[PATCH v2 3/3] vfio: use __aligned_u64 in struct vfio_device_ioeventfd

Stefan Hajnoczi
2023-08-29T18:29:18Z

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>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
include/uapi/linux/vfio.h | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
index 777374dd7725..032e41b56506 100644
--- 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)
--
2.41.0