linux-kernel - Re: [PATCH] vfio: align capability structures

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Date:   Wed, 9 Aug 2023 16:24:58 -0400
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Alex Williamson <alex.williamson@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vfio: align capability structures

On Tue, Aug 08, 2023 at 08:29:25PM -0300, Jason Gunthorpe wrote:
> 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.

I will send a separate series that switches the struct definitions to
__aligned_u64.

> 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;

Okay, will fix.

Stefan

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