linux/fs/char_dev.c
Logan Gunthorpe 233ed09d7f chardev: add helper function to register char devs with a struct device
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:

2f0157f char_dev: pin parent kobject

and the first instance of the fix was then done in the input subsystem
in the following commit:

4a215aa Input: 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:

0d5b7da iio: Prevent race between IIO chardev opening and IIO device
(by Lars-Peter Clausen in 2013)

ba0ef85 tpm: 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>
2017-03-21 06:44:32 +01:00

661 lines
16 KiB
C

/*
* linux/fs/char_dev.c
*
* Copyright (C) 1991, 1992 Linus Torvalds
*/
#include <linux/init.h>
#include <linux/fs.h>
#include <linux/kdev_t.h>
#include <linux/slab.h>
#include <linux/string.h>
#include <linux/major.h>
#include <linux/errno.h>
#include <linux/module.h>
#include <linux/seq_file.h>
#include <linux/kobject.h>
#include <linux/kobj_map.h>
#include <linux/cdev.h>
#include <linux/mutex.h>
#include <linux/backing-dev.h>
#include <linux/tty.h>
#include "internal.h"
static struct kobj_map *cdev_map;
static DEFINE_MUTEX(chrdevs_lock);
static struct char_device_struct {
struct char_device_struct *next;
unsigned int major;
unsigned int baseminor;
int minorct;
char name[64];
struct cdev *cdev; /* will die */
} *chrdevs[CHRDEV_MAJOR_HASH_SIZE];
/* index in the above */
static inline int major_to_index(unsigned major)
{
return major % CHRDEV_MAJOR_HASH_SIZE;
}
#ifdef CONFIG_PROC_FS
void chrdev_show(struct seq_file *f, off_t offset)
{
struct char_device_struct *cd;
if (offset < CHRDEV_MAJOR_HASH_SIZE) {
mutex_lock(&chrdevs_lock);
for (cd = chrdevs[offset]; cd; cd = cd->next)
seq_printf(f, "%3d %s\n", cd->major, cd->name);
mutex_unlock(&chrdevs_lock);
}
}
#endif /* CONFIG_PROC_FS */
/*
* Register a single major with a specified minor range.
*
* If major == 0 this functions will dynamically allocate a major and return
* its number.
*
* If major > 0 this function will attempt to reserve the passed range of
* minors and will return zero on success.
*
* Returns a -ve errno on failure.
*/
static struct char_device_struct *
__register_chrdev_region(unsigned int major, unsigned int baseminor,
int minorct, const char *name)
{
struct char_device_struct *cd, **cp;
int ret = 0;
int i;
cd = kzalloc(sizeof(struct char_device_struct), GFP_KERNEL);
if (cd == NULL)
return ERR_PTR(-ENOMEM);
mutex_lock(&chrdevs_lock);
/* temporary */
if (major == 0) {
for (i = ARRAY_SIZE(chrdevs)-1; i > 0; i--) {
if (chrdevs[i] == NULL)
break;
}
if (i < CHRDEV_MAJOR_DYN_END)
pr_warn("CHRDEV \"%s\" major number %d goes below the dynamic allocation range\n",
name, i);
if (i == 0) {
ret = -EBUSY;
goto out;
}
major = i;
}
cd->major = major;
cd->baseminor = baseminor;
cd->minorct = minorct;
strlcpy(cd->name, name, sizeof(cd->name));
i = major_to_index(major);
for (cp = &chrdevs[i]; *cp; cp = &(*cp)->next)
if ((*cp)->major > major ||
((*cp)->major == major &&
(((*cp)->baseminor >= baseminor) ||
((*cp)->baseminor + (*cp)->minorct > baseminor))))
break;
/* Check for overlapping minor ranges. */
if (*cp && (*cp)->major == major) {
int old_min = (*cp)->baseminor;
int old_max = (*cp)->baseminor + (*cp)->minorct - 1;
int new_min = baseminor;
int new_max = baseminor + minorct - 1;
/* New driver overlaps from the left. */
if (new_max >= old_min && new_max <= old_max) {
ret = -EBUSY;
goto out;
}
/* New driver overlaps from the right. */
if (new_min <= old_max && new_min >= old_min) {
ret = -EBUSY;
goto out;
}
}
cd->next = *cp;
*cp = cd;
mutex_unlock(&chrdevs_lock);
return cd;
out:
mutex_unlock(&chrdevs_lock);
kfree(cd);
return ERR_PTR(ret);
}
static struct char_device_struct *
__unregister_chrdev_region(unsigned major, unsigned baseminor, int minorct)
{
struct char_device_struct *cd = NULL, **cp;
int i = major_to_index(major);
mutex_lock(&chrdevs_lock);
for (cp = &chrdevs[i]; *cp; cp = &(*cp)->next)
if ((*cp)->major == major &&
(*cp)->baseminor == baseminor &&
(*cp)->minorct == minorct)
break;
if (*cp) {
cd = *cp;
*cp = cd->next;
}
mutex_unlock(&chrdevs_lock);
return cd;
}
/**
