Re: [eigen] CholmodSupport.h in Eigen 3.1 alpha returns a mapped matrix then frees the storage being mapped.

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 07.12.2011 00:00, Douglas Bates wrote:
> I mentioned this once before but did not get around to creating an
> test case to show the problem.  In CholmodSupport.h the internal
> _solve functions assign a mapped dense or sparse matrix to the dest
> reference and then free the storage that is mapped.
>
> For example, in the dense case the _solve function ends with
>
>
>        dest = Matrix<Scalar,Dest::RowsAtCompileTime,Dest::ColsAtCompileTime>::Map(reinterpret_cast<Scalar*>(x_cd->x),b.rows(),b.cols());
>        cholmod_free_dense(&x_cd,&m_cholmod);

Yes, but that means the values of x_cd are copied to dest -- even if 
dest is also a Map-type, operator= copies everything by value. This is 
not optimal, but I don't see a critical problem with that. Though, maybe 
I'm missing something?

> One of the side-effects of cholmod_free_dense is to free the storage
> to which x_cd->x points.
>
> I think that the tests in eigen/unsupported/test/sparse_llt.cpp pass
> because the value is tested immediately and the freed storage has not
> been overwritten.
>
> I'm not exactly sure how to create a test case to show the problem.
> By default Cholmod uses malloc/free for dynamic storage and I'm not
> sure how this interacts with the C++ dynamic storage allocation for
> objects.  I would like to ensure that the freed chunk of storage
> pointed to by x_cd->x is overwritten after the result has been
> returned but that involves knowing more about malloc, etc. than I do.

The easiest way to test is to run the test with valgrind (assuming you 
run linux), which would give you (at least) a warning as soon as freed 
memory is accessed.

You could also modify the _solve-function and overwrite the values in 
x_cd->x immediately before cholmod_free_dense is called.



Christoph



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Dipl.-Inf. Christoph Hertzberg
Cartesium 0.051
Universität Bremen
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