Hi,

Here are an example module of djprobe and a simple helper script.

NOTE:
Currently, this helper script can ONLY measure the *LENGTH* of the
instruction-block which will be overwritten by a jump code. It can
*NOT* check whether this instruction-block can be executed out of
line and no branch jumps into the target area.
However, now we're developing more useful helper tool which can
check it.

Here is the example of usage;
1) Analyze the kernel code by using the helper script.

$ ./disym.sh sys_symlink
sys_symlink
0xc017bbe0

/lib/modules/2.6.19-rc1-mm1/build/vmlinux:     file format elf32-i386

Disassembly of section .text:

c017bbe0 <sys_symlink>:
c017bbe0:       83 ec 0c                sub    $0xc,%esp
c017bbe3:       8b 44 24 14             mov    0x14(%esp),%eax

Please be sure that the above-disassembled instructions are relocatable.
Parameter: addr=0xc017bbe0 size=7


2) If the instructions can be executed out of line (ex. load/store,
 compare, add/sub, etc.) and no branch jumps into it (you can dump whole
 of the function by using disym.sh with '-a' option),
 Install the example module with the above parameters.

$ sudo /sbin/insmod ./djprobe_ex.ko addr=0xc017bbe0 size=7


3) and test it.

$ ln -s hoge huga
$ dmesg | tail -n 4
probe install at c017bbe0, size 7
Stopping tasks: =======================================|
Restarting tasks... done
probe call:c017bbe0, caller:c01030c5

$ rm huga
$ ln -s hoge huga
$ dmesg | tail -n 5
probe install at c017bbe0, size 7
Stopping tasks: =======================================|
Restarting tasks... done
probe call:c017bbe0, caller:c01030c5
probe call:c017bbe0, caller:c01030c5

4) Finally, remove the module.

$ sudo /sbin/rmmod djprobe_ex.ko
$ dmesg | tail -n 8
probe install at c017bbe0, size 7
Stopping tasks: =======================================|
Restarting tasks... done
probe call:c017bbe0, caller:c01030c5
probe call:c017bbe0, caller:c01030c5
probe uninstall at c017bbe0
Stopping tasks: =======================================|
Restarting tasks... done


Thanks,
-- 
Masami HIRAMATSU
Linux Technology Center
Hitachi, Ltd., Systems Development Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com