如何用LKM的导出符号编译内核？

Question

I'm a Kernel newbie and needed some help compiling the Linux Kernel with exported symbols of a Kernel Module.

I tried the following - EXPORT_SYMBOL(func_name) in LKM source, LKM compiles and symbol is generated in its Module.symvers. However, when I try to compile the Kernel with the Kernel invoking the exported symbol, it says undefined reference to 'func_name'.

How should I get the Kernel compilation to see the exported symbols? MOst example I see on Google are of Module-to-Module exporting.

What am I missing? Any help is appreciated!

EDIT: I basically want to call a function which is in LKM from the kernel.

Answer 1

When exporting kernel symbols, you have to consider the order of dependencies. It sounds like you are trying to export a symbol from your module, and expecting that the kernel monolith will be able to use that exported symbol. But remember that the kernel is statically linked and therefore must be able to resolve all its symbols when it is built. If you are adding some code to the monolith which is attempting to reference a symbol exported by a module, then it simply won't build - the linker has no knowledge of module symbols, since modules will always load after the monolith.

In short, the monolith may export symbols which may be referenced by modules. Also modules may export symbols which are referenced by other dependent modules. But the opposite direction is impossible.

It sounds like you need to refactor your code somewhat so that you don't have to export from your module. Could you perhaps instead have the monolith export a function pointer which your module would populate when it is loaded? You would have to set it up so that the the monolith would only call the function pointed to by the function pointer if the module is in fact loaded.