RE: [PATCH v3] vfio: align capability structures

[Tian, Kevin]

> From: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
> Sent: Thursday, August 10, 2023 4:32 AM
> 
> The VFIO_DEVICE_GET_INFO, VFIO_DEVICE_GET_REGION_INFO, and
> VFIO_IOMMU_GET_INFO ioctls fill in an info struct followed by capability
> structs:
> 
>   +------+---------+---------+-----+
>   | info | caps[0] | caps[1] | ... |
>   +------+---------+---------+-----+
> 
> Both the info and capability struct sizes are not always multiples of
> sizeof(u64), leaving u64 fields in later capability structs misaligned.
> 
> Userspace applications currently need to handle misalignment manually in
> order to support CPU architectures and programming languages with strict
> alignment requirements.
> 
> Make life easier for userspace by ensuring alignment in the kernel. This
> is done by padding info struct definitions and by copying out zeroes
> after capability structs that are not aligned.
> 
> The new layout is as follows:
> 
>   +------+---------+---+---------+-----+
>   | info | caps[0] | 0 | caps[1] | ... |
>   +------+---------+---+---------+-----+
> 
> In this example caps[0] has a size that is not multiples of sizeof(u64),
> so zero padding is added to align the subsequent structure.
> 
> Adding zero padding between structs does not break the uapi. The memory
> layout is specified by the info.cap_offset and caps[i].next fields
> filled in by the kernel. Applications use these field values to locate
> structs and are therefore unaffected by the addition of zero padding.
> 
> Note that code that copies out info structs with padding is updated to
> always zero the struct and copy out as many bytes as userspace
> requested. This makes the code shorter and avoids potential information
> leaks by ensuring padding is initialized.
> 
> Originally-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Signed-off-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx>

Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>