diff mbox series

[v7,12/12] rpmsg: Fix kfree() of static memory on setting driver_override

Message ID 20220419113435.246203-13-krzysztof.kozlowski@linaro.org (mailing list archive)
State New
Headers show
Series Fix broken usage of driver_override (and kfree of static memory) | expand

Commit Message

Krzysztof Kozlowski April 19, 2022, 11:34 a.m. UTC
The driver_override field from platform driver should not be initialized
from static memory (string literal) because the core later kfree() it,
for example when driver_override is set via sysfs.

Use dedicated helper to set driver_override properly.

Fixes: 950a7388f02b ("rpmsg: Turn name service into a stand alone driver")
Fixes: c0cdc19f84a4 ("rpmsg: Driver for user space endpoint interface")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
---
 drivers/rpmsg/rpmsg_internal.h | 13 +++++++++++--
 drivers/rpmsg/rpmsg_ns.c       | 14 ++++++++++++--
 include/linux/rpmsg.h          |  6 ++++--
 3 files changed, 27 insertions(+), 6 deletions(-)

Comments

Marek Szyprowski April 29, 2022, 12:29 p.m. UTC | #1
Hi Krzysztof,

On 19.04.2022 13:34, Krzysztof Kozlowski wrote:
> The driver_override field from platform driver should not be initialized
> from static memory (string literal) because the core later kfree() it,
> for example when driver_override is set via sysfs.
>
> Use dedicated helper to set driver_override properly.
>
> Fixes: 950a7388f02b ("rpmsg: Turn name service into a stand alone driver")
> Fixes: c0cdc19f84a4 ("rpmsg: Driver for user space endpoint interface")
> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
> Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>

This patch landed recently in linux-next as commit 42cd402b8fd4 ("rpmsg: 
Fix kfree() of static memory on setting driver_override"). In my tests I 
found that it triggers the following issue during boot of the 
DragonBoard410c SBC (arch/arm64/boot/dts/qcom/apq8016-sbc.dtb):

------------[ cut here ]------------
DEBUG_LOCKS_WARN_ON(lock->magic != lock)
WARNING: CPU: 1 PID: 8 at kernel/locking/mutex.c:582 
__mutex_lock+0x1ec/0x430
Modules linked in:
CPU: 1 PID: 8 Comm: kworker/u8:0 Not tainted 5.18.0-rc4-next-20220429 #11815
Hardware name: Qualcomm Technologies, Inc. APQ 8016 SBC (DT)
Workqueue: events_unbound deferred_probe_work_func
pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : __mutex_lock+0x1ec/0x430
lr : __mutex_lock+0x1ec/0x430
..
Call trace:
  __mutex_lock+0x1ec/0x430
  mutex_lock_nested+0x38/0x64
  driver_set_override+0x124/0x150
  qcom_smd_register_edge+0x2a8/0x4ec
  qcom_smd_probe+0x54/0x80
  platform_probe+0x68/0xe0
  really_probe.part.0+0x9c/0x29c
  __driver_probe_device+0x98/0x144
  driver_probe_device+0xac/0x14c
  __device_attach_driver+0xb8/0x120
  bus_for_each_drv+0x78/0xd0
  __device_attach+0xd8/0x180
  device_initial_probe+0x14/0x20
  bus_probe_device+0x9c/0xa4
  deferred_probe_work_func+0x88/0xc4
  process_one_work+0x288/0x6bc
  worker_thread+0x248/0x450
  kthread+0x118/0x11c
  ret_from_fork+0x10/0x20
irq event stamp: 3599
hardirqs last  enabled at (3599): [<ffff80000919053c>] 
_raw_spin_unlock_irqrestore+0x98/0x9c
hardirqs last disabled at (3598): [<ffff800009190ba4>] 
_raw_spin_lock_irqsave+0xc0/0xcc
softirqs last  enabled at (3554): [<ffff800008010470>] _stext+0x470/0x5e8
softirqs last disabled at (3549): [<ffff8000080a4514>] 
__irq_exit_rcu+0x180/0x1ac
---[ end trace 0000000000000000 ]---

I don't see any direct relation between the $subject and the above log, 
but reverting the $subject on top of linux next-20220429 hides/fixes it. 
Maybe there is a kind of memory trashing somewhere there and your change 
only revealed it?

