Re: [PATCH 0/2] mm: introduce MAP_FIXED_SAFE

[John Hubbard]

On 12/05/2017 11:35 PM, Florian Weimer wrote:
> On 12/06/2017 08:33 AM, John Hubbard wrote:
>> In that case, maybe:
>>
>> ÂÂÂÂ MAP_EXACT
>>
>> ? ...because that's the characteristic behavior.
> 
> Is that true? mmap still silently rounding up the length to the page size, I assume, so even that name is misleading.

Hi Florian,

Not as far as I can tell, it's not doing that.

For both MAP_FIXED, and this new flag, the documented (and actual)
behavior is *not* to do any such rounding. Instead, the requested
input address is required to be page-aligned itself, and mmap()
should be honoring the exact addr.

>From the mmap(2) man page:

   MAP_FIXED
          Don't  interpret  addr  as  a  hint: place the mapping at
          exactly that address.  addr must be  a  multiple  of  the
          page  size. 


And from what I can see, the do_mmap() implementation leaves addr
unchanged, in the MAP_FIXED case:

do_mmap(...)
{
        /* ... */
	if (!(flags & MAP_FIXED))
		addr = round_hint_to_min(addr);

...although it does look like device drivers have the opportunity
to break that:

mmap_region(...)
{
		/* Can addr have changed??
		 *
		 * Answer: Yes, several device drivers can do it in their
		 *         f_op->mmap method. -DaveM
		 * Bug: If addr is changed, prev, rb_link, rb_parent should
		 *      be updated for vma_link()
		 */
		WARN_ON_ONCE(addr != vma->vm_start);

		addr = vma->vm_start;
   

--
thanks,
John Hubbard
NVIDIA

> 
> Thanks,
> Florian