On Tue, 1 Aug 2023 11:38:46 -0400
Stefan Hajnoczi <stefanha@xxxxxxxxxx> wrote:
> Hi,
> It appears that ioctl(VFIO_IOMMU_GET_INFO) can produce misaligned
> capability structures. Userspace workarounds exist but I wanted to ask
> whether the kernel can align capability structures to save all userspace
> programs the trouble?
>
> The issue is:
>
> struct vfio_iommu_type1_info_dma_avail {
> struct vfio_info_cap_header header; /* 0 8 */
> __u32 avail; /* 8 4 */
>
> /* size: 12, cachelines: 1, members: 2 */
> /* last cacheline: 12 bytes */
> };
>
> Once this capability is added, the next capability will be 4-byte
> aligned but not 8-byte aligned. If there are __u64 fields in the next
> capability, then they will be misaligned.
>
> This was noticed when investigating a bug in userspace code that uses
> ioctl(VFIO_IOMMU_GET_INFO):
> https://gitlab.com/pci-driver/pci-driver/-/merge_requests/2#note_1495734084
>
> One possible solution is to modify vfio_info_cap_add() so that
> capability structures are always rounded up to 8 bytes. This does not
> break the uapi because capability structure offsets are described at
> runtime via the cap_offset and header->next fields. Existing userspace
> programs would continue to work and all programs would find that
> capability structures are now aligned.
Yes, I think the helpers should automatically align each added
capability. Thanks,
Alex