* register_chrdev_region() - register a range of device numbers
* @from: the first in the desired range of device numbers; must include
* the major number.
* @count: the number of consecutive device numbers required
* @name: the name of the device or driver.
*
* Return value is zero on success, a negative error code on failure.
*/
int register_chrdev_region(dev_t from, unsigned count, const char *name)
{
struct char_device_struct *cd;
dev_t to = from + count;
dev_t n, next;
for (n = from; n < to; n = next) {
next = MKDEV(MAJOR(n)+1, 0);
if (next > to)
next = to;
cd = __register_chrdev_region(MAJOR(n), MINOR(n),
next - n, name);
if (IS_ERR(cd))
goto fail;
}
return 0;
fail:
to = n;
for (n = from; n < to; n = next) {
next = MKDEV(MAJOR(n)+1, 0);
kfree(__unregister_chrdev_region(MAJOR(n), MINOR(n), next - n));
}
return PTR_ERR(cd);
}
/**
* alloc_chrdev_region() - register a range of char device numbers
* @dev: output parameter for first assigned number
* @baseminor: first of the requested range of minor numbers
* @count: the number of minor numbers required
* @name: the name of the associated device or driver
*
* Allocates a range of char device numbers. The major number will be
* chosen dynamically, and returned (along with the first minor number)
* in @dev. Returns zero or a negative error code.
*/
int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count,
const char *name)
{
struct char_device_struct *cd;
cd = __register_chrdev_region(0, baseminor, count, name);
if (IS_ERR(cd))
return PTR_ERR(cd);
*dev = MKDEV(cd->major, cd->baseminor);
return 0;
}
/**
* __register_chrdev() - create and register a cdev occupying a range of minors
* @major: major device number or 0 for dynamic allocation
* @baseminor: first of the requested range of minor numbers
* @count: the number of minor numbers required
* @name: name of this range of devices
* @fops: file operations associated with this devices
*
* If @major == 0 this functions will dynamically allocate a major and return
* its number.
*
* If @major > 0 this function will attempt to reserve a device with the given
* major number and will return zero on success.
*
* Returns a -ve errno on failure.
*
* The name of this device has nothing to do with the name of the device in
* /dev. It only helps to keep track of the different owners of devices. If
* your module name has only one type of devices it's ok to use e.g. the name
* of the module here.
*/
int __register_chrdev(unsigned int major, unsigned int baseminor,
unsigned int count, const char *name,
const struct file_operations *fops)
{
struct char_device_struct *cd;
struct cdev *cdev;
int err = -ENOMEM;
cd = __register_chrdev_region(major, baseminor, count, name);
if (IS_ERR(cd))
return PTR_ERR(cd);
cdev = cdev_alloc();
if (!cdev)
goto out2;
cdev->owner = fops->owner;
cdev->ops = fops;
kobject_set_name(&cdev->kobj, "%s", name);
err = cdev_add(cdev, MKDEV(cd->major, baseminor), count);
if (err)
goto out;
cd->cdev = cdev;
return major ? 0 : cd->major;
out:
kobject_put(&cdev->kobj);
out2:
kfree(__unregister_chrdev_region(cd->major, baseminor, count));
return err;
}
/**
* unregister_chrdev_region() - unregister a range of device numbers
* @from: the first in the range of numbers to unregister
* @count: the number of device numbers to unregister
*
* This function will unregister a range of @count device numbers,
* starting with @from. The caller should normally be the one who
* allocated those numbers in the first place...
*/
void unregister_chrdev_region(dev_t from, unsigned count)
{
dev_t to = from + count;
dev_t n, next;
for (n = from; n < to; n = next) {
next = MKDEV(MAJOR(n)+1, 0);
if (next > to)
next = to;
kfree(__unregister_chrdev_region(MAJOR(n), MINOR(n), next - n));
}
}
/**
* __unregister_chrdev - unregister and destroy a cdev
* @major: major device number
* @baseminor: first of the range of minor numbers
* @count: the number of minor numbers this cdev is occupying
* @name: name of this range of devices
*
* Unregister and destroy the cdev occupying the region described by
* @major, @baseminor and @count. This function undoes what
