Re: [PATCH] drivers/char/mem: mmap readonly MAP_SHARED-/dev/zero correctly

[Lorenzo Stoakes (ARM)]

On Mon, Sep 28, 2026 at 01:59:47PM +0200, David Hildenbrand (Arm) wrote:
>
> >
> > It then becomes a question about how to get readonly memory.
> >
> >>
> >> We can
> >>
> >> a) Disallow allocating an anon_vma and fail gracefully. So only a shared
> >>    zeropage could ever get mapped there. Might break the s390x
> >>    mm_forbids_zeropage(). But given that's only used in hypervisors like QEMU,
> >>    unlikely.
> >
> > Something like:
> >
> > 	/* about to maybe prep anon_vma */
> >
> > 	if (!vma_cow_mapping(vma) && vma_test(vma, VMA_MAYSHARE_BIT)) {
> > 		/* don't prep anon give zero page */
> > 	}
> >
> > ?
>
> Something like that, but really just failing directly from anon-vma preparation
> in a !vma_cow_mapping(vma), maybe using a special error code.
>
> But the less we have to hack around this special case, the better.
>
> >
> > I think though it's surely the only case (I hope!) where you can possibly be
> > both anon (as in missing vm_ops) and !CoW? I hope? :)
> >
> > So it feels better to fix it at the source.
> >
> > OTOH maybe it's worth special-casing so we don't allocate on read.
> >
> > But that brings me to c)...
> >
> >
> >>
> >> b) Do what Lorenzo proposes. This will allocate real memory. Someone decided to
> >>    use MAP_SHARED, for unknown reasons, so I'd assume it's unlikely that
> >>    something breaks, but you have a point.
> >
> > I would say this patch is the right fix for the moment to fix the assert, and we
> > can chase up with other approaches afterwards.
>
> It will get backported, so we better be careful.
>
> IIUC, the real change is only for /dev/zero users that
>
> fd = open("/dev/zero", O_RDONLY);
> mmap(fd, MAP_SHARED, PROT_READ);
>
> Which is rather something odd to do indeed.
>
> Letting AI do some digging ... