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Date:   Tue, 8 Aug 2023 20:29:25 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Alex Williamson <alex.williamson@redhat.com>
Cc:     Stefan Hajnoczi <stefanha@redhat.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] vfio: align capability structures
Message-ID: <ZNLP1dU1Ijzm/NPE@ziepe.ca>
References: <20230803144109.2331944-1-stefanha@redhat.com>
 <20230803151823.4e5943e6.alex.williamson@redhat.com>
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On Thu, Aug 03, 2023 at 03:18:23PM -0600, Alex Williamson wrote:

> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> index 902f06e52c48..2d074cbd371d 100644
> --- a/drivers/vfio/vfio_main.c
> +++ b/drivers/vfio/vfio_main.c
> @@ -1362,6 +1362,8 @@ struct vfio_info_cap_header *vfio_info_cap_add(struct vfio_info_cap *caps,
>  	void *buf;
>  	struct vfio_info_cap_header *header, *tmp;
>  
> +	size = ALIGN(size, sizeof(u64));
> +
>  	buf = krealloc(caps->buf, caps->size + size, GFP_KERNEL);
>  	if (!buf) {
>  		kfree(caps->buf);
> @@ -1395,6 +1397,8 @@ void vfio_info_cap_shift(struct vfio_info_cap *caps, size_t offset)
>  	struct vfio_info_cap_header *tmp;
>  	void *buf = (void *)caps->buf;
>  
> +	WARN_ON(!IS_ALIGNED(offset, sizeof(u64)));
> +
>  	for (tmp = buf; tmp->next; tmp = buf + tmp->next - offset)
>  		tmp->next += offset;
>  }
> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> index fa06e3eb4955..fd2761841ffe 100644
> --- a/include/uapi/linux/vfio.h
> +++ b/include/uapi/linux/vfio.h
> @@ -217,6 +217,7 @@ struct vfio_device_info {
>  	__u32	num_regions;	/* Max region index + 1 */
>  	__u32	num_irqs;	/* Max IRQ index + 1 */
>  	__u32   cap_offset;	/* Offset within info struct of first cap */
> +	__u32	pad;		/* Size must be aligned for caps */
>  };
>  #define VFIO_DEVICE_GET_INFO		_IO(VFIO_TYPE, VFIO_BASE + 7)
>  
> @@ -1444,6 +1445,7 @@ struct vfio_iommu_type1_info {
>  #define VFIO_IOMMU_INFO_CAPS	(1 << 1)	/* Info supports caps */
>  	__u64	iova_pgsizes;	/* Bitmap of supported page sizes */
>  	__u32   cap_offset;	/* Offset within info struct of first cap */
> +	__u32	pad;		/* Size must be aligned for caps */
>  };

IMHO this is partially being caused by not using __aligned_u64 for the
other __u64's in the same struct..

Both of these structs have u64s in them and many arches will
automatically add the above padding. __aligned_u64 will force the
reset to do it, and then making padding explicit as you have done will
make it really true.

This is a subtle x64/x32 compatability issue also. It is probably best
just to do the change across the whole header file.

Please also include the matching hunk for iommufd:

--- a/drivers/iommu/iommufd/vfio_compat.c
+++ b/drivers/iommu/iommufd/vfio_compat.c
@@ -483,6 +483,8 @@ static int iommufd_vfio_iommu_get_info(struct iommufd_ctx *ictx,
                        rc = cap_size;
                        goto out_put;
                }
+               cap_size = ALIGN(cap_size, sizeof(u64));
+
                if (last_cap && info.argsz >= total_cap_size &&
                    put_user(total_cap_size, &last_cap->next)) {
                        rc = -EFAULT;

Thanks,
Jason