linux-kernel - RE: [PATCH v2 0/3] vfio: use __aligned

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Message-ID: <3e8b6e0503a84c93b6dd44c0d311abfe@AcuMS.aculab.com>
Date:   Tue, 29 Aug 2023 21:10:06 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Stefan Hajnoczi' <stefanha@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>
Subject: RE: [PATCH v2 0/3] vfio: use __aligned_u64 for ioctl structs

From: Stefan Hajnoczi
> Sent: 29 August 2023 19:27
> 
> v2:
> - Rebased onto https://github.com/awilliam/linux-vfio.git next to get the
>   vfio_iommu_type1_info pad field [Kevin]
> - Fixed min(minsz, sizeof(dmabuf)) -> min(dmabuf.argsz, sizeof(dmabuf)) [Jason, Kevin]

You managed to use min_t() instead of fixing the types to match.

> - Squashed Patch 3 (vfio_iommu_type1_info) into Patch 1 since it is trivial now
>   that the padding field is already there.
> 
> Jason Gunthorpe <jgg@...dia.com> pointed out that u64 VFIO ioctl struct fields
> have architecture-dependent alignment. iommufd already uses __aligned_u64 to
> avoid this problem.
> 
> See the __aligned_u64 typedef in <uapi/linux/types.h> for details on why it is
> a good idea for kernel<->user interfaces.
> 
> This series modifies the VFIO ioctl structs to use __aligned_u64. Some of the
> changes preserve the existing memory layout on all architectures, so I put them
> together into the first patch. The remaining patches are for structs where
> explanation is necessary about why changing the memory layout does not break
> the uapi.

But you are extending a field in the middle of the uapi structure.
This completely breaks any applications.

You could add code to detect the length of the user-provided
structure and use the correct kernel structure that matches
the length of the user-provided one.
That needs the opposite of __aligned_u64 - a 64bit integer with
32bit alignment on x64-64.

	David

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