Re: quotactl hook

gregat_private

On Sat, Sep 01, 2001 at 08:48:23PM -0400, jmjonesat_private wrote:
> 
> How?
> 
> If the "blob" is not allocated and the module gets it, how do I lock it
> prior to one of the threads allocating and starting it flying?

Here's one way to do it:

This is all in your module code:

static struct semaphore task_semaphore;

int my_module_init_function (void)
{
	init_MUTEX (&task_semaphore);

	/* now register my module with the lsm framework */
	...
}

int my_module_task_alloc_security (struct task_struct *p)
{
	down (&task_semaphore);
	if (p->security == NULL) {
		p->security = kmalloc (sizeof (struct my_task_security_struct));
		if (p->security == NULL) {
			up (&task_semaphore);
			return -ENOMEM;
		}
		init_MUTEX (&p->security->semaphore);
		/* init the rest of your stuff */
		...
	}
	up (&task_semaphore);
	return 0;
}

int my_module_task_setgroups (int gidsetsize, gid_t *grouplist)
{
	if (current->security == NULL) {
		ret = my_module_task_alloc_security (current);
		if (ret)
			return ret;
	}

	down (&current->security->semaphore);
	/* do my stuff that changes the contents of current->security */
	...
	up (&current->security->semaphore);

	return 0;
}

So you have a semaphore that protects you initializing the task
structure's security blob.  This has to be a semaphore as you will be
calling kmalloc which can sleep (can't hold a spinlock across a sleep).

And you check the value of p->security _after_ you get the lock in case
you were sleeping on the lock when another call was allocating the same
structure.

Then you have a local semaphore (or can be a spinlock, just don't call
any sleepable functions while it is held.  these are functions like
kmalloc, copy_*_user, printk, and others) that protects your local data
within your blob.  A read_write lock might be good to use for your
internal data if you find that you are mostly reading things and not
writing them.  Check out the research done in the netfilter locks if you
want to see some truly looney read_write locks that are very cool (paper
was presented at OLS 2001 about this.)

Did that help?  Much easier than keeping a list of places you've been.

Usual disclaimer applies, this is not tested code, was written first
thing in the morning before really waking up, etc...

greg k-h

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