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233ed09d7f
Credit for this patch goes is shared with Dan Williams [1]. I've taken things one step further to make the helper function more useful and clean up calling code. There's a common pattern in the kernel whereby a struct cdev is placed in a structure along side a struct device which manages the life-cycle of both. In the naive approach, the reference counting is broken and the struct device can free everything before the chardev code is entirely released. Many developers have solved this problem by linking the internal kobjs in this fashion: cdev.kobj.parent = &parent_dev.kobj; The cdev code explicitly gets and puts a reference to it's kobj parent. So this seems like it was intended to be used this way. Dmitrty Torokhov first put this in place in 2012 with this commit:2f0157fchar_dev: pin parent kobject and the first instance of the fix was then done in the input subsystem in the following commit:4a215aaInput: fix use-after-free introduced with dynamic minor changes Subsequently over the years, however, this issue seems to have tripped up multiple developers independently. For example, see these commits:0d5b7daiio: Prevent race between IIO chardev opening and IIO device (by Lars-Peter Clausen in 2013)ba0ef85tpm: Fix initialization of the cdev (by Jason Gunthorpe in 2015)5b28dde[media] media: fix use-after-free in cdev_put() when app exits after driver unbind (by Shauh Khan in 2016) This technique is similarly done in at least 15 places within the kernel and probably should have been done so in another, at least, 5 places. The kobj line also looks very suspect in that one would not expect drivers to have to mess with kobject internals in this way. Even highly experienced kernel developers can be surprised by this code, as seen in [2]. To help alleviate this situation, and hopefully prevent future wasted effort on this problem, this patch introduces a helper function to register a char device along with its parent struct device. This creates a more regular API for tying a char device to its parent without the developer having to set members in the underlying kobject. This patch introduce cdev_device_add and cdev_device_del which replaces a common pattern including setting the kobj parent, calling cdev_add and then calling device_add. It also introduces cdev_set_parent for the few cases that set the kobject parent without using device_add. [1] https://lkml.org/lkml/2017/2/13/700 [2] https://lkml.org/lkml/2017/2/10/370 Signed-off-by: Logan Gunthorpe <logang@deltatee.com> Signed-off-by: Dan Williams <dan.j.williams@intel.com> Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com> Reviewed-by: Alexandre Belloni <alexandre.belloni@free-electrons.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
39 lines
787 B
C
39 lines
787 B
C
#ifndef _LINUX_CDEV_H
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#define _LINUX_CDEV_H
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#include <linux/kobject.h>
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#include <linux/kdev_t.h>
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#include <linux/list.h>
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#include <linux/device.h>
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struct file_operations;
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struct inode;
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struct module;
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struct cdev {
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struct kobject kobj;
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struct module *owner;
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const struct file_operations *ops;
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struct list_head list;
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dev_t dev;
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unsigned int count;
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};
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void cdev_init(struct cdev *, const struct file_operations *);
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struct cdev *cdev_alloc(void);
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void cdev_put(struct cdev *p);
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int cdev_add(struct cdev *, dev_t, unsigned);
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void cdev_set_parent(struct cdev *p, struct kobject *kobj);
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int cdev_device_add(struct cdev *cdev, struct device *dev);
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void cdev_device_del(struct cdev *cdev, struct device *dev);
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void cdev_del(struct cdev *);
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void cd_forget(struct inode *);
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#endif
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