On Wed, 7 Apr 2010, Peter Zijlstra wrote:

quoted text
> On Tue, 2010-04-06 at 11:28 -0700, Linus Torvalds wrote:
> > Just as an example of the kind of code that makes me worry:
> > 
> >         void unlink_anon_vmas(struct vm_area_struct *vma)
> >         {
> >                 struct anon_vma_chain *avc, *next;
> >                         
> >                 /* Unlink each anon_vma chained to the VMA. */
> >                 list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
> >                         anon_vma_unlink(avc);
> >                         list_del(&avc->same_vma);
> >                         anon_vma_chain_free(avc);
> >                 }
> >         }
> > 
> > Now, think about what happens for the *last* entry in that avc chain. It 
> > will call that "anon_vma_unlink()" thing, which will delete perhaps the 
> > last entry in the "same_anon_vma" one, and then it does
> > 
> >         if (empty)
> >                 anon_vma_free(anon_vma);
> > 
> > *before* unlink_anon_vma's has actually does that
> > 
> >         list_del(&avc->same_vma);
> > 
> > and what we essentially have is a stale anon_vma_chain entry that still 
> > exists on that same_vma list, and points to an anon_vma that already got 
> > deleted.
> > 
> > Does it matter? I really can't see that it does. 
> 
> I think it does, the anon_vma thing has an RCU destroyed slab, but that
> doesn't mean the anon_vma object itself is rcu delayed. The moment we
> free it it can be re-used. So the above use after free is a bug.

Well, it's not really a "use after free" - it's just that a stale pointer 
still exists in a live data structure that is linked into the list. I 
don't think there is a real bug there, simply because I don't think 
anybody will be accessing that list (we should hopefully have all the 
sufficient mutual exclusion in place).

So I just think it is bad form to potentially free something before we get 
rid of all pointers to it. So to me it's a cleanliness issue: good code 
shouldn't do things like that, and it would be much cleaner to remove the 
AVC entry that has a pointer to the anon_vma _before_ we might be freeing 
the anon_vma.

Maybe I'm just anal.

		Linus
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