linux-kernel - Re: [Xen-devel] [PATCH v4 8/9] xen/gntdev.c: Convert to use vm_map

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Date:   Sun, 28 Jul 2019 20:06:11 +0200
From:   Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>
To:     Souptick Joarder <jrdr.linux@...il.com>
Cc:     akpm@...ux-foundation.org, willy@...radead.org, mhocko@...e.com,
        boris.ostrovsky@...cle.com, jgross@...e.com, linux@...linux.org.uk,
        robin.murphy@....com, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [Xen-devel] [PATCH v4 8/9] xen/gntdev.c: Convert to use
 vm_map_pages()

On Fri, Feb 15, 2019 at 08:18:31AM +0530, Souptick Joarder wrote:
> Convert to use vm_map_pages() to map range of kernel
> memory to user vma.
> 
> map->count is passed to vm_map_pages() and internal API
> verify map->count against count ( count = vma_pages(vma))
> for page array boundary overrun condition.

This commit breaks gntdev driver. If vma->vm_pgoff > 0, vm_map_pages
will:
 - use map->pages starting at vma->vm_pgoff instead of 0
 - verify map->count against vma_pages()+vma->vm_pgoff instead of just
   vma_pages().

In practice, this breaks using a single gntdev FD for mapping multiple
grants.

It looks like vm_map_pages() is not a good fit for this code and IMO it
should be reverted.

> Signed-off-by: Souptick Joarder <jrdr.linux@...il.com>
> Reviewed-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> ---
>  drivers/xen/gntdev.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> index 5efc5ee..5d64262 100644
> --- a/drivers/xen/gntdev.c
> +++ b/drivers/xen/gntdev.c
> @@ -1084,7 +1084,7 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
>  	int index = vma->vm_pgoff;
>  	int count = vma_pages(vma);
>  	struct gntdev_grant_map *map;
> -	int i, err = -EINVAL;
> +	int err = -EINVAL;
>  
>  	if ((vma->vm_flags & VM_WRITE) && !(vma->vm_flags & VM_SHARED))
>  		return -EINVAL;
> @@ -1145,12 +1145,9 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
>  		goto out_put_map;
>  
>  	if (!use_ptemod) {
> -		for (i = 0; i < count; i++) {
> -			err = vm_insert_page(vma, vma->vm_start + i*PAGE_SIZE,
> -				map->pages[i]);
> -			if (err)
> -				goto out_put_map;
> -		}
> +		err = vm_map_pages(vma, map->pages, map->count);
> +		if (err)
> +			goto out_put_map;
>  	} else {
>  #ifdef CONFIG_X86
>  		/*

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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