Hello Robert....
On Sun, Jun 28, 2009 at 10:47 PM, Robert P. J. Day<rpjday@xxxxxxxxxxxxxx> wrote:
>
> as a followup to my earlier post about writing a kernel newbies
> column for linux.com, i'm hoping to use this mailing list to ask the
> occasional question for someone to clarify when, quite simply, i don't
> have the time since i have about eight things on the go right now and
> it would be ever so convenient if others decided to dig into some of
> the more arcane details that i'd add to a future piece.
>
> right now, i'm trying to clarify what you can and can't do with a
> module that's been loaded and marked as "[permanent]". as i read it
> (can't remember where now), if you manually load a module that has no
> registered module_exit() routine, that module will show under "lsmod"
> as "[permanent]" and is no longer unloadable. you can see that from
> this snippet from kernel/module.c in the print_unload_info() routine:
>
> if (mod->init != NULL && mod->exit == NULL) {
> printed_something = 1;
> seq_printf(m, "[permanent],");
> }
>
> i did test that with a trivial module and, sure enough, that's how
> lsmod displayed it, and trying to unload it normally with "rmmod" told
> me it was busy (even though no one else was using it). fair enough.
> but is that the whole story?
In kernel/module.c, I see function which is declared like this:
SYSCALL_DEFINE2(delete_module, const char __user *, name_user,
unsigned int, flags)
and few lines below that, you'll see:
/* If it has an init func, it must have an exit func to unload */
if (mod->init && !mod->exit) {
forced = try_force_unload(flags);
if (!forced) {
/* This module can't be removed */
ret = -EBUSY;
goto out;
}
}
let's assume the code doesn't reach "goto out". Then few lines below
that we shall see a call to free_module(). Brief check of the code
inside that function (theoritically) convince me that you can wipe
your "permanent" module. However, it won't guarantee that it will exit
cleanly i.e still grabbing a lock or unused page not being kfree()-d.
You got the idea anyway.
regards,
Mulyadi.
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