Compile linux kernel module for ARM - "unknown relocation: 27"

* First of all, check your kernel configuration.

- CONFIG_MODULES=y
- CONFIG_MODULE_UNLOAD=y    <- Optional

* Create your Kernel module code and Makefile

If you have luck, everything should be OK. But...

CASE: error at insmod?

insmod: init_module 'hello.ko' failed (Exec format error) in android
kernel message : unknown relocation: 27

Hmm, then you should check kernel functions you are using in the module (ex. printk)

You can check relocation type with following command (ex. printk)

readelf -r hello.ko | grep prink

Then, result may like below

xxxxxxxx  xxxxxxxx R_ARM_PLT32       00000000   printk

For your information : number 27 indicates R_ARM_PLT32.

Then, what's the problem?

ELF for the ARM Architecture(IHI0044E_aaelf.pdf) says that R_ARM_PLT32 is deprecated.

And, Linux kernel doesn't handle R_ARM_PLT32 at module.c : apply_relocation() function

We have two workaround for this issue.

* Update toolchains.

In my case, I faced this issue when I used gcc-4.4.3. And, gcc-4.7 resolved this issue.

(You can see, relocation type is changed into R_ARM_CALL instead of R_ARM_PLT32.)

* fpic / fno-pic HACK.

If you are in trouble with upgrading your toolchains, here is workaround (tested on gcc-4.4.3).

Add, -fpic or -fno-pic at CFLAGS_MODULE in your module Makefile like below.

make CFLAGS_MODULE=-fpic xxxx modules

Then, like above - updating toolchains -, relocation type is changed :-)