Subject: [PATCH] Fix for the arm64 kern_addr_valid() function

From: [email-masked] (Catalin Marinas)
Date: Tue, 29 Apr 2014 15:25:42 +0100

On Wed, Apr 16, 2014 at 08:51:44AM +0100, Will Deacon wrote:
> On Tue, Apr 15, 2014 at 06:53:24PM +0100, Dave Anderson wrote:
> >  Fix for the arm64 kern_addr_valid() function to recognize
> >  virtual addresses in the kernel logical memory map.  The
> >  function fails as written because it does not check whether
> >  the addresses in that region are mapped at the pmd level to
> >  2MB or 512MB pages, continues the page table walk to the
> >  pte level, and issues a garbage value to pfn_valid().
> > 
> >  Tested on 4K-page and 64K-page kernels.
> > 
> > Signed-off-by: Dave Anderson <[email-masked]>
> > ---
> >  arch/arm64/mm/mmu.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > index 6b7e895..0a472c4 100644
> > --- a/arch/arm64/mm/mmu.c
> > +++ b/arch/arm64/mm/mmu.c
> > @@ -374,6 +374,9 @@ int kern_addr_valid(unsigned long addr)
> >  	if (pmd_none(*pmd))
> >  		return 0;
> >  
> > +	if (pmd_sect(*pmd))
> > +		return pfn_valid(pmd_pfn(*pmd));
> > +
> >  	pte = pte_offset_kernel(pmd, addr);
> >  	if (pte_none(*pte))
> >  		return 0;
> 
> Whilst this patch looks fine to me, I wonder whether walking the page tables
> is really necessary for this function? The only user is fs/proc/kcore.c,
> which basically wants to know if a lowmem address is actually backed by
> physical memory. Our current implementation of kern_addr_valid will return
> true even for MMIO mappings,

There is still a pfn_valid() check, so MMIO mappings wouldn't return
true.

> whilst I think we could actually just do
> something like:
> 
> 
> 	if ((((long)addr) >> VA_BITS) != -1UL)
> 		return 0;
> 
> 	return pfn_valid(__pa(addr) >> PAGE_SHIFT);
> 
> Am I missing something here?

__pa(addr) isn't valid for vmalloc/ioremap addresses (which would pass
the VA_BITS test above).

I would go with Dave's original patch for now. We've discussing change
the memory map a bit for the kernel at some point in the future with
PHYS_OFFSET always 0 and the kernel text/data mapped at a different
address from PAGE_OFFSET (similar to x86_64). If we get there, this
function would work unmodified.

-- 
Catalin