> ---
>   drivers/rpmsg/rpmsg_internal.h | 13 +++++++++++--
>   drivers/rpmsg/rpmsg_ns.c       | 14 ++++++++++++--
>   include/linux/rpmsg.h          |  6 ++++--
>   3 files changed, 27 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/rpmsg/rpmsg_internal.h b/drivers/rpmsg/rpmsg_internal.h
> index d4b23fd019a8..3e81642238d2 100644
> --- a/drivers/rpmsg/rpmsg_internal.h
> +++ b/drivers/rpmsg/rpmsg_internal.h
> @@ -94,10 +94,19 @@ int rpmsg_release_channel(struct rpmsg_device *rpdev,
>    */
>   static inline int rpmsg_ctrldev_register_device(struct rpmsg_device *rpdev)
>   {
> +	int ret;
> +
>   	strcpy(rpdev->id.name, "rpmsg_ctrl");
> -	rpdev->driver_override = "rpmsg_ctrl";
> +	ret = driver_set_override(&rpdev->dev, &rpdev->driver_override,
> +				  rpdev->id.name, strlen(rpdev->id.name));
> +	if (ret)
> +		return ret;
> +
> +	ret = rpmsg_register_device(rpdev);
> +	if (ret)
> +		kfree(rpdev->driver_override);
>   
> -	return rpmsg_register_device(rpdev);
> +	return ret;
>   }
>   
>   #endif
> diff --git a/drivers/rpmsg/rpmsg_ns.c b/drivers/rpmsg/rpmsg_ns.c
> index 762ff1ae279f..8eb8f328237e 100644
> --- a/drivers/rpmsg/rpmsg_ns.c
> +++ b/drivers/rpmsg/rpmsg_ns.c
> @@ -20,12 +20,22 @@
>    */
>   int rpmsg_ns_register_device(struct rpmsg_device *rpdev)
>   {
> +	int ret;
> +
>   	strcpy(rpdev->id.name, "rpmsg_ns");
> -	rpdev->driver_override = "rpmsg_ns";
> +	ret = driver_set_override(&rpdev->dev, &rpdev->driver_override,
> +				  rpdev->id.name, strlen(rpdev->id.name));
> +	if (ret)
> +		return ret;
> +
>   	rpdev->src = RPMSG_NS_ADDR;
>   	rpdev->dst = RPMSG_NS_ADDR;
>   
> -	return rpmsg_register_device(rpdev);
> +	ret = rpmsg_register_device(rpdev);
> +	if (ret)
> +		kfree(rpdev->driver_override);
> +
> +	return ret;
>   }
>   EXPORT_SYMBOL(rpmsg_ns_register_device);
>   
> diff --git a/include/linux/rpmsg.h b/include/linux/rpmsg.h
> index 02fa9116cd60..20c8cd1cde21 100644
> --- a/include/linux/rpmsg.h
> +++ b/include/linux/rpmsg.h
> @@ -41,7 +41,9 @@ struct rpmsg_channel_info {
>    * rpmsg_device - device that belong to the rpmsg bus
>    * @dev: the device struct
>    * @id: device id (used to match between rpmsg drivers and devices)
> - * @driver_override: driver name to force a match
> + * @driver_override: driver name to force a match; do not set directly,
> + *                   because core frees it; use driver_set_override() to
> + *                   set or clear it.
>    * @src: local address
>    * @dst: destination address
>    * @ept: the rpmsg endpoint of this channel
> @@ -51,7 +53,7 @@ struct rpmsg_channel_info {
>   struct rpmsg_device {
>   	struct device dev;
>   	struct rpmsg_device_id id;
> -	char *driver_override;
> +	const char *driver_override;
>   	u32 src;
>   	u32 dst;
>   	struct rpmsg_endpoint *ept;

