Re: [PATCH] vfs: Fix possible NULL pointer dereference in inode_permission()

[Matthew Wilcox <matthew@wil.cx>]

On Thu, Jan 09, 2014 at 04:27:31PM -0500, Steven Rostedt wrote:
> Note, the crash came from stressing the deletion and reading of debugfs
> files. I was not able to recreate this via normal files. But I'm not
> sure they are safe. It may just be that the race window is much harder
> to hit.

But "normal" files have a 'destroy_inode' method.  So you've basically
only fixed it for debugfs (and maybe a few other unusual filesystems).
Why doesn't the code look like this:

static void i_callback(struct rcu_head *head)
{
	struct inode *inode = container_of(head, struct inode, i_rcu);
	__destroy_inode(inode);
	if (inode->i_sb->s_op->destroy_inode)
		inode->i_sb->s_op->destroy_inode(inode);
	else
		kmem_cache_free(inode_cachep, inode);
}

static void destroy_inode(struct inode *inode)
{
	BUG_ON(!list_empty(&inode->i_lru));
	call_rcu(&inode->i_rcu, i_callback);
}

We'd then have to get rid of all the call_rcu() invocations in individual
filesystems' destroy_inode methods, but that doesn't sound like a bad
thing to me.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."