[$] The proper use of EXPORT_SYMBOL_GPL()
The kernel, in theory, puts strict limits on which functions and data
structures are available to loadable kernel modules; only those that have
been explicitly exported with EXPORT_SYMBOL() or
EXPORT_SYMBOL_GPL() are accessible. In the case of
EXPORT_SYMBOL_GPL(), only modules that declare a GPL-compatible
license will be able to see the symbol. There have been questions about
when EXPORT_SYMBOL_GPL() should be used for almost as long as it
has existed. The latest attempt to answer those questions was a session
run by Greg Kroah-Hartman at the 2018 Kernel Maintainers Summit; that
session offered little in the way of general guidance, but it did address
one specific case.