Best regards
Krzysztof Kozlowski April 29, 2022, 2:16 p.m. UTC | #2
On 29/04/2022 14:29, Marek Szyprowski wrote:
> Hi Krzysztof,
> 
> On 19.04.2022 13:34, Krzysztof Kozlowski wrote:
>> The driver_override field from platform driver should not be initialized
>> from static memory (string literal) because the core later kfree() it,
>> for example when driver_override is set via sysfs.
>>
>> Use dedicated helper to set driver_override properly.
>>
>> Fixes: 950a7388f02b ("rpmsg: Turn name service into a stand alone driver")
>> Fixes: c0cdc19f84a4 ("rpmsg: Driver for user space endpoint interface")
>> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
>> Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
> 
> This patch landed recently in linux-next as commit 42cd402b8fd4 ("rpmsg: 
> Fix kfree() of static memory on setting driver_override"). In my tests I 
> found that it triggers the following issue during boot of the 
> DragonBoard410c SBC (arch/arm64/boot/dts/qcom/apq8016-sbc.dtb):
> 
> ------------[ cut here ]------------
> DEBUG_LOCKS_WARN_ON(lock->magic != lock)
> WARNING: CPU: 1 PID: 8 at kernel/locking/mutex.c:582 
> __mutex_lock+0x1ec/0x430
> Modules linked in:
> CPU: 1 PID: 8 Comm: kworker/u8:0 Not tainted 5.18.0-rc4-next-20220429 #11815
> Hardware name: Qualcomm Technologies, Inc. APQ 8016 SBC (DT)
> Workqueue: events_unbound deferred_probe_work_func
> pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> pc : __mutex_lock+0x1ec/0x430
> lr : __mutex_lock+0x1ec/0x430
> ..
> Call trace:
>   __mutex_lock+0x1ec/0x430
>   mutex_lock_nested+0x38/0x64
>   driver_set_override+0x124/0x150
>   qcom_smd_register_edge+0x2a8/0x4ec
>   qcom_smd_probe+0x54/0x80
>   platform_probe+0x68/0xe0
>   really_probe.part.0+0x9c/0x29c
>   __driver_probe_device+0x98/0x144
>   driver_probe_device+0xac/0x14c
>   __device_attach_driver+0xb8/0x120
>   bus_for_each_drv+0x78/0xd0
>   __device_attach+0xd8/0x180
>   device_initial_probe+0x14/0x20
>   bus_probe_device+0x9c/0xa4
>   deferred_probe_work_func+0x88/0xc4
>   process_one_work+0x288/0x6bc
>   worker_thread+0x248/0x450
>   kthread+0x118/0x11c
>   ret_from_fork+0x10/0x20
> irq event stamp: 3599
> hardirqs last  enabled at (3599): [<ffff80000919053c>] 
> _raw_spin_unlock_irqrestore+0x98/0x9c
> hardirqs last disabled at (3598): [<ffff800009190ba4>] 
> _raw_spin_lock_irqsave+0xc0/0xcc
> softirqs last  enabled at (3554): [<ffff800008010470>] _stext+0x470/0x5e8
> softirqs last disabled at (3549): [<ffff8000080a4514>] 
> __irq_exit_rcu+0x180/0x1ac
> ---[ end trace 0000000000000000 ]---
> 
> I don't see any direct relation between the $subject and the above log, 
> but reverting the $subject on top of linux next-20220429 hides/fixes it. 
> Maybe there is a kind of memory trashing somewhere there and your change 
> only revealed it?

Thanks for the report. I think the error path of my patch is wrong - I
should not kfree(rpdev->driver_override) from the rpmsg code. That's the
only thing I see now...

Could you test following patch and tell if it helps?
https://pastebin.ubuntu.com/p/rp3q9Z5fXj/

-----

diff --git a/drivers/rpmsg/rpmsg_internal.h b/drivers/rpmsg/rpmsg_internal.h
index 3e81642238d2..1e2ad944e2ec 100644
--- a/drivers/rpmsg/rpmsg_internal.h
+++ b/drivers/rpmsg/rpmsg_internal.h
@@ -102,11 +102,7 @@ static inline int
rpmsg_ctrldev_register_device(struct rpmsg_device *rpdev)
        if (ret)
                return ret;

-       ret = rpmsg_register_device(rpdev);
-       if (ret)
-               kfree(rpdev->driver_override);
-
-       return ret;
+       return rpmsg_register_device(rpdev);
 }

