vfio: use __aligned_u64 in struct vfio_device_ioeventfd

The memory layout of struct vfio_device_ioeventfd 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 that 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).

The code that uses struct vfio_device_ioeventfd already works correctly
when the struct size grows, so only the struct definition needs to be
changed.

Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/20230918205617.1478722-4-stefanha@redhat.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
index 8a3f83e0bdc1500c6b89feb08dae65e4adff9973..7f5fb010226d8cb80a4e435209b3b69ba6e80e35 100644 (file)
--- a/include/uapi/linux/vfio.h
+++ b/include/uapi/linux/vfio.h
@@ -864,9 +864,10 @@ struct vfio_device_ioeventfd {
 #define VFIO_DEVICE_IOEVENTFD_32       (1 << 2) /* 4-byte write */
 #define VFIO_DEVICE_IOEVENTFD_64       (1 << 3) /* 8-byte write */
 #define VFIO_DEVICE_IOEVENTFD_SIZE_MASK        (0xf)
-       __u64   offset;                 /* device fd offset of write */
-       __u64   data;                   /* data to be written */
+       __aligned_u64   offset;         /* device fd offset of write */
+       __aligned_u64   data;           /* data to be written */
        __s32   fd;                     /* -1 for de-assignment */
+       __u32   reserved;
 };
 
 #define VFIO_DEVICE_IOEVENTFD          _IO(VFIO_TYPE, VFIO_BASE + 16)