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Date:   Tue, 16 May 2023 18:53:48 -0400
From:   "Liam R. Howlett" <Liam.Howlett@Oracle.com>
To:     Peter Xu <peterx@redhat.com>
Cc:     Lorenzo Stoakes <lstoakes@gmail.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>
Subject: Re: [PATCH v2] mm: userfaultfd: avoid passing an invalid range to
 vma_merge()
Message-ID: <20230516225348.yrh5qpwwodezebak@revolver>
Mail-Followup-To: "Liam R. Howlett" <Liam.Howlett@Oracle.com>,
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        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@kernel.org>,
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* Peter Xu <peterx@redhat.com> [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.