 #endif
diff --git a/drivers/rpmsg/rpmsg_ns.c b/drivers/rpmsg/rpmsg_ns.c
index 8eb8f328237e..f26078467899 100644
--- a/drivers/rpmsg/rpmsg_ns.c
+++ b/drivers/rpmsg/rpmsg_ns.c
@@ -31,11 +31,7 @@ int rpmsg_ns_register_device(struct rpmsg_device *rpdev)
        rpdev->src = RPMSG_NS_ADDR;
        rpdev->dst = RPMSG_NS_ADDR;

-       ret = rpmsg_register_device(rpdev);
-       if (ret)
-               kfree(rpdev->driver_override);
-
-       return ret;
+       return rpmsg_register_device(rpdev);
 }
 EXPORT_SYMBOL(rpmsg_ns_register_device);
Marek Szyprowski April 29, 2022, 2:51 p.m. UTC | #3
On 29.04.2022 16:16, Krzysztof Kozlowski wrote:
> On 29/04/2022 14:29, Marek Szyprowski wrote:
>> On 19.04.2022 13:34, Krzysztof Kozlowski wrote:
>>> The driver_override field from platform driver should not be initialized
>>> from static memory (string literal) because the core later kfree() it,
>>> for example when driver_override is set via sysfs.
>>>
>>> Use dedicated helper to set driver_override properly.
>>>
>>> Fixes: 950a7388f02b ("rpmsg: Turn name service into a stand alone driver")
>>> Fixes: c0cdc19f84a4 ("rpmsg: Driver for user space endpoint interface")
>>> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
>>> Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
>> This patch landed recently in linux-next as commit 42cd402b8fd4 ("rpmsg:
>> Fix kfree() of static memory on setting driver_override"). In my tests I
>> found that it triggers the following issue during boot of the
>> DragonBoard410c SBC (arch/arm64/boot/dts/qcom/apq8016-sbc.dtb):
>>
>> ------------[ cut here ]------------
>> DEBUG_LOCKS_WARN_ON(lock->magic != lock)
>> WARNING: CPU: 1 PID: 8 at kernel/locking/mutex.c:582
>> __mutex_lock+0x1ec/0x430
>> Modules linked in:
>> CPU: 1 PID: 8 Comm: kworker/u8:0 Not tainted 5.18.0-rc4-next-20220429 #11815
>> Hardware name: Qualcomm Technologies, Inc. APQ 8016 SBC (DT)
>> Workqueue: events_unbound deferred_probe_work_func
>> pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>> pc : __mutex_lock+0x1ec/0x430
>> lr : __mutex_lock+0x1ec/0x430
>> ..
>> Call trace:
>>    __mutex_lock+0x1ec/0x430
>>    mutex_lock_nested+0x38/0x64
>>    driver_set_override+0x124/0x150
>>    qcom_smd_register_edge+0x2a8/0x4ec
>>    qcom_smd_probe+0x54/0x80
>>    platform_probe+0x68/0xe0
>>    really_probe.part.0+0x9c/0x29c
>>    __driver_probe_device+0x98/0x144
>>    driver_probe_device+0xac/0x14c
>>    __device_attach_driver+0xb8/0x120
>>    bus_for_each_drv+0x78/0xd0
>>    __device_attach+0xd8/0x180
>>    device_initial_probe+0x14/0x20
>>    bus_probe_device+0x9c/0xa4
>>    deferred_probe_work_func+0x88/0xc4
>>    process_one_work+0x288/0x6bc
>>    worker_thread+0x248/0x450
>>    kthread+0x118/0x11c
>>    ret_from_fork+0x10/0x20
>> irq event stamp: 3599
>> hardirqs last  enabled at (3599): [<ffff80000919053c>]
>> _raw_spin_unlock_irqrestore+0x98/0x9c
>> hardirqs last disabled at (3598): [<ffff800009190ba4>]
>> _raw_spin_lock_irqsave+0xc0/0xcc
>> softirqs last  enabled at (3554): [<ffff800008010470>] _stext+0x470/0x5e8
>> softirqs last disabled at (3549): [<ffff8000080a4514>]
>> __irq_exit_rcu+0x180/0x1ac
>> ---[ end trace 0000000000000000 ]---
>>
>> I don't see any direct relation between the $subject and the above log,
>> but reverting the $subject on top of linux next-20220429 hides/fixes it.
>> Maybe there is a kind of memory trashing somewhere there and your change
>> only revealed it?
> Thanks for the report. I think the error path of my patch is wrong - I
> should not kfree(rpdev->driver_override) from the rpmsg code. That's the
> only thing I see now...
>
> Could you test following patch and tell if it helps?
> https://pastebin.ubuntu.com/p/rp3q9Z5fXj/

This doesn't help, the issue is still reported.

