Re: [PATCH] mm/mmap: Fix MAP_FIXED address return on VMA merge

[David Hildenbrand]

> > >    mm/mmap.c | 15 +++++++--------
> > >    1 file changed, 7 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > index 42cd2c260898..22010e13f1a1 100644
> > > --- a/mm/mmap.c
> > > +++ b/mm/mmap.c
> > > @@ -2625,14 +2625,14 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> > >    		if (error)
> > >    			goto unmap_and_free_vma;
> > > -		/* Can addr have changed??
> > > -		 *
> > > -		 * Answer: Yes, several device drivers can do it in their
> > > -		 *         f_op->mmap method. -DaveM
> > > +		/*
> > > +		 * Expansion is handled above, merging is handled below.
> > > +		 * Drivers should not alter the address of the VMA.
> > >    		 */
> > > -		WARN_ON_ONCE(addr != vma->vm_start);
> > > -
> > > -		addr = vma->vm_start;
> > > +		if (WARN_ON((addr != vma->vm_start))) {
> > > +			error = -EINVAL;
> > > +			goto close_and_free_vma;
> > > +		}
> >
> > If this is something that user space can trigger, WARN_* is the wrong
> > choice. But what I understand from the comment change is that this must not
> > happen at that point unless there is a real issue.
>
> The VMA start address could be changed in call_mmap() which is a driver
> call.  I guess someone could write a driver to mmap by a users action?
> I don't think it can be reached other ways.  In any case, I'm changing a
> WARN_ON_ONCE() to a WARN_ON() and undoing the badness instead of
> marching forwards.

WARN_ON_ONCE() can also be used in conditionals if that's what you were 
concerned about, but ...

> > Why not "if (WARN_ON_ONCE)" ?
>
> I was thinking it was harder to ignore if it happen more frequently?
> There isn't a driver that does this now, but I'm not picky over which
> variant to call.

.. I think the assumption really is that we won't see (m)any these 
calls. And if we do, it's a bad bad driver. So WARN_ON() might be just fine.

If this would be easy to trigger by any user space, WARN* would have 
been the wrong choice, that's why I was asking.

--
Thanks,

David / dhildenb