Re: [PATCH v2] mm: userfaultfd: avoid passing an invalid range to vma_merge()

[Liam R. Howlett]

* Peter Xu <peterx@xxxxxxxxxx> [230516 18:52]:
> On Tue, May 16, 2023 at 06:38:30PM -0400, Liam R. Howlett wrote:
> > > It seems to me what you're trying to explain is we shouldn't handle any
> > > split in vma_merge() so we should move cases 4 & 5 out of vma_merge().  If
> > > we split first then merge, cases 4 & 5 will become case 2 & 3 after split.
> > 
> > We don't split in case 4 or 5 - we adjust the existing VMA limits.  We
> > don't actually handle any splits in vma_merge().  I think splitting
> > first would change 4 & 5 to 7 & 8?  2 & 3 would require a split and
> > munmap, right?
> 
> Right, I referenced to the wrong numbers..  2 & 3 are when CUR (CCCC) is
> empty and newly mapped in, if split happened it means CUR (CCCC) exists
> which is 7 & 8 correspondingly.
> 
> > 
> > > My question would be: if it worked perfect in the past few years and it
> > > looks all good enough, why bother..
> > 
> > I suspect, but it's not clear (like all of this), that the other
> > arguments to vma_merge() is ruling out this potential hazard I thought
> > existed.
> 
> Some more elaborations on this one would be appreciated.

I just responded in the other thread, as the context is more complete
there.