Re: [PATCH 5/5] uprobes: kill insert_vm_struct()->uprobe_mmap()

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* Oleg Nesterov <oleg@redhat.com> [2012-07-13 15:29:16]:

> On 07/13, Srikar Dronamraju wrote:
> >
> > * Oleg Nesterov <oleg@redhat.com> [2012-07-08 22:30:11]:
> >
> > > Kill insert_vm_struct()->uprobe_mmap(). It is not needed, nobody
> > > except arch/ia64/kernel/perfmon.c uses insert_vm_struct(vma) with
> > > vma->vm_file != NULL.
> > >
> >
> > Right, but somebody else might start using this later.
> 
> Unlikely, I think...
> 
> > I cant think of a use case though.
> 
> Yes.
> 
> > > And it is wrong. Again, get_user_pages() can not succeed before
> > > vma_link(vma) makes is visible to find_vma(). And even if this
> > > worked, we must not insert the new bp before this mapping is
> > > visible to vma_prio_tree_foreach() for uprobe_unregister().
> > >
> >
> > Agree, we are wrong to do it before vma_link.
> >
> >
> > > Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> > > ---
> > >  mm/mmap.c |    3 ---
> > >  1 files changed, 0 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > index e5a4614..4fe2697 100644
> > > --- a/mm/mmap.c
> > > +++ b/mm/mmap.c
> > > @@ -2345,9 +2345,6 @@ int insert_vm_struct(struct mm_struct * mm, struct vm_area_struct * vma)
> > >  	     security_vm_enough_memory_mm(mm, vma_pages(vma)))
> > >  		return -ENOMEM;
> > >
> > > -	if (vma->vm_file && uprobe_mmap(vma))
> > > -		return -EINVAL;
> > > -
> > >  	vma_link(mm, vma, prev, rb_link, rb_parent);
> > >  	return 0;
> > >  }
> >
> > Can we do something like:
> >
> > 	vma_link(mm, vma, prev, rb_link, rb_parent);
> >
> > 	if (vma->vm_file && uprobe_mmap(vma)) {
> > 		/* FIXME: dont know if calling unmap_region is fine here */
> > 		unmap_region(mm, vma, prev, vma->vm_start, vma->vm_end);
> > 		return -EINVAL;
> > 	}
> 
> Yes, I was thinking about the possible fix, but afaics this is not
> enough. At least this needs vm_unacct_memory(). And I am not sure
> about unmap_region...
> 
> The main problem is that I have no idea how could I test the fix.
> Once again, currently this file can't be probed.
> 
> So. Can't we kill this obviously wrong and unneeded (at least currently)
> code? Currently uprobe_mmap/munmap logic is not correct (I'll try to send
> more fixes after I return from vacation), it would be nice to remove the
> callsite.
> 
> If somebody else will use insert_vm_struct() to mmap the can-be-uprobed
> file then yes, we will need to add uprobe_mmap() somewhere. But until
> then, the right fix is not clear and not testable.

Yes, for now your solution should be the way to go.
May be we should probably add a TODO/Fixme comment in insert_vm_struct
saying anybody trying to use insert_vm_struct to look at uprobe_mmap().

-- 
Thanks and Regards
Srikar