Best regards
Krzysztof Kozlowski April 29, 2022, 6:29 p.m. UTC | #4
On 29/04/2022 16:51, Marek Szyprowski wrote:
> On 29.04.2022 16:16, Krzysztof Kozlowski wrote:
>> On 29/04/2022 14:29, Marek Szyprowski wrote:
>>> On 19.04.2022 13:34, Krzysztof Kozlowski wrote:
>>>> The driver_override field from platform driver should not be initialized
>>>> from static memory (string literal) because the core later kfree() it,
>>>> for example when driver_override is set via sysfs.
>>>>
>>>> Use dedicated helper to set driver_override properly.
>>>>
>>>> Fixes: 950a7388f02b ("rpmsg: Turn name service into a stand alone driver")
>>>> Fixes: c0cdc19f84a4 ("rpmsg: Driver for user space endpoint interface")
>>>> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
>>>> Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
>>> This patch landed recently in linux-next as commit 42cd402b8fd4 ("rpmsg:
>>> Fix kfree() of static memory on setting driver_override"). In my tests I
>>> found that it triggers the following issue during boot of the
>>> DragonBoard410c SBC (arch/arm64/boot/dts/qcom/apq8016-sbc.dtb):
>>>
>>> ------------[ cut here ]------------
>>> DEBUG_LOCKS_WARN_ON(lock->magic != lock)
>>> WARNING: CPU: 1 PID: 8 at kernel/locking/mutex.c:582
>>> __mutex_lock+0x1ec/0x430
>>> Modules linked in:
>>> CPU: 1 PID: 8 Comm: kworker/u8:0 Not tainted 5.18.0-rc4-next-20220429 #11815
>>> Hardware name: Qualcomm Technologies, Inc. APQ 8016 SBC (DT)
>>> Workqueue: events_unbound deferred_probe_work_func
>>> pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>>> pc : __mutex_lock+0x1ec/0x430
>>> lr : __mutex_lock+0x1ec/0x430
>>> ..
>>> Call trace:
>>>    __mutex_lock+0x1ec/0x430
>>>    mutex_lock_nested+0x38/0x64
>>>    driver_set_override+0x124/0x150
>>>    qcom_smd_register_edge+0x2a8/0x4ec
>>>    qcom_smd_probe+0x54/0x80
>>>    platform_probe+0x68/0xe0
>>>    really_probe.part.0+0x9c/0x29c
>>>    __driver_probe_device+0x98/0x144
>>>    driver_probe_device+0xac/0x14c
>>>    __device_attach_driver+0xb8/0x120
>>>    bus_for_each_drv+0x78/0xd0
>>>    __device_attach+0xd8/0x180
>>>    device_initial_probe+0x14/0x20
>>>    bus_probe_device+0x9c/0xa4
>>>    deferred_probe_work_func+0x88/0xc4
>>>    process_one_work+0x288/0x6bc
>>>    worker_thread+0x248/0x450
>>>    kthread+0x118/0x11c
>>>    ret_from_fork+0x10/0x20
>>> irq event stamp: 3599
>>> hardirqs last  enabled at (3599): [<ffff80000919053c>]
>>> _raw_spin_unlock_irqrestore+0x98/0x9c
>>> hardirqs last disabled at (3598): [<ffff800009190ba4>]
>>> _raw_spin_lock_irqsave+0xc0/0xcc
>>> softirqs last  enabled at (3554): [<ffff800008010470>] _stext+0x470/0x5e8
>>> softirqs last disabled at (3549): [<ffff8000080a4514>]
>>> __irq_exit_rcu+0x180/0x1ac
>>> ---[ end trace 0000000000000000 ]---
>>>
>>> I don't see any direct relation between the $subject and the above log,
>>> but reverting the $subject on top of linux next-20220429 hides/fixes it.
>>> Maybe there is a kind of memory trashing somewhere there and your change
>>> only revealed it?
>> Thanks for the report. I think the error path of my patch is wrong - I
>> should not kfree(rpdev->driver_override) from the rpmsg code. That's the
>> only thing I see now...
>>
>> Could you test following patch and tell if it helps?
>> https://pastebin.ubuntu.com/p/rp3q9Z5fXj/
> 
> This doesn't help, the issue is still reported.

