Kernel 模块使用导出的符号，具体取决于加载的其他模块

Question

By definition a module won't load (or complains) if required symbols do not exist. I'd like to ask if there is a way to somehow overcome the following scenario:

I have module X which uses symbols from modules Y1 or Y2. Only one of Y1 or Y2 is ever loaded at the same time. Like, for one system configuration I would load Y1, for other Y2. The problem is, that X won't load since it would be complaining about missing symbols from other (not loaded) Y driver.

Example: Y1 exports a symbol: EXPORT_SYMBOL_GPL(y1); Y2 exports a symbol: EXPORT_SYMBOL_GPL(y2);

X would do:

if (config1)
   y1(); // Available only when Y1 is loaded.
else
   y2(); // Available only when Y2 is loaded.

At boot, only one of Y1 or Y2 is loaded depending on configuration. This all compiles, but when loading X in config1 it would say:

Unknown symbol y2

Does anyone have an idea if this can be solved somehow? Or I will just have to build two X modules, X1 and X2 depending on configuration?

Thank you very much, your comments are much appreciated.

Answer 1

You can achieve this by not using exported symbols directly, but by looking for them in the runtime using kallsyms_lookup_name() . That way your module X will not have explicit dependencies on symbols exported by modules Y1 and Y2, so it will be able to load even if one (or even both) of those modules are not loaded.

This also means that your module will be responsible for checking that all the symbols required for its operation are present. It will also need to manually increment refcount for modules providing those symbols via try_module_get() , and then decrement it on module exit via module_put() .