* __register_chrdev() did.
*/
void __unregister_chrdev(unsigned int major, unsigned int baseminor,
unsigned int count, const char *name)
{
struct char_device_struct *cd;
cd = __unregister_chrdev_region(major, baseminor, count);
if (cd && cd->cdev)
cdev_del(cd->cdev);
kfree(cd);
}
static DEFINE_SPINLOCK(cdev_lock);
static struct kobject *cdev_get(struct cdev *p)
{
struct module *owner = p->owner;
struct kobject *kobj;
if (owner && !try_module_get(owner))
return NULL;
kobj = kobject_get(&p->kobj);
if (!kobj)
module_put(owner);
return kobj;
}
void cdev_put(struct cdev *p)
{
if (p) {
struct module *owner = p->owner;
kobject_put(&p->kobj);
module_put(owner);
}
}
/*
* Called every time a character special file is opened
*/
static int chrdev_open(struct inode *inode, struct file *filp)
{
const struct file_operations *fops;
struct cdev *p;
struct cdev *new = NULL;
int ret = 0;
spin_lock(&cdev_lock);
p = inode->i_cdev;
if (!p) {
struct kobject *kobj;
int idx;
spin_unlock(&cdev_lock);
kobj = kobj_lookup(cdev_map, inode->i_rdev, &idx);
if (!kobj)
return -ENXIO;
new = container_of(kobj, struct cdev, kobj);
spin_lock(&cdev_lock);
/* Check i_cdev again in case somebody beat us to it while
we dropped the lock. */
p = inode->i_cdev;
if (!p) {
inode->i_cdev = p = new;
list_add(&inode->i_devices, &p->list);
new = NULL;
} else if (!cdev_get(p))
ret = -ENXIO;
} else if (!cdev_get(p))
ret = -ENXIO;
spin_unlock(&cdev_lock);
cdev_put(new);
if (ret)
return ret;
ret = -ENXIO;
fops = fops_get(p->ops);
if (!fops)
goto out_cdev_put;
replace_fops(filp, fops);
if (filp->f_op->open) {
ret = filp->f_op->open(inode, filp);
if (ret)
goto out_cdev_put;
}
return 0;
out_cdev_put:
cdev_put(p);
return ret;
}
void cd_forget(struct inode *inode)
{
spin_lock(&cdev_lock);
list_del_init(&inode->i_devices);
inode->i_cdev = NULL;
inode->i_mapping = &inode->i_data;
spin_unlock(&cdev_lock);
}
static void cdev_purge(struct cdev *cdev)
{
spin_lock(&cdev_lock);
while (!list_empty(&cdev->list)) {
struct inode *inode;
inode = container_of(cdev->list.next, struct inode, i_devices);
list_del_init(&inode->i_devices);
inode->i_cdev = NULL;
}
spin_unlock(&cdev_lock);
}
/*
* Dummy default file-operations: the only thing this does
* is contain the open that then fills in the correct operations
* depending on the special file...
*/
const struct file_operations def_chr_fops = {
.open = chrdev_open,
.llseek = noop_llseek,
};
static struct kobject *exact_match(dev_t dev, int *part, void *data)
{
struct cdev *p = data;
return &p->kobj;
}
static int exact_lock(dev_t dev, void *data)
{
struct cdev *p = data;
return cdev_get(p) ? 0 : -1;
}
/**
* cdev_add() - add a char device to the system
* @p: the cdev structure for the device
* @dev: the first device number for which this device is responsible
* @count: the number of consecutive minor numbers corresponding to this
* device
*
* cdev_add() adds the device represented by @p to the system, making it
* live immediately. A negative error code is returned on failure.
*/
int cdev_add(struct cdev *p, dev_t dev, unsigned count)
{
int error;
p->dev = dev;
p->count = count;
error = kobj_map(cdev_map, dev, count, NULL,
exact_match, exact_lock, p);
if (error)
return error;
kobject_get(p->kobj.parent);
return 0;
}
/**
* cdev_set_parent() - set the parent kobject for a char device
* @p: the cdev structure
* @kobj: the kobject to take a reference to
*
* cdev_set_parent() sets a parent kobject which will be referenced
* appropriately so the parent is not freed before the cdev. This
* should be called before cdev_add.
*/
void cdev_set_parent(struct cdev *p, struct kobject *kobj)
{
WARN_ON(!kobj->state_initialized);
p->kobj.parent = kobj;
}
/**
* cdev_device_add() - add a char device and it's corresponding
* struct device, linkink
* @dev: the device structure
* @cdev: the cdev structure
*
* cdev_device_add() adds the char device represented by @cdev to the system,
* just as cdev_add does. It then adds @dev to the system using device_add
* The dev_t for the char device will be taken from the struct device which
