c - control a function pointer in the kernel -

if have controlled function pointer in kernel pointing somewhere want, let's make point own designed function evil in user land.

err = writepage(page) //->writepage kernel function pointer pointing evil in userland 

there printf in evil, there kernel panic if kernel dereferences function pointer? since evil runs in kernel mode (correct me if i'm wrong), kernel not printf is.

int evil() {    printf("i don't think printf executed because evil executed kernel mode") } 

the kernel never sees "printf"; sees call different address, program doesn't call function name, sets registers containing parameters accordingly , calls function.

that won't work, because address printf call points relative userland process' memory, , doesn't exist in kernel memory.

you have realize processes run in virtual memory of own -- none of addresses used in program need make sense different process.

so can't call function in userland process; you'd first have find out in memory kernel sees it, , call it. of course, it'd run in kernel mode, that's not surprising -- no sane os allow userland process bend internal function calls in manner.