On Mon, 24 Mar 2003 19:06:52 +0530
"Omanakuttan" <omanakuttan@tataelxsi.co.in> wrote:
> Hi,
> I am trying to debug some code I have written. I could not understand some
> code in the kernel.
> function sys_create_module() in module.c
> As far as I understand the functionality is as follows.
>
> in this mod is defined as a struct module *.
> checks the user is capable of loading the module.
> locks the kernel.
> the module name is taken from userspace into the variable 'name', defined
> locally.
> memset the vmalloc-ed (module_map) pointer with zeros.
> set the size_of_struct to size of the module structure.
> Next statement
> mod->name = (char *) (mod + 1) ; puzzles me.
> what does this statement do?
> if my knowledge of C is corrrect, since mod is a pointer of type struct module
> , mod+1 will point to ((void *) &mod)+(sizeof (*mod)), which is an invalid
> pointer here.
> Then how does this piece of code work?
> any idea?
Quite simple. Let's see it..
[..]
if ((namelen = get_mod_name(name_user, &name)) < 0) {
error = namelen;
goto err0;
}
if (size < sizeof(struct module)+namelen) {
error = -EINVAL;
goto err1;
}
As you can see, size can't be less than sizeof(struct module)+ namelen so you are
vmalloc'ing enough space for a (struct module) and LKM name.
[..]
memset(mod, 0, sizeof(*mod));
mod->size_of_struct = sizeof(*mod);
mod->name = (char *)(mod + 1);
Now mod->name points to the first char below struct module....
mod->size = size;
memcpy((char*)(mod+1), name, namelen+1);
.... and you memcpy module name just there.
--
Regards,
Angelo Dell'Aera 'buffer'
Emails : <buffer@antifork.org>
<buffer@users.sourceforge.net>
Antifork Research, Inc.
http://www.antifork.org
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