Re: system call logging in userspace

[Pavel Machek]

Man strace, or http://subterfugue.org

> 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                                 [e-mail]
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