linux-kernel - Re: [PATCH v4 4/5] mm/mseal: simplify and rename VMA gap check

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Message-ID: <53222f36-ec38-4b3b-95b9-23204a69da66@lucifer.local>
Date: Fri, 25 Jul 2025 19:22:58 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Jeff Xu <jeffxu@...omium.org>, Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Pedro Falcato <pfalcato@...e.de>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Kees Cook <kees@...nel.org>
Subject: Re: [PATCH v4 4/5] mm/mseal: simplify and rename VMA gap check

On Fri, Jul 25, 2025 at 08:10:11PM +0200, David Hildenbrand wrote:
> On 25.07.25 19:43, Lorenzo Stoakes wrote:
> > >
> > > Contact is a great way to hide implementation details. Could you
> > > please keep check_mm_seal() in mseal.c and create
> > > range_contains_unmapped() in vma.c. Then you can refactor as needed.
> >
> > Wait what?
>
> do_mseal() calls range_contains_unmapped(), so I don't see the problem.

Thanks.

>
> We could add a comment above the range_contains_unmapped(), call stating
> *why* we do that, which is much more relevant than some check_XXX function.
>
> /*
>  * mseal() is documented to reject ranges that contain unmapped ranges
>  * (VMA holes): we can only seal VMAs, so nothing would stop mmap() etc.
>  * from succeeding on these unmapped ranged later, and we would not
>  * actually be sealing the requested range.
>  */
>
> Something like that.

Actually this is useful, as it explains to me why we disallow gaps (which I
found silly).

Though I'm not sure I still agree with this (under what circumstances
exactly you'd map within an mseal()'d range afterwards and then assume that
mapping is sealed, I don't know).

Anyway that's sort of besides the point.

I'll send a fix-patch to include this.

>
> --
> Cheers,
>
> David / dhildenb
>

Cheers, Lorenzo