[PATCH v2] media: v4l2-ioctl: zero the ext control built for VIDIOC_{G,S}_CTRL

[Nick Rogers <nick@getfieldwork.ai>]

When a driver implements the extended control ioctls but has no control
handler, v4l_g_ctrl() and v4l_s_ctrl() pass VIDIOC_G_CTRL and
VIDIOC_S_CTRL on as a single struct v4l2_ext_control built on the stack.
Only its id and value are set, and check_ext_ctrls() clears reserved[0]
and reserved2[0]; the control's size and the rest of both structures are
left uninitialized.

A driver that forwards the controls rather than handling them through
the control framework sees that stack garbage. The virtio-media driver
under review takes a nonzero size as a payload to copy from userspace,
so VIDIOC_G_CTRL and VIDIOC_S_CTRL fail with -EINVAL through it whenever
the stack is dirty. GStreamer's V4L2 encoders set their profile with
VIDIOC_S_CTRL, and cannot negotiate against such a device.

Zero-initialize both structures.

Assisted-by: LLM
Signed-off-by: Nick Rogers <nick@getfieldwork.ai>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
---
Changes in v2:
- Assisted-by: LLM, per Documentation/process/coding-assistants.rst
  (Alexandre)
- Collected Nicolas's Reviewed-by

v1: https://lore.kernel.org/all/20260923160936.33445-1-nick@getfieldwork.ai/

Alexandre asked whether drivers should fill the structure themselves.
The core builds it and passes it down, and a driver can't tell a
translated G_CTRL/S_CTRL from a real extended control call, so I think
it's the core's to zero. He's right that virtio-media will meet kernels
without this, though, so the driver now guards against it too:
https://lore.kernel.org/all/20260925080140.44696-1-nick@getfieldwork.ai/

Found running the virtio-media v9 series [1] in a VMM with a host-side
stateful encoder: GStreamer's v4l2h264enc fails to negotiate because
VIDIOC_S_CTRL returns -EINVAL. Tested on 6.18 with that series applied:
VIDIOC_G_CTRL and VIDIOC_S_CTRL now reach the device intact, and
v4l2-compliance 1.30.1 reports the same results with and without this
patch. Build-tested on media.git next (arm64, W=1, no new warnings).

[1] https://lore.kernel.org/all/20260917171921.2810550-1-briandaniels@google.com/

 drivers/media/v4l2-core/v4l2-ioctl.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/media/v4l2-core/v4l2-ioctl.c b/drivers/media/v4l2-core/v4l2-ioctl.c
index 17ba1ae70..b7d248ab7 100644
--- a/drivers/media/v4l2-core/v4l2-ioctl.c
+++ b/drivers/media/v4l2-core/v4l2-ioctl.c
@@ -2357,8 +2357,8 @@ static int v4l_g_ctrl(const struct v4l2_ioctl_ops *ops, struct file *file,
 	struct video_device *vfd = video_devdata(file);
 	struct v4l2_control *p = arg;
 	struct v4l2_fh *vfh = file_to_v4l2_fh(file);
-	struct v4l2_ext_controls ctrls;
-	struct v4l2_ext_control ctrl;
+	struct v4l2_ext_controls ctrls = {};
+	struct v4l2_ext_control ctrl = {};
 
 	if (vfh && vfh->ctrl_handler)
 		return v4l2_g_ctrl(vfh->ctrl_handler, p);
@@ -2388,8 +2388,8 @@ static int v4l_s_ctrl(const struct v4l2_ioctl_ops *ops, struct file *file,
 	struct video_device *vfd = video_devdata(file);
 	struct v4l2_control *p = arg;
 	struct v4l2_fh *vfh = file_to_v4l2_fh(file);
-	struct v4l2_ext_controls ctrls;
-	struct v4l2_ext_control ctrl;
+	struct v4l2_ext_controls ctrls = {};
+	struct v4l2_ext_control ctrl = {};
 	int ret;
 
 	if (vfh && vfh->ctrl_handler)
-- 
2.54.0 (Apple Git-157)