* needs to be initialized first. This helper function correctly takes a
* reference to the parent device so the parent will not get released until
* all references to the cdev are released.
*
* This helper uses dev->devt for the device number. If it is not set
* it will not add the cdev and it will be equivalent to device_add.
*
* This function should be used whenever the struct cdev and the
* struct device are members of the same structure whose lifetime is
* managed by the struct device.
*
* NOTE: Callers must assume that userspace was able to open the cdev and
* can call cdev fops callbacks at any time, even if this function fails.
*/
int cdev_device_add(struct cdev *cdev, struct device *dev)
{
int rc = 0;
if (dev->devt) {
cdev_set_parent(cdev, &dev->kobj);
rc = cdev_add(cdev, dev->devt, 1);
if (rc)
return rc;
}
rc = device_add(dev);
if (rc)
cdev_del(cdev);
return rc;
}
/**
* cdev_device_del() - inverse of cdev_device_add
* @dev: the device structure
* @cdev: the cdev structure
*
* cdev_device_del() is a helper function to call cdev_del and device_del.
* It should be used whenever cdev_device_add is used.
*
* If dev->devt is not set it will not remove the cdev and will be equivalent
* to device_del.
*
* NOTE: This guarantees that associated sysfs callbacks are not running
* or runnable, however any cdevs already open will remain and their fops
* will still be callable even after this function returns.
*/
void cdev_device_del(struct cdev *cdev, struct device *dev)
{
device_del(dev);
if (dev->devt)
cdev_del(cdev);
}
static void cdev_unmap(dev_t dev, unsigned count)
{
kobj_unmap(cdev_map, dev, count);
}
/**
* cdev_del() - remove a cdev from the system
* @p: the cdev structure to be removed
*
* cdev_del() removes @p from the system, possibly freeing the structure
* itself.
*
* NOTE: This guarantees that cdev device will no longer be able to be
* opened, however any cdevs already open will remain and their fops will
* still be callable even after cdev_del returns.
*/
void cdev_del(struct cdev *p)
{
cdev_unmap(p->dev, p->count);
kobject_put(&p->kobj);
}
static void cdev_default_release(struct kobject *kobj)
{
struct cdev *p = container_of(kobj, struct cdev, kobj);
struct kobject *parent = kobj->parent;
cdev_purge(p);
kobject_put(parent);
}
static void cdev_dynamic_release(struct kobject *kobj)
{
struct cdev *p = container_of(kobj, struct cdev, kobj);
struct kobject *parent = kobj->parent;
cdev_purge(p);
kfree(p);
kobject_put(parent);
}
static struct kobj_type ktype_cdev_default = {
.release = cdev_default_release,
};
static struct kobj_type ktype_cdev_dynamic = {
.release = cdev_dynamic_release,
};
/**
* cdev_alloc() - allocate a cdev structure
*
* Allocates and returns a cdev structure, or NULL on failure.
*/
struct cdev *cdev_alloc(void)
{
struct cdev *p = kzalloc(sizeof(struct cdev), GFP_KERNEL);
if (p) {
INIT_LIST_HEAD(&p->list);
kobject_init(&p->kobj, &ktype_cdev_dynamic);
}
return p;
}
/**
* cdev_init() - initialize a cdev structure
* @cdev: the structure to initialize
* @fops: the file_operations for this device
*
* Initializes @cdev, remembering @fops, making it ready to add to the
* system with cdev_add().
*/
void cdev_init(struct cdev *cdev, const struct file_operations *fops)
{
memset(cdev, 0, sizeof *cdev);
INIT_LIST_HEAD(&cdev->list);
kobject_init(&cdev->kobj, &ktype_cdev_default);
cdev->ops = fops;
}
static struct kobject *base_probe(dev_t dev, int *part, void *data)
{
if (request_module("char-major-%d-%d", MAJOR(dev), MINOR(dev)) > 0)
/* Make old-style 2.4 aliases work */
request_module("char-major-%d", MAJOR(dev));
return NULL;
}
void __init chrdev_init(void)
{
cdev_map = kobj_map_init(base_probe, &chrdevs_lock);
}
/* Let modules do char dev stuff */
EXPORT_SYMBOL(register_chrdev_region);
EXPORT_SYMBOL(unregister_chrdev_region);
EXPORT_SYMBOL(alloc_chrdev_region);
EXPORT_SYMBOL(cdev_init);
EXPORT_SYMBOL(cdev_alloc);
EXPORT_SYMBOL(cdev_del);
EXPORT_SYMBOL(cdev_add);
EXPORT_SYMBOL(cdev_set_parent);
EXPORT_SYMBOL(cdev_device_add);
EXPORT_SYMBOL(cdev_device_del);
EXPORT_SYMBOL(__register_chrdev);
EXPORT_SYMBOL(__unregister_chrdev);