linux-kernel - Re: [PATCH 03/16] mm: add vma_desc_size(), vma_desc

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Message-ID: <b1a1f4be-8f1c-4fc1-8f60-a5f02836bd12@lucifer.local>
Date: Mon, 8 Sep 2025 21:24:30 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Jason Gunthorpe <jgg@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>, Matthew Wilcox <willy@...radead.org>,
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Subject: Re: [PATCH 03/16] mm: add vma_desc_size(), vma_desc_pages() helpers

On Mon, Sep 08, 2025 at 07:36:59PM +0200, David Hildenbrand wrote:
> On 08.09.25 17:56, Jason Gunthorpe wrote:
> > On Mon, Sep 08, 2025 at 05:50:18PM +0200, David Hildenbrand wrote:
> >
> > > So in practice there is indeed not a big difference between a private and
> > > cow mapping.
> >
> > Right and most drivers just check SHARED.
> >
> > But if we are being documentative why they check shared is because the
> > driver cannot tolerate COW.
> >
> > I think if someone is cargo culting a diver and sees
> > 'vma_never_cowable' they will have a better understanding of the
> > driver side issues.
> >
> > Driver's don't actually care about private vs shared, except this
> > indirectly implies something about cow.
>
> I recall some corner cases, but yes, most drivers don't clear MAP_MAYWRITE so
> is_cow_mapping() would just rule out what they wanted to rule out (no anon
> pages / cow semantics).
>
> FWIW, I recalled some VM_MAYWRITE magic in memfd, but it's really just for
> !cow mappings, so the following should likely work:

I was invovled in these dark arts :)

Since we gate the check_write_seal() function (which is the one that removes
VM_MAYWRITE) on the mapping being shared, then obviously we can't remove
VM_MAYWRITE in the first place.

The only other way VM_MAYWRITE could be got rid of is if it already a MAP_SHARED
or MAP_SHARED_VALIDATE mapping without write permission, and then it'd fail this
check anyway.

So I think the below patch is fine!

>
> diff --git a/mm/memfd.c b/mm/memfd.c
> index 1de610e9f2ea2..2a3aa26444bbb 100644
> --- a/mm/memfd.c
> +++ b/mm/memfd.c
> @@ -346,14 +346,11 @@ static int check_write_seal(vm_flags_t *vm_flags_ptr)
>         vm_flags_t vm_flags = *vm_flags_ptr;
>         vm_flags_t mask = vm_flags & (VM_SHARED | VM_WRITE);
> -       /* If a private mapping then writability is irrelevant. */
> -       if (!(mask & VM_SHARED))
> +       /* If a CoW mapping then writability is irrelevant. */
> +       if (is_cow_mapping(vm_flags))
>                 return 0;
> -       /*
> -        * New PROT_WRITE and MAP_SHARED mmaps are not allowed when
> -        * write seals are active.
> -        */
> +       /* New PROT_WRITE mappings are not allowed when write-sealed. */
>         if (mask & VM_WRITE)
>                 return -EPERM;

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

Cheers, Lorenzo