Re: [PATCH] char: misc: make misc_open() and misc_register() killable

Wedson Almeida Filho
2022-07-04T13:58:08Z

On Mon, Jul 04, 2022 at 10:48:32PM +0900, Tetsuo Handa wrote:
> On 2022/07/04 21:59, Wedson Almeida Filho wrote:
> >> @@ -139,6 +139,10 @@ static int misc_open(struct inode *inode, struct file *file)
> >>
> >> err = 0;
> >> replace_fops(file, new_fops);
> >> + if (iter->unlocked_open && file->f_op->open) {
> >> + mutex_unlock(&misc_mtx);
> >> + return file->f_op->open(inode, file);
> >> + }
> >
> > One of the invariants of miscdev is that once misc_deregister() returns,
> > no new calls to f_op->open() are made. (Although, of course, you can
> > still have open files but that's a whole different problem.)
>
> The point of this change is that file->f_op after mutex_unlock(&misc_mtx) is
> from new_fops which is guaranteed to hold a ref on "struct file_operations"
> via new_fops = fops_get("struct miscdevice"->fops).
> That is, a module ref remains valid even after mutex_unlock(&misc_mtx).
>
> And as with major_names_lock case quoted below, this change assumes that
> misc_deregister() is called from module's __exit function, and fops_get()
> is preventing the module owning new_fops from calling __exit function.

Your assumption is not sound. misc_deregister() can be (and is)
legitimately called from other places, for example, a driver's remove()
callback. In fact, when I grep for misc_deregister(), the second
instance is such a case.

Anyway, I agree that something should be done about the issue you
report, I'm just arguing that this proposal isn't a good one because it
breaks an invariant that makes it really hard for drivers to properly
cleanup.

>
> + if ((*n)->major != major || !(*n)->probe)
> + continue;
> + if (!try_module_get((*n)->owner))
> + break;
> + /*
> + * Calling probe function with major_names_lock held causes
> + * circular locking dependency problem. Thus, call it after
> + * releasing major_names_lock.
> + */
> + probe_fn = (*n)->probe;
> + mutex_unlock(&major_names_lock);
> + /*
> + * Assuming that unregister_blkdev() is called from module's
> + * __exit function, a module refcount taken above allows us
> + * to safely call probe function without major_names_lock held.
> + */
> + probe_fn(devt);
> + module_put((*n)->owner);
> + return;
>