On Monday 24 March 2003 07:34 pm, Angelo Dell'Aera wanted us to know:
> 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.
Thanks,
In my hurry, I overlooked this.
Thanks a lot.
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