linux-kernel - Re: [PATCH v2 2/3] vfio: use __aligned_u64 in struct vfio_device_gfx_plane

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Message-ID: <ZPn5d8VFAo5PQmpj@ziepe.ca>
Date:   Thu, 7 Sep 2023 13:25:27 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Stefan Hajnoczi <stefanha@...hat.com>
Cc:     kvm@...r.kernel.org, David Laight <David.Laight@...lab.com>,
        linux-kernel@...r.kernel.org, "Tian, Kevin" <kevin.tian@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [PATCH v2 2/3] vfio: use __aligned_u64 in struct
 vfio_device_gfx_plane_info

On Tue, Aug 29, 2023 at 02:27:19PM -0400, Stefan Hajnoczi wrote:
> The memory layout of struct vfio_device_gfx_plane_info is
> architecture-dependent due to a u64 field and a struct size that is not
> a multiple of 8 bytes:
> - On x86_64 the struct size is padded to a multiple of 8 bytes.
> - On x32 the struct size is only a multiple of 4 bytes, not 8.
> - Other architectures may vary.
> 
> Use __aligned_u64 to make memory layout consistent. This reduces the
> chance of 32-bit userspace on a 64-bit kernel breakage.
> 
> This patch increases the struct size on x32 but this is safe because of
> the struct's argsz field. The kernel may grow the struct as long as it
> still supports smaller argsz values from userspace (e.g. applications
> compiled against older kernel headers).
> 
> Suggested-by: Jason Gunthorpe <jgg@...pe.ca>
> Signed-off-by: Stefan Hajnoczi <stefanha@...hat.com>
> ---
>  include/uapi/linux/vfio.h        | 3 ++-
>  drivers/gpu/drm/i915/gvt/kvmgt.c | 4 +++-
>  samples/vfio-mdev/mbochs.c       | 6 ++++--
>  samples/vfio-mdev/mdpy.c         | 4 +++-
>  4 files changed, 12 insertions(+), 5 deletions(-)

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason