/usr/include/dune/pdelab/backend/istl/ is in libdune-pdelab-dev 2.5.0~rc1-2build1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/dune/pdelab/backend/istl/bcrsmatrix.hh
/usr/include/dune/pdelab/backend/istl/bcrsmatrixbackend.hh
/usr/include/dune/pdelab/backend/istl/bcrspattern.hh
/usr/include/dune/pdelab/backend/istl/blockmatrixdiagonal.hh
/usr/include/dune/pdelab/backend/istl/cg_to_dg_prolongation.hh
/usr/include/dune/pdelab/backend/istl/descriptors.hh
/usr/include/dune/pdelab/backend/istl/dunefunctions.hh
/usr/include/dune/pdelab/backend/istl/ensureistlinclude.hh
/usr/include/dune/pdelab/backend/istl/forwarddeclarations.hh
/usr/include/dune/pdelab/backend/istl/istlsolverbackend.hh
/usr/include/dune/pdelab/backend/istl/matrixhelpers.hh
/usr/include/dune/pdelab/backend/istl/novlpistlsolverbackend.hh
/usr/include/dune/pdelab/backend/istl/ovlp_amg_dg_backend.hh
/usr/include/dune/pdelab/backend/istl/ovlpistlsolverbackend.hh
/usr/include/dune/pdelab/backend/istl/parallelhelper.hh
/usr/include/dune/pdelab/backend/istl/patternstatistics.hh
/usr/include/dune/pdelab/backend/istl/seq_amg_dg_backend.hh
/usr/include/dune/pdelab/backend/istl/seqistlsolverbackend.hh
/usr/include/dune/pdelab/backend/istl/tags.hh
/usr/include/dune/pdelab/backend/istl/utility.hh
/usr/include/dune/pdelab/backend/istl/vector.hh
/usr/include/dune/pdelab/backend/istl/vectorhelpers.hh
/usr/include/dune/pdelab/backend/istl/vectoriterator.hh