Re: [PATCH 1/3] driver core: generalize driver_override in struct device

On Mon, Mar 2, 2026 at 8:27 AM Danilo Krummrich <[email protected]> wrote:
>
> Currently, there are 12 busses (including platform and PCI) that
> duplicate the driver_override logic for their individual devices.
>
> All of them seem to be prone to the bug described in [1].
>
> While this could be solved for every bus individually using a separate
> lock, solving this in the driver-core generically results in less (and
> cleaner) changes overall.
>
> Thus, move driver_override to struct device, provide corresponding
> accessors for busses and handle locking with a separate lock internally.
>
> In particular, add device_set_driver_override(),
> device_has_driver_override(), device_match_driver_override() and a
> helper, DEVICE_ATTR_DRIVER_OVERRIDE(), to declare the corresponding
> sysfs store() and show() callbacks.
>
> Until all busses have migrated, keep driver_set_override() in place.
>
> Note that we can't use the device lock for the reasons described in [2].
>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=220789 [1]
> Link: https://lore.kernel.org/driver-core/[email protected]/ [2]
> Signed-off-by: Danilo Krummrich <[email protected]>

Applying the following diff fixes the KASAN issue. It was just a minor bug.

diff --git a/drivers/base/core.c b/drivers/base/core.c
index a8cb90577d10..09b98f02f559 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -2556,6 +2556,7 @@ static void device_release(struct kobject *kobj)
        devres_release_all(dev);

        kfree(dev->dma_range_map);
+       kfree(dev->driver_override.name);

        if (dev->release)
                dev->release(dev);
@@ -2566,7 +2567,6 @@ static void device_release(struct kobject *kobj)
        else
                WARN(1, KERN_ERR "Device '%s' does not have a
release() function, it is broken and must be fixed. See
Documentation/core-api/kobject.rst.\n",
                        dev_name(dev));
-       kfree(dev->driver_override.name);
        kfree(p);
 }

With this applied, along with the PCI driver diff from the WIP patch,
the issue is resolved. I tested this on PCI and both PoCs no longer
trigger KASAN. I also ran with other debug options enabled (lockdep,
sleep inside atomic, etc.) and hit no warnings.

I was working on a similar patch recently, but your version is better.
Not returning the string directly provides better encapsulation and
makes the API much harder to misuse.

Tested-by: Gui-Dong Han <[email protected]>
Reviewed-by: Gui-Dong Han <[email protected]>

> ---
>  drivers/base/core.c    |  2 ++
>  drivers/base/dd.c      | 60 +++++++++++++++++++++++++++++++
>  include/linux/device.h | 81 ++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 143 insertions(+)
>
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index 791f9e444df8..a8cb90577d10 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -2566,6 +2566,7 @@ static void device_release(struct kobject *kobj)
>         else
>                 WARN(1, KERN_ERR "Device '%s' does not have a release() function, it is broken and must be fixed. See Documentation/core-api/kobject.rst.\n",
>                         dev_name(dev));
> +       kfree(dev->driver_override.name);
>         kfree(p);
>  }
>
> @@ -3159,6 +3160,7 @@ void device_initialize(struct device *dev)
>         kobject_init(&dev->kobj, &device_ktype);
>         INIT_LIST_HEAD(&dev->dma_pools);
>         mutex_init(&dev->mutex);
> +       spin_lock_init(&dev->driver_override.lock);
>         lockdep_set_novalidate_class(&dev->mutex);
>         spin_lock_init(&dev->devres_lock);
>         INIT_LIST_HEAD(&dev->devres_head);
> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index 0354f209529c..697e36e63cab 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -381,6 +381,66 @@ static void __exit deferred_probe_exit(void)
>  }
>  __exitcall(deferred_probe_exit);
>
> +int __device_set_driver_override(struct device *dev, const char *s, size_t len)
> +{
> +       const char *new, *old;
> +       char *cp;
> +
> +       if (!s)
> +               return -EINVAL;
> +
> +       /*
> +        * The stored value will be used in sysfs show callback (sysfs_emit()),
> +        * which has a length limit of PAGE_SIZE and adds a trailing newline.
> +        * Thus we can store one character less to avoid truncation during sysfs
> +        * show.
> +        */
> +       if (len >= (PAGE_SIZE - 1))
> +               return -EINVAL;
> +
> +       /*
> +        * Compute the real length of the string in case userspace sends us a
> +        * bunch of \0 characters like python likes to do.
> +        */
> +       len = strlen(s);
> +
> +       if (!len) {
> +               /* Empty string passed - clear override */
> +               spin_lock(&dev->driver_override.lock);
> +               old = dev->driver_override.name;
> +               dev->driver_override.name = NULL;
> +               spin_unlock(&dev->driver_override.lock);
> +               kfree(old);
> +
> +               return 0;
> +       }
> +
> +       cp = strnchr(s, len, '\n');
> +       if (cp)
> +               len = cp - s;
> +
> +       new = kstrndup(s, len, GFP_KERNEL);
> +       if (!new)
> +               return -ENOMEM;
> +
> +       spin_lock(&dev->driver_override.lock);
> +       old = dev->driver_override.name;
> +       if (cp != s) {
> +               dev->driver_override.name = new;
> +               spin_unlock(&dev->driver_override.lock);
> +       } else {
> +               /* "\n" passed - clear override */
> +               dev->driver_override.name = NULL;
> +               spin_unlock(&dev->driver_override.lock);
> +
> +               kfree(new);
> +       }
> +       kfree(old);
> +
> +       return 0;
> +}
> +EXPORT_SYMBOL_GPL(__device_set_driver_override);
> +
>  /**
>   * device_is_bound() - Check if device is bound to a driver
>   * @dev: device to check
> diff --git a/include/linux/device.h b/include/linux/device.h
> index 0be95294b6e6..4599156d5cbd 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -266,6 +266,33 @@ ssize_t device_show_string(struct device *dev, struct device_attribute *attr,
>         struct dev_ext_attribute dev_attr_##_name = \
>                 { __ATTR(_name, (_mode) & ~0222, device_show_string, NULL), (_var) }
>
> +/**
> + * DEVICE_ATTR_DRIVER_OVERRIDE - Define sysfs driver_override attribute callbacks
> + *
> + * Generates the standard driver_override_show() and driver_override_store()
> + * sysfs callbacks and the static DEVICE_ATTR_RW(driver_override) declaration.
> + */
> +#define DEVICE_ATTR_DRIVER_OVERRIDE()                                          \
> +static ssize_t driver_override_store(struct device *dev,                       \
> +                                    struct device_attribute *attr,             \
> +                                    const char *buf, size_t count)             \
> +{                                                                              \
> +       int ret;                                                                \
> +                                                                               \
> +       ret = __device_set_driver_override(dev, buf, count);                    \
> +       if (ret)                                                                \
> +               return ret;                                                     \
> +                                                                               \
> +       return count;                                                           \
> +}                                                                              \
> +static ssize_t driver_override_show(struct device *dev,                                \
> +                                   struct device_attribute *attr, char *buf)   \
> +{                                                                              \
> +       guard(spinlock)(&dev->driver_override.lock);                            \
> +       return sysfs_emit(buf, "%s\n", dev->driver_override.name);              \
> +}                                                                              \
> +static DEVICE_ATTR_RW(driver_override)
> +
>  #define DEVICE_ATTR_IGNORE_LOCKDEP(_name, _mode, _show, _store) \
>         struct device_attribute dev_attr_##_name =              \
>                 __ATTR_IGNORE_LOCKDEP(_name, _mode, _show, _store)
> @@ -483,6 +510,8 @@ struct device_physical_location {
>   *             on.  This shrinks the "Board Support Packages" (BSPs) and
>   *             minimizes board-specific #ifdefs in drivers.
>   * @driver_data: Private pointer for driver specific info.
> + * @driver_override: Driver name to force a match.  Do not touch directly; use
> + *                  device_set_driver_override() instead.
>   * @links:     Links to suppliers and consumers of this device.
>   * @power:     For device power management.
>   *             See Documentation/driver-api/pm/devices.rst for details.
> @@ -576,6 +605,10 @@ struct device {
>                                            core doesn't touch it */
>         void            *driver_data;   /* Driver data, set and get with
>                                            dev_set_drvdata/dev_get_drvdata */
> +       struct {
> +               const char      *name;
> +               spinlock_t      lock;
> +       } driver_override;
>         struct mutex            mutex;  /* mutex to synchronize calls to
>                                          * its driver.
>                                          */
> @@ -701,6 +734,54 @@ struct device_link {
>
>  #define kobj_to_dev(__kobj)    container_of_const(__kobj, struct device, kobj)
>
> +int __device_set_driver_override(struct device *dev, const char *s, size_t len);
> +
> +/**
> + * device_set_driver_override() - Helper to set or clear driver override.
> + * @dev: Device to change
> + * @s: NUL-terminated string, new driver name to force a match, pass empty
> + *     string to clear it ("" or "\n", where the latter is only for sysfs
> + *     interface).
> + *
> + * Helper to set or clear driver override of a device.
> + *
> + * Returns: 0 on success or a negative error code on failure.
> + */
> +static inline int device_set_driver_override(struct device *dev, const char *s)
> +{
> +       return __device_set_driver_override(dev, s, strlen(s));
> +}
> +
> +/**
> + * device_has_driver_override() - Check if a driver override has been set.
> + * @dev: device to check
> + *
> + * Returns true if a driver override has been set for this device.
> + */
> +static inline bool device_has_driver_override(struct device *dev)
> +{
> +       guard(spinlock)(&dev->driver_override.lock);
> +       return !!dev->driver_override.name;
> +}
> +
> +/**
> + * device_match_driver_override() - Match a driver against the device's driver_override.
> + * @dev: device to check
> + * @drv: driver to match against
> + *
> + * Returns > 0 if a driver override is set and matches the given driver, 0 if a
> + * driver override is set but does not match, or < 0 if a driver override is not
> + * set at all.
> + */
> +static inline int device_match_driver_override(struct device *dev,
> +                                              const struct device_driver *drv)
> +{
> +       guard(spinlock)(&dev->driver_override.lock);
> +       if (dev->driver_override.name)
> +               return !strcmp(dev->driver_override.name, drv->name);
> +       return -1;
> +}
> +
>  /**
>   * device_iommu_mapped - Returns true when the device DMA is translated
>   *                      by an IOMMU
> --
> 2.53.0
>