I think I screwed this part of code. The new helper uses device_lock()
(the mutexes you see in backtrace) but in rpmsg it is called before
device_register() which initializes the device.

I don't have a device using qcom-smd rpmsg, so it's a bit tricky to
reproduce.

Best regards,
Krzysztof
diff mbox series

Patch

diff --git a/drivers/rpmsg/rpmsg_internal.h b/drivers/rpmsg/rpmsg_internal.h
index d4b23fd019a8..3e81642238d2 100644
--- a/drivers/rpmsg/rpmsg_internal.h
+++ b/drivers/rpmsg/rpmsg_internal.h
@@ -94,10 +94,19 @@  int rpmsg_release_channel(struct rpmsg_device *rpdev,
  */
 static inline int rpmsg_ctrldev_register_device(struct rpmsg_device *rpdev)
 {
+	int ret;
+
 	strcpy(rpdev->id.name, "rpmsg_ctrl");
-	rpdev->driver_override = "rpmsg_ctrl";
+	ret = driver_set_override(&rpdev->dev, &rpdev->driver_override,
+				  rpdev->id.name, strlen(rpdev->id.name));
+	if (ret)
+		return ret;
+
+	ret = rpmsg_register_device(rpdev);
+	if (ret)
+		kfree(rpdev->driver_override);
 
-	return rpmsg_register_device(rpdev);
+	return ret;
 }
 
 #endif
diff --git a/drivers/rpmsg/rpmsg_ns.c b/drivers/rpmsg/rpmsg_ns.c
index 762ff1ae279f..8eb8f328237e 100644
--- a/drivers/rpmsg/rpmsg_ns.c
+++ b/drivers/rpmsg/rpmsg_ns.c
@@ -20,12 +20,22 @@ 
  */
 int rpmsg_ns_register_device(struct rpmsg_device *rpdev)
 {
+	int ret;
+
 	strcpy(rpdev->id.name, "rpmsg_ns");
-	rpdev->driver_override = "rpmsg_ns";
+	ret = driver_set_override(&rpdev->dev, &rpdev->driver_override,
+				  rpdev->id.name, strlen(rpdev->id.name));
+	if (ret)
+		return ret;
+
 	rpdev->src = RPMSG_NS_ADDR;
 	rpdev->dst = RPMSG_NS_ADDR;
 
-	return rpmsg_register_device(rpdev);
+	ret = rpmsg_register_device(rpdev);
+	if (ret)
+		kfree(rpdev->driver_override);
+
+	return ret;
 }
 EXPORT_SYMBOL(rpmsg_ns_register_device);
 
diff --git a/include/linux/rpmsg.h b/include/linux/rpmsg.h
index 02fa9116cd60..20c8cd1cde21 100644
--- a/include/linux/rpmsg.h
+++ b/include/linux/rpmsg.h
@@ -41,7 +41,9 @@  struct rpmsg_channel_info {
  * rpmsg_device - device that belong to the rpmsg bus
  * @dev: the device struct
  * @id: device id (used to match between rpmsg drivers and devices)
- * @driver_override: driver name to force a match
+ * @driver_override: driver name to force a match; do not set directly,
+ *                   because core frees it; use driver_set_override() to
+ *                   set or clear it.
  * @src: local address
  * @dst: destination address
  * @ept: the rpmsg endpoint of this channel
@@ -51,7 +53,7 @@  struct rpmsg_channel_info {
 struct rpmsg_device {
 	struct device dev;
 	struct rpmsg_device_id id;
-	char *driver_override;
+	const char *driver_override;
 	u32 src;
 	u32 dst;
 	struct rpmsg_endpoint *ept;