[PATCH 3/5] arm64: mm: Copy over arch_get_unmapped_area

[Steve.Capper@arm.com (Steve Capper)]

On Fri, Sep 07, 2018 at 03:04:39PM +0100, Steve Capper wrote:
> On Fri, Sep 07, 2018 at 02:15:32AM -0400, Jon Masters wrote:
> > On 08/29/2018 08:45 AM, Steve Capper wrote:
> > 
> > > In order to support 52-bit VAs for userspace we need to alter the mmap
> > > area choosing logic to give 52-bit VAs where "high" addresses are
> > > requested.
> > 
> > <snip>
> > 
> > > +unsigned long
> > > +arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
> > > +			  const unsigned long len, const unsigned long pgoff,
> > > +			  const unsigned long flags)
> > 
> > <snip>
> > 
> > > +	/* requested length too big for entire address space */
> > > +	if (len > TASK_SIZE - mmap_min_addr)
> > > +		return -ENOMEM;
> > > +
> > > +	if (flags & MAP_FIXED)
> > > +		return addr;
> > 
> > arch/x86/mm/mmap.c:
> > 
> >  *  With 5-level paging this request would be granted and result in a
> >  *  mapping which crosses the border of the 47-bit virtual address
> >  *  space. If the application cannot handle addresses above 47-bit this
> >  *  will lead to misbehaviour and hard to diagnose failures.
> >  *
> >  * Therefore ignore address hints which would result in a mapping
> >  * crossing the 47-bit virtual address boundary.
> > 
> > You'll probably want something similar above.
> >
> 
> Thanks Jon, I'll roll this into a future patch.
>

Hi Jon,
Going through this again, I believe the logic in the core code
arch_get_unmapped_area_topdown and arch_get_unmapped_area has equivalent
checks to mmap_address_hint_valid (if we substitute TASK_SIZE for a
limit depending on hint address).

Cheers,
-- 
Steve