diff mbox series

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

Message ID 20230829182720.331083-4-stefanha@redhat.com (mailing list archive)
State New, archived
Headers show
Series vfio: use __aligned_u64 for ioctl structs | expand

Commit Message

Stefan Hajnoczi Aug. 29, 2023, 6:27 p.m. UTC 
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(-)

Comments

Tian, Kevin Sept. 11, 2023, 6:08 a.m. UTC | #1 
> From: Stefan Hajnoczi <stefanha@redhat.com>
> Sent: Wednesday, August 30, 2023 2:27 AM
> 
> 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>

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
diff mbox series

Patch

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)