linux-kernel - Re: [PATCH v3 07/33] drm/vkms: Introduce configfs for plane name

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Message-ID: <da5db513-1b0c-4ba9-8513-a616895405de@bootlin.com>
Date: Mon, 29 Dec 2025 15:40:23 +0100
From: Louis Chauvet <louis.chauvet@...tlin.com>
To: Luca Ceresoli <luca.ceresoli@...tlin.com>,
 Haneen Mohammed <hamohammed.sa@...il.com>, Simona Vetter <simona@...ll.ch>,
 Melissa Wen <melissa.srw@...il.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, jose.exposito89@...il.com,
 Jonathan Corbet <corbet@....net>
Cc: victoria@...tem76.com, sebastian.wick@...hat.com,
 thomas.petazzoni@...tlin.com, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v3 07/33] drm/vkms: Introduce configfs for plane name



On 12/23/25 12:14, Luca Ceresoli wrote:
> On Mon Dec 22, 2025 at 11:11 AM CET, Louis Chauvet wrote:
>> Planes can have name, create a plane attribute to configure it. Currently
>> plane name is mainly used in logs.
>>
>> Signed-off-by: Louis Chauvet <louis.chauvet@...tlin.com>
>> ---
>>   Documentation/ABI/testing/configfs-vkms |  6 +++++
>>   Documentation/gpu/vkms.rst              |  3 ++-
>>   drivers/gpu/drm/vkms/vkms_configfs.c    | 43 +++++++++++++++++++++++++++++++++
>>   3 files changed, 51 insertions(+), 1 deletion(-)
>>
>> diff --git a/Documentation/ABI/testing/configfs-vkms b/Documentation/ABI/testing/configfs-vkms
>> index 0beaa25f30ba..6fe375d1636f 100644
>> --- a/Documentation/ABI/testing/configfs-vkms
>> +++ b/Documentation/ABI/testing/configfs-vkms
>> @@ -103,6 +103,12 @@ Description:
>>           Plane type. Possible values: 0 - overlay, 1 - primary,
>>           2 - cursor.
>>
>> +What:		/sys/kernel/config/vkms/<device>/planes/<plane>/name
>> +Date:		Nov 2025
>> +Contact:	dri-devel@...ts.freedesktop.org
>> +Description:
>> +        Name of the plane.
>> +
>>   What:		/sys/kernel/config/vkms/<device>/planes/<plane>/possible_crtcs
>>   Date:		Nov 2025
>>   Contact:	dri-devel@...ts.freedesktop.org
>> diff --git a/Documentation/gpu/vkms.rst b/Documentation/gpu/vkms.rst
>> index 1e79e62a6bc4..79f1185d8645 100644
>> --- a/Documentation/gpu/vkms.rst
>> +++ b/Documentation/gpu/vkms.rst
>> @@ -87,10 +87,11 @@ Start by creating one or more planes::
>>
>>     sudo mkdir /config/vkms/my-vkms/planes/plane0
>>
>> -Planes have 1 configurable attribute:
>> +Planes have 2 configurable attributes:
>>
>>   - type: Plane type: 0 overlay, 1 primary, 2 cursor (same values as those
>>     exposed by the "type" property of a plane)
>> +- name: Name of the plane. Allowed characters are [A-Za-z1-9_-]
>>
>>   Continue by creating one or more CRTCs::
>>
>> diff --git a/drivers/gpu/drm/vkms/vkms_configfs.c b/drivers/gpu/drm/vkms/vkms_configfs.c
>> index 506666e21c91..989788042191 100644
>> --- a/drivers/gpu/drm/vkms/vkms_configfs.c
>> +++ b/drivers/gpu/drm/vkms/vkms_configfs.c
>> @@ -324,10 +324,53 @@ static ssize_t plane_type_store(struct config_item *item, const char *page,
>>   	return (ssize_t)count;
>>   }
>>
>> +static ssize_t plane_name_show(struct config_item *item, char *page)
>> +{
>> +	struct vkms_configfs_plane *plane;
>> +	const char *name;
>> +
>> +	plane = plane_item_to_vkms_configfs_plane(item);
>> +
>> +	scoped_guard(mutex, &plane->dev->lock)
>> +		name = vkms_config_plane_get_name(plane->config);
> 
> vkms_config_plane_get_name() returns a pointer to the name string, not a
> copy. Unless I'm missing something, that string might be freed before the
> next lines, where it is used:
> 
>> +
>> +	if (name)
>> +		return sprintf(page, "%s\n", name);
>> +	return sprintf(page, "\n");
> 
> So for safety the above 3 lines whould go inside the scoped_guard().

Good catch!

This also raised some questions on the whole locking synchronization 
between configfs / config / DRM core. I will work on this topic and 
maybe move the mutex / add a refcount to vkms_config.

>> +}
>> +
>> +static ssize_t plane_name_store(struct config_item *item, const char *page,
>> +				size_t count)
>> +{
>> +	struct vkms_configfs_plane *plane;
>> +	size_t str_len;
>> +
>> +	plane = plane_item_to_vkms_configfs_plane(item);
>> +
>> +	// strspn is not lenght-protected, ensure that page is a null-terminated string.
>> +	str_len = strnlen(page, count);
>> +	if (str_len >= count)
>> +		return -EINVAL;
>> +
>> +	if (strspn(page, "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_-") != count - 1)
>> +		return -EINVAL;
> 
> I see you effor to make this as clean as possible, thanks. Still this is a
> tad ugly, and should be moved to some common place at some point IMO. For
> now it's fine, but if you need to add more user-passed strings, that could
> be the moment to move this code.

There are multiple "user strings" in this file (notably group names), 
but currently without limitation.

I can create a tiny helper and limit all user strings to a-zA-Z0-9_-
It will technically break the ABI, but I don't think this is a big issue.

Do you or José think this is a good idea? If so I can extract the helper 
for v4 and send a separate series to do the limitation on other strings.

> Luca
> 
> --
> Luca Ceresoli, Bootlin
> Embedded Linux and Kernel engineering
> https://bootlin.com