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

Tetsuo Handa
2022-07-04T13:49:13Z

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.

+ 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;