[PATCH v2] drm/qxl: use correct offset when stamping drawable mm_time

[Aldo Ariel Panzardo <qwe.aldo@gmail.com>]

qxl_process_single_command() maps the page containing the current
release slot and copies the userspace command into it at the
release_offset within the page:

    fb_cmd = qxl_bo_kmap_atomic_page(qdev, cmd_bo,
                (release->release_offset & PAGE_MASK));
    copy_from_user(..., fb_cmd + sizeof(union qxl_release_info) +
                   (release->release_offset & ~PAGE_MASK), ...);

However, the subsequent mm_time stamp casts the page-start pointer
directly:

    struct qxl_drawable *draw = fb_cmd;

This writes mm_time at the page start rather than into the drawable
that was just copied. The drawable begins at the page-relative
release_offset (struct qxl_drawable embeds the release_info header
as its first member, so no additional sizeof skip is needed for the
struct pointer).

Apply the page-relative offset so mm_time lands in the right place.

Fixes: f64122c1f6ad ("drm: add new QXL driver. (v1.4)")
Cc: stable@vger.kernel.org
Reported-by: Sashiko <sashiko-bot@kernel.org>
Signed-off-by: Aldo Ariel Panzardo <qwe.aldo@gmail.com>
---
v2: drop the sizeof(union qxl_release_info) term — qxl_drawable
    embeds release_info as its first member, so the struct pointer
    only needs the page-relative offset (found by Sashiko AI review).

 drivers/gpu/drm/qxl/qxl_ioctl.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/qxl/qxl_ioctl.c b/drivers/gpu/drm/qxl/qxl_ioctl.c
index 591b026..65e9bfd 100644
--- a/drivers/gpu/drm/qxl/qxl_ioctl.c
+++ b/drivers/gpu/drm/qxl/qxl_ioctl.c
@@ -188,7 +188,8 @@ static int qxl_process_single_command(struct qxl_device *qdev,
 		 u64_to_user_ptr(cmd->command), cmd->command_size);
 
 	{
-		struct qxl_drawable *draw = fb_cmd;
+		struct qxl_drawable *draw = (void *)fb_cmd +
+			(release->release_offset & ~PAGE_MASK);
 
 		draw->mm_time = qdev->rom->mm_clock;
 	}
-- 
2.43.0