Re: [PATCH v9 11/12] x86, mpx: cleanup unused bound tables

[Thomas Gleixner <tglx@linutronix.de>]

On Mon, 3 Nov 2014, Dave Hansen wrote:
> On 10/27/2014 01:49 PM, Thomas Gleixner wrote:
> > Errm. Before user space can use the bounds table for the new mapping
> > it needs to add the entries, right? So:
> > 
> > CPU 0					CPU 1
> > 
> > down_write(mm->bd_sem);
> > mpx_pre_unmap();
> >    clear bounds directory entries	
> > unmap();
> > 					map()
> > 					write_bounds_entry()
> > 					trap()
> > 					  down_read(mm->bd_sem);
> > mpx_post_unmap(); 
> > up_write(mm->bd_sem);
> > 					  allocate_bounds_table();
> > 
> > That's the whole point of bd_sem.
> 
> This does, indeed, seem to work for the normal munmap() cases.  However,
> take a look at shmdt().  We don't know the size of the segment being
> unmapped until after we acquire mmap_sem for write, so we wouldn't be
> able to do do a mpx_pre_unmap() for those.

That's not really true. You can evaluate that information with
mmap_sem held for read as well. Nothing can change the mappings until
you drop it. So you could do:

   down_write(mm->bd_sem);
   down_read(mm->mmap_sem;
   evaluate_size_of_shm_to_unmap();
   clear_bounds_directory_entries();
   up_read(mm->mmap_sem);
   do_the_real_shm_unmap();
   up_write(mm->bd_sem);

That should still be covered by the above scheme.
 
> mremap() is similar.  We don't know if the area got expanded (and we
> don't need to modify bounds tables) or moved (and we need to free the
> old location's tables) until well after we've taken mmap_sem for write.

See above.
 
> I propose we keep mm->bd_sem.  But, I think we need to keep a list
> during each of the unmapping operations of VMAs that got unmapped, and
> then keep them on a list without freeing then.  At up_write() time, we
> look at the list, if it is empty, we just do an up_write() and we are done.
> 
> If *not* empty, downgrade_write(mm->mmap_sem), and do the work you
> spelled out in mpx_pre_unmap() above: clear the bounds directory entries
> and gather the VMAs while still holding mm->bd_sem for write.
> 
> Here's the other wrinkle: This would invert the ->bd_sem vs. ->mmap_sem
> ordering (bd_sem nests outside mmap_sem with the above scheme).  We
> _could_ always acquire bd_sem for write whenever mmap_sem is acquired,
> although that seems a bit heavyweight.  I can't think of anything better
> at the moment, though.

That works as well. If it makes stuff simpler I'm all for it. But then
we should really replace down_write(mmap_sem) with a helper function
and add something to checkpatch.pl and to the coccinelle scripts to
catch new instances of open coded 'down_write(mmap_sem)'.

Thanks,

	tglx

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