  The propagation of particles accelerated at supernova remnant shocks and
escaping the parent remnants is likely to proceed in a strongly non-linear
regime, due to the efficient self-generation of Alfv\'en waves excited through
streaming instability near the sources. Depending on the amount of neutral
hydrogen present in the regions around the sites of supernova explosions,
cosmic rays may accumulate an appreciable grammage in the same regions and get
self-confined for non-negligible times, which in turn results in an enhanced
rate of production of secondaries. Here we calculate the contribution to the
diffuse gamma-ray background due to the overlap along lines of sight of several
of these extended halos as due to pion production induced by self-confined
cosmic rays. We find that if the density of neutrals is low, the halos can
account for a substantial fraction of the diffuse emission observed by
Fermi-LAT, depending on the orientation of the line of sight with respect to
the direction of the Galactic centre.
