system call logging in userspace

[Chad Hogan <Chad.Hogan@inphinity.com>]

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Hello,

I'm not very experienced with dealing directly with the kernel, so I was 
hoping for a little advice...

I'd like to implement some sort of rudimentary (file)system-call logging.  
Specifically, I'd like information about write, open, creat, unlink, and 
maybe a few others to be pushed into userspace.  Mostly, I'd just like to 
know what files are being created, modified, and deleted as it happens.

It seems quite easy to me -- I was thinking of doing this with a module.  
I'll just grab the pointer from sys_call_table[__NR_open] and replace it with 
my own little wrapper that does nothing but call the original function, and 
then log the call in some manner.

================

asmlinkage int my_sys_open(const char *fname, int flags, int mode)
{
         [preliminary stuff]

         returnval = real_sys_open(fname, flags, mode);

         [log information based on returnval, fname, whatever];

         return returnval;
}


int init_module()
{
         [other stuff]

         real_sys_open = sys_call_table[__NR_open];
         sys_call_table[__NR_open] = my_sys_open;
         return 0;
}

init cleanup_module()
{
         sys_call_table[__NR_open] = real_sys_open;
}

===========

The simplicity of the whole thing is what scares me a little bit.  Am I being 
horribly naive about something here?  It seems like an obviously useful 
module to have around, and yet I've never seen it and I couldn't find anyone 
who had done it already.  Is there a much better way to accomplish this than 
loading in a module?  Am I risking serious fs corruption?

It occurs to me that I may have some problems if something else changes the 
sys_call_table[__NR_open] and the two modules don't cooperate...

Thanks.
- -- 
Chad Hogan                                 chad.hogan@inphinity.com
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