Re: [PATCH] nommu: remap_pfn_range: fix addr parameter check

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 13 Sep 2012 10:40:57 +0800
Bob Liu <lliubbo@gmail.com> wrote:

> The addr parameter may not page aligned eg. when it's come from
> vfb_mmap():vma->vm_start in video driver.
> 
> This patch fix the check in remap_pfn_range() else some driver like v4l2 will
> fail in this function while calling mmap() on nommu arch like blackfin and st.
> 
> Reported-by: Bhupesh SHARMA <bhupesh.sharma@st.com>
> Reported-by: Scott Jiang <scott.jiang.linux@gmail.com>
> Signed-off-by: Bob Liu <lliubbo@gmail.com>
> ---
>  mm/nommu.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/nommu.c b/mm/nommu.c
> index d4b0c10..5d6068b 100644
> --- a/mm/nommu.c
> +++ b/mm/nommu.c
> @@ -1819,7 +1819,7 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
>  int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
>  		unsigned long pfn, unsigned long size, pgprot_t prot)
>  {
> -	if (addr != (pfn << PAGE_SHIFT))
> +	if ((addr & PAGE_MASK) != (pfn << PAGE_SHIFT))
>  		return -EINVAL;
>  
>  	vma->vm_flags |= VM_IO | VM_RESERVED | VM_PFNMAP;

hm, what is the right thing to do here?

Yes, the MMU version of remap_pfn_range() does permit non-page-aligned
`addr' (at least, if the userspace maaping is a non-COW one).  But I
suspect that was an implementation accident - it is a nonsensical thing
to do, isn't it?  The MMU cannot map a bunch of kernel pages onto a
non-page-aligned userspace address.

So I'm thinking that we should declare ((addr & ~PAGE_MASK) != 0) to be
a caller bug, and fix up this regrettably unidentified v4l driver?

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