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Re: trapping execve()

On Thu, May 23, 2002 at 03:12:50AM +0530, Sridhar N wrote:

> 1) why isn't modifying syscalltable safe under module unloading ?

Consider code like this :

asmlinkage static long my_sys_init_module(char const * name_user, struct
module * mod_user)
{
        long ret;
 
        ret = old_sys_init_module(name_user, mod_user);
 
	/* some code ... */
        return ret;
}

Now consider what happens if a process is sleeping in
old_sys_init_module somewhere (i.e. sleeping in the kernel), and
somebody does a rmmod. This code (my_sys_init_module) is unmapped. Now
the process wakes up and tries to return to "some code" above.
Unfortunately, at this point the vfat module has been autoloaded and
/its/ code is now taking up this space. You just trashed your windows
partition.

You might think that module use counts will help here, but there is
always a tiny window between the decrement of the use count, and the end
of a process needing the code to be mapped[1]

> 2) why isn't the argument to execve not a pointer ?

Well, there's nothing stopping it being a pointer as far as I can see,
but the fact is, it is not, and your code must cope with this as result.

Hope this helps.

regards
john

[1] in fact my tests have /never/ caused this race in this manner, but
that's not the point

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