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev