[PATCH] virtio-mmio: fix device release warning on module unload

[PATCH] virtio-mmio: fix device release warning on module unload

[Johan Hovold]

Driver core expects devices to be allocated dynamically and complains
loudly when a device that lacks a release function is freed.

Use __root_device_register() to allocate and register the root device
instead of open coding using a static device.

Note that root_device_register(), which also creates a link to the
module, cannot be used as the device is registered when parsing the
module parameters which happens before the module kobject has been set
up.

Fixes: 81a054ce0b46 ("virtio-mmio: Devices parameter parsing")
Cc: stable@vger.kernel.org	# 3.5
Cc: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/virtio/virtio_mmio.c | 20 ++++++++------------
 1 file changed, 8 insertions(+), 12 deletions(-)

diff --git a/drivers/virtio/virtio_mmio.c b/drivers/virtio/virtio_mmio.c
index 595c2274fbb5..1b580de81e82 100644
--- a/drivers/virtio/virtio_mmio.c
+++ b/drivers/virtio/virtio_mmio.c
@@ -662,9 +662,7 @@ static void virtio_mmio_remove(struct platform_device *pdev)
 
 #if defined(CONFIG_VIRTIO_MMIO_CMDLINE_DEVICES)
 
-static struct device vm_cmdline_parent = {
-	.init_name = "virtio-mmio-cmdline",
-};
+static struct device *vm_cmdline_parent;
 
 static int vm_cmdline_parent_registered;
 static int vm_cmdline_id;
@@ -672,7 +670,6 @@ static int vm_cmdline_id;
 static int vm_cmdline_set(const char *device,
 		const struct kernel_param *kp)
 {
-	int err;
 	struct resource resources[2] = {};
 	char *str;
 	long long base, size;
@@ -704,11 +701,10 @@ static int vm_cmdline_set(const char *device,
 	resources[1].start = resources[1].end = irq;
 
 	if (!vm_cmdline_parent_registered) {
-		err = device_register(&vm_cmdline_parent);
-		if (err) {
-			put_device(&vm_cmdline_parent);
+		vm_cmdline_parent = __root_device_register("virtio-mmio-cmdline", NULL);
+		if (IS_ERR(vm_cmdline_parent)) {
 			pr_err("Failed to register parent device!\n");
-			return err;
+			return PTR_ERR(vm_cmdline_parent);
 		}
 		vm_cmdline_parent_registered = 1;
 	}
@@ -719,7 +715,7 @@ static int vm_cmdline_set(const char *device,
 		       (unsigned long long)resources[0].end,
 		       (int)resources[1].start);
 
-	pdev = platform_device_register_resndata(&vm_cmdline_parent,
+	pdev = platform_device_register_resndata(vm_cmdline_parent,
 			"virtio-mmio", vm_cmdline_id++,
 			resources, ARRAY_SIZE(resources), NULL, 0);
 
@@ -743,7 +739,7 @@ static int vm_cmdline_get_device(struct device *dev, void *data)
 static int vm_cmdline_get(char *buffer, const struct kernel_param *kp)
 {
 	buffer[0] = '\0';
-	device_for_each_child(&vm_cmdline_parent, buffer,
+	device_for_each_child(vm_cmdline_parent, buffer,
 			vm_cmdline_get_device);
 	return strlen(buffer) + 1;
 }
@@ -766,9 +762,9 @@ static int vm_unregister_cmdline_device(struct device *dev,
 static void vm_unregister_cmdline_devices(void)
 {
 	if (vm_cmdline_parent_registered) {
-		device_for_each_child(&vm_cmdline_parent, NULL,
+		device_for_each_child(vm_cmdline_parent, NULL,
 				vm_unregister_cmdline_device);
-		device_unregister(&vm_cmdline_parent);
+		root_device_unregister(vm_cmdline_parent);
 		vm_cmdline_parent_registered = 0;
 	}
 }
-- 
2.53.0

Re: [PATCH] virtio-mmio: fix device release warning on module unload

[Jason Wang]

On Fri, Apr 24, 2026 at 6:48 PM Johan Hovold <johan@kernel.org> wrote:
>
> Driver core expects devices to be allocated dynamically and complains
> loudly when a device that lacks a release function is freed.
>
> Use __root_device_register() to allocate and register the root device
> instead of open coding using a static device.
>
> Note that root_device_register(), which also creates a link to the
> module, cannot be used as the device is registered when parsing the
> module parameters which happens before the module kobject has been set
> up.
>
> Fixes: 81a054ce0b46 ("virtio-mmio: Devices parameter parsing")
> Cc: stable@vger.kernel.org      # 3.5
> Cc: Pawel Moll <pawel.moll@arm.com>
> Signed-off-by: Johan Hovold <johan@kernel.org>
> ---
>  drivers/virtio/virtio_mmio.c | 20 ++++++++------------
>  1 file changed, 8 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/virtio/virtio_mmio.c b/drivers/virtio/virtio_mmio.c
> index 595c2274fbb5..1b580de81e82 100644
> --- a/drivers/virtio/virtio_mmio.c
> +++ b/drivers/virtio/virtio_mmio.c
> @@ -662,9 +662,7 @@ static void virtio_mmio_remove(struct platform_device *pdev)
>
>  #if defined(CONFIG_VIRTIO_MMIO_CMDLINE_DEVICES)
>
> -static struct device vm_cmdline_parent = {
> -       .init_name = "virtio-mmio-cmdline",
> -};
> +static struct device *vm_cmdline_parent;

vm_cmdline_get() is the .get callback for the device module parameter.
It is invoked when userspace reads
/sys/module/virtio_mmio/parameters/device. This function uses
vm_cmdline_parent unconditionally, without checking whether the device
has been registered. This would cause NULL pointer dereference.

Thanks

Re: [PATCH] virtio-mmio: fix device release warning on module unload

[Johan Hovold]

On Mon, Apr 27, 2026 at 12:16:47PM +0800, Jason Wang wrote:
> On Fri, Apr 24, 2026 at 6:48 PM Johan Hovold <johan@kernel.org> wrote:
> >
> > Driver core expects devices to be allocated dynamically and complains
> > loudly when a device that lacks a release function is freed.
> >
> > Use __root_device_register() to allocate and register the root device
> > instead of open coding using a static device.

> > -static struct device vm_cmdline_parent = {
> > -       .init_name = "virtio-mmio-cmdline",
> > -};
> > +static struct device *vm_cmdline_parent;
> 
> vm_cmdline_get() is the .get callback for the device module parameter.
> It is invoked when userspace reads
> /sys/module/virtio_mmio/parameters/device. This function uses
> vm_cmdline_parent unconditionally, without checking whether the device
> has been registered. This would cause NULL pointer dereference.

Indeed, Sashiko flagged this as well. Just sent a v2 here:

	https://lore.kernel.org/r/20260427143710.14702-1-johan@kernel.org

Johan