内核导出符号vs全局符号vs静态全局？

Question

I am little bit stack with kernel symbols type meaning. Simple static symbols have the same meaning like C static. So local static variable have local scope and static allocation. Static functions scope is a file. But what about static exported symbols? How to deal with EXPORT_SYMBOL() , EXPORT_PER_CPU_SYMBOL() , EXPORT_UNUSED_SYMBOL() if macro export static symbol? What is the difference between global and exported symbols? Is it linker responsibility to add additional info for exported symbols? Is global static variable built-in kernel visible in all the kernel and loadable module?

Kernel exported symbols can be accessed from loadable module. Is it good style touch such symbols inside kernel. When kernel resolve symbols is it lookup thru kernel symbols table?

Answer 1

Conceptually , using static keyword with function declaration means internal linkage -- so such function is only visible within single translation unit (*.o file). This may involve inlining of that function (in which case it will be unusable further), but since EXPORT_SYMBOL() takes address of static function, compiler should disable inlining optimization.

Implementation is a bit more complicated. This internal and external linkage rules are only apply to static ld linker which works when vmlinux or kernel module is built. Normally symbol with external linkage is added to symtab ELF section and when dynamic linker ld.so loads shared object it reads that section.

But when module is loaded Linux Kernel uses separate symbol table ksymtab . EXPORT_SYMBOL() adds symbol to that table, but this process is completely transparent to compiler-linker toolchain thus it is not related with internal and external linkage at all.