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

José Expósito
2025-12-29T16:01:23Z

On Mon, Dec 29, 2025 at 03:40:23PM +0100, Louis Chauvet wrote:
>
>
> 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@bootlin.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@lists.freedesktop.org
> > > +Description:
> > > + Name of the plane.
> > > +
> > > What: /sys/kernel/config/vkms/<device>/planes/<plane>/possible_crtcs
> > > Date: Nov 2025
> > > Contact: dri-devel@lists.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.

From the top of my head, I think that at the moment the device name is the only
user facing string that is not limted in code? It is limited by the allowed
characters in filenames. But I might be forgetting other custom strings.

Technically, it'd be an ABI break... So, while I think it shouldn't be an issue,
I'd prefer to avoid breaking the ABI, but I leave the decision to you.

Jose

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