copy_from_user problem

[Benjamin Herrenschmidt]

On Tue, 2008-02-26 at 08:49 -0600, Maynard Johnson wrote:
> 2. Compile C program as 32-bit; then run it.  While the program is 
> waiting for input, obtain its PID and do 'cat /proc/<pid>/maps' to
> get 
> the address of where libc is loaded.
> 3. From the dir where you build the uaccess_test kernel module:
>          'insmod ./uaccess_test.ko lib_addr=0x<mem_loc_libc>'
>     This should succeed.  dmesg to verify.
> 4. Unload the module.
> 5. Recompile your C program with -m64; start it up and obtain the 
> address of libc again (now a 64-bit address).
> 6. Load the uaccess_test kernel module and pass 
> 'lib_addr=0x<mem_loc_libc>'.  Note that this time, the load fails. 
> dmesg to see debug printk's.

Sounds to me that your kernel module will try to copy_from_user() from
the user context of ... insmod :-)

You need to do your copy_from_user() from within the context of the
program you try to access the memory from !

If you need to access another context than the current one, you then
need to use a different mechanism, such as get_user_pages(), though
beware that you can only do that for memory, not SPE local store or
register mappings.

Ben.