Re: [patch] mmap bug with drivers that adjust vm_start

[Andrea Arcangeli <andrea@suse.de>]

On Tue, Mar 26, 2002 at 01:57:03PM -0500, Benjamin LaHaise wrote:
> On Tue, Mar 26, 2002 at 05:42:36PM +0100, Andrea Arcangeli wrote:
> > However if the patch is needed it means the ->mmap also must do the
> > do_munmap stuff by hand internally, which is very ugly given we also did
> > our own do_munmap in a completly different region (the one requested by
> > the user).
> 
> At least my own code checks for that and fails if there is a mapping 
> already placed at the fixed address it needs to use.  If we're paranoid, 

Ok, so it's safe.

> we could BUG() on getting a vma back from the new find_vma_prepare call.

yes, it sounds a good idea to verify there's no other mapping in the way
of the relocation (until a better fix is implemented), it's a slow path
so we won't hurt performance.

> 
> > Our do_munmap should not happen if we place the mapping
> > elsewhere. If possible I would prefer to change those drivers to
> > advertise their enforced vm_start with a proper callback, the current
> > way is halfway broken still. BTW, which are those drivers, and why they
> > needs to enforce a certain vm_start (also despite MAP_FIXED that they
> > cannot check within the ->mmap callback)?
> 
> Video drivers, others that require specific alignment (4MB pages for 
> example).  Historically, the mmap call has been the hook for doing this, 
> hence the comment in do_mmap from davem.  Unless there's a really good 
> reason for changing the hook, I don't see doing so as providing much 
> benefit other than making source compatibility hard.

The good reason, is that currently we're literally corrupting the
userspace with the senseless do_munmap call in the add<->addr+len area
before the ->mmap lowlevel callback. And such an munmap is certainly not
required to maintain source and binary compatibility (otherwise it would
be insane in the first place :).

Andrea
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