Re: [PATCH] drm/syncobj: Fix handle <-> fd ioctls with dirty stack

[Christian König <christian.koenig@amd.com>]

On 3/1/26 13:34, Julian Orth wrote:
> Consider the following application:
> 
>     #include <fcntl.h>
>     #include <string.h>
>     #include <drm/drm.h>
>     #include <sys/ioctl.h>
> 
>     int main(void) {
>         int fd = open("/dev/dri/renderD128", O_RDWR);
>         struct drm_syncobj_create arg1;
>         ioctl(fd, DRM_IOCTL_SYNCOBJ_CREATE, &arg1);
>         struct drm_syncobj_handle arg2;
>         memset(&arg2, 1, sizeof(arg2)); // simulate dirty stack
>         arg2.handle = arg1.handle;
>         arg2.flags = 0;
>         arg2.fd = 0;
>         arg2.pad = 0;
>         // arg2.point = 0; // userspace is required to set point to 0
>         ioctl(fd, DRM_IOCTL_SYNCOBJ_HANDLE_TO_FD, &arg2);
>     }
> 
> The last ioctl returns EINVAL because args->point is not 0. However,
> userspace developed against older kernel versions is not aware of the
> new point field and might therefore not initialize it.
> 
> The correct check would be
> 
>     if (args->flags & DRM_SYNCOBJ_FD_TO_HANDLE_FLAGS_TIMELINE)
>         return -EINVAL;
> 
> However, there might already be userspace that relies on this not
> returning an error as long as point == 0. Therefore use the more lenient
> check.
> 
> Fixes: c2d3a7300695 ("drm/syncobj: Extend EXPORT_SYNC_FILE for timeline syncobjs")
> Signed-off-by: Julian Orth <ju.orth@gmail.com>

Good catch, Reviewed-by: Christian König <christian.koenig@amd.com>

As long as nobody objects I'm going to push this to drm-misc-fixes later today.

Thanks,
Christian.

> ---
> This patch fixes a regression that would cause conversions between
> syncobj handles and fds to fail if userspace did not initialize a
> recently-added field to 0.
> ---
>  drivers/gpu/drm/drm_syncobj.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
> index 250734dee928..49eccb43ce63 100644
> --- a/drivers/gpu/drm/drm_syncobj.c
> +++ b/drivers/gpu/drm/drm_syncobj.c
> @@ -875,7 +875,7 @@ drm_syncobj_handle_to_fd_ioctl(struct drm_device *dev, void *data,
>                 return drm_syncobj_export_sync_file(file_private, args->handle,
>                                                     point, &args->fd);
> 
> -       if (args->point)
> +       if (point)
>                 return -EINVAL;
> 
>         return drm_syncobj_handle_to_fd(file_private, args->handle,
> @@ -909,7 +909,7 @@ drm_syncobj_fd_to_handle_ioctl(struct drm_device *dev, void *data,
>                                                           args->handle,
>                                                           point);
> 
> -       if (args->point)
> +       if (point)
>                 return -EINVAL;
> 
>         return drm_syncobj_fd_to_handle(file_private, args->fd,
> 
> ---
> base-commit: eb71ab2bf72260054677e348498ba995a057c463
> change-id: 20260301-point-4305b6417f55
> 
> Best regards,
> --
> Julian Orth <ju.orth@gmail.com>
>