On Wed, May 21, 2014 at 05:48:28PM +0100, Dave P Martin wrote:
> Hi there,
>
> I have a quick debugfs question:
>
> How can I protect against a module being unloaded while debugfs files it
> provides are open?
>
> I can do something like the following:
>
> static int my_debugfs_file_open(struct inode *, struct file *)
> {
> if (!try_module_get())
> return -EIO;
>
> /* ... */
> }
>
> static int my_debugfs_file_release(struct inode *, struct file *)
> {
> /* ... */
>
> module_put();
>
> return 0;
> }
>
> static const struct file_operations my_debugfs_fops = {
> /* ... */
>
> .open = my_debugfs_file_open,
> .release = my_debugfs_file_release,
>
> /* ... */
> };
>
> static struct device_driver my_driver {
> /* ... */
>
> .owner = THIS_MODULE;
>
> /* ... */
> };
>
> static int my_module_init(void)
> {
> driver_register(&my_driver);
> debugfs_create_file(..., &my_debugfs_fops);
> }
>
> static void my_module_exit(void)
> {
> debugfs_remove_recusrive(...);
> driver_unregister(&my_driver);
> }
>
>
> ... but it doesn't quite work. debugfs_remove_recursive() prevents any
> new file handles being opened, but files already open remain open -- so
> it's still possible for the module text to get unloaded between the
> module refcount being decreased inside module_put() and the time
> my_debugfs_file_release() returns.
>
> The scenario to consider is when a request to unload the module races
> with closure of the last debugfs file.
>
>
> The only obvious way I can see to solve this without changing the debugfs
> code is to make the module impossible to unload by calling __module_get()
> during initialisation, before any debugfs file is created.
>
>
> A similar dependency problem exists when a pointer to some device
> instance data is passed to debugfs_create_file(). For pluggable
> devices, the device might go away at any time. I hoped this could
> be solved by calling get_device() ... put_device() in the debugfs file
> open and release methods, but I found that a device instance can
> get removed, and the module unloaded, even though the struct device
> refcount is not zero.
>
>
> Am I missing something?
Nope, you are not, it's a known issue. Al Viro is doing some core VFS
work to help mitigate this, and I am working on converting debugfs to
use kernfs which should also help resolve this problem.
Don't unload modules automatically and you should be fine :)
thanks,
greg k-h
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