[ILUG] kernel programming curiosity

[Fergal Daly]

Hi,
	I was just playing about with some simple Kernel programming and I ran 
into something I thought was odd. I had a simple module which used the 
put_user function, the module compiled fine but whenever I insmodded I got 
an error of "unresolved symbol __put_user_X" and sure enough if you do an 
nm on the module you'll see

          U __put_user_1
          U __put_user_2
          U __put_user_4
          U __put_user_X

as part of the output. I can't find tail of trace of __put_user_X in any 
header files or in /proc/ksyms although __put_user_x does get a mention in 
/usr/include/linux/asm/uaccess.h

The funny thing is that if I add -O to the gcc options the problem goes 
away. All the __put_user_ functions are optimised away except for 
__put_user_1 which is a valid kernel symbol and I can now insmod the module 
(although the example below is NOT a module you should try insmodding, I've 
just cut the whole thing down to a tiny example that still reproduces the 
problem).

What's the story? I thought -O wasn't meant to make any difference except 
speedwise. Is there something that says that all kernel code must be 
compiled with a -O switch? Where is gcc getting this capital X instead of a 
small x?

Any light shed would be appreciated, the details are below,

Fergal

Without -O

 > kgcc -DMODULE -D__KERNEL__ -c broken.c
/home/fergal/tmp/ccAONDH5.s: Assembler messages:
/home/fergal/tmp/ccAONDH5.s:9: Warning: Ignoring changed section attributes 
for .modinfo
 > nm broken.o
00000000 b Message_Ptr
00000000 ? __module_kernel_version
          U __put_user_1
          U __put_user_2
          U __put_user_4
          U __put_user_X
00000000 t gcc2_compiled.
00000000 T init_module

with -O

 > kgcc -DMODULE -D__KERNEL__ -O -c broken.c
/home/fergal/tmp/cc4hqC3e.s: Assembler messages:
/home/fergal/tmp/cc4hqC3e.s:9: Warning: Ignoring changed section attributes 
for .modinfo
 > nm broken.o
00000000 b Message_Ptr
00000000 ? __module_kernel_version
          U __put_user_1
00000000 t gcc2_compiled.
00000000 T init_module

 > cat broken.c
#include <linux/kernel.h>
#include <linux/module.h>
#include <asm/uaccess.h>

static char *Message_Ptr;

int init_module()
{
     char *buffer;
     put_user(*(Message_Ptr++), buffer++);
}