anon_inodes: use fops->owner for module refcount

There is an imbalance for anonymous inodes. If the fops->owner field is set,
the module reference count of owner is decreases on release.
("filp_close" --> "__fput" ---> "fops_put")

On the other hand, anon_inode_getfd does not increase the module reference
count of owner. This causes two problems:

- if owner is set, the module refcount goes negative
- if owner is not set, the module can be unloaded while code is running

This patch changes anon_inode_getfd to be symmetric regarding fops->owner
handling.

I have checked all existing users of anon_inode_getfd. Noone sets fops->owner,
thats why nobody has seen the module refcount negative. The refcounting was
tested with a patched and unpatched KVM module.(see patch 2/2) I also did an
epoll_open/close test.
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Davide Libenzi <davidel@xmailserver.org>
Signed-off-by: Avi Kivity <avi@redhat.com>

fs/anon_inodes.c

@@ -79,9 +79,12 @@ int anon_inode_getfd(const char *name, const struct file_operations *fops,
 	if (IS_ERR(anon_inode_inode))
 		return -ENODEV;
 
+	if (fops->owner && !try_module_get(fops->owner))
+		return -ENOENT;
+
 	error = get_unused_fd_flags(flags);
 	if (error < 0)
-		return error;
+		goto err_module;
 	fd = error;
 
 	/*
@@ -128,6 +131,8 @@ int anon_inode_getfd(const char *name, const struct file_operations *fops,
 	dput(dentry);
 err_put_unused_fd:
 	put_unused_fd(fd);
+err_module:
+	module_put(fops->owner);
 	return error;
 }
 EXPORT_SYMBOL_GPL(anon_inode_getfd);