VFIO_IOMMU_GET_INFO capability struct alignment

[Stefan Hajnoczi <stefanha@redhat.com>]

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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.

Stefan

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