"The kernel's loadable module mechanism does not give modules access to all parts of the kernel. Instead, any kernel symbol which is intended to be usable by loadable modules must be explicitly exported to them via one of the variants of the EXPORT_SYMBOL() macro. The idea behind this restriction is to place limits on the reach of modules and to provide a relatively well-defined module API. In practice, there have been few limits placed on the exporting of symbols, with the result that many thousands of symbols are available to modules. Loadable modules can access many of the obviously useful symbols (printk(), say, or kmalloc()), but they can also get at generic symbols like edd, tpm_pm_suspend(), vr41xx_set_irq_trigger(), or flexcop_dump_reg()..."
