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

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Message-ID: <BN9PR11MB5276D9562D1ADE41AF4324268CF2A@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Mon, 11 Sep 2023 06:07:16 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Stefan Hajnoczi <stefanha@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:     David Laight <David.Laight@...LAB.COM>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>
Subject: RE: [PATCH v2 2/3] vfio: use __aligned_u64 in struct
 vfio_device_gfx_plane_info

> From: Stefan Hajnoczi <stefanha@...hat.com>
> Sent: Wednesday, August 30, 2023 2:27 AM
> 
> 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>

Reviewed-by: Kevin Tian <kevin.tian@...el.com>