kernel symbol marked with "T" in /proc/kallsyms is not exported

Question

Symbol machine_power_off is marked with "T" in /proc/kallsyms:

$ grep -w machine_power_off /proc/kallsyms 
ffffffff8102391b T machine_power_off

But it is not exported. Is "T" in kallsyms necessary and sufficient for a symbol to be exported? Is being exported necessary and sufficient for using in other modules?

My module using it is compiled with a warning:

WARNING: "machine_power_off" [/path/to/module.ko] undefined!

On host machine (3.2.0-4-amd64) I can load this module, but on VirtualBox (3.16.0-4-amd64) it produces the following message:

insmod: ERROR: could not insert module module.ko: Unknown symbol in module

Why is this module loaded in my host system, but not in VirtualBox?

Answer 1

Mark "T" in /proc/kallsyms means that symbol is globally visible, and can be used in other kernel's code (e.g. by drivers, compiled built-in).

But for being usable in kernel module's code, symbol is needed to be exported using EXPORT_SYMBOL or similar. List of exported symbols is maintained separately from list of all symbols in the kernel.

Exported symbols can be found in file /lib/modules/<kernel-version>/build/Module.symvers.

(this file should exist for possibility to build kernel modules against given kernel).

Answer 2

To use kernel symbols that are global, but not exported (such as the machine_power_off symbol that you mention), you can use kallsyms_lookup in your module code:

#include <linux/kallsyms.h>

static void (*machine_power_off_p)(void);
machine_power_off_p = (void*) kallsyms_lookup_name("machine_power_off");

Now you can call the machine_power_off function via the machine_power_off_p pointer:

(*machine_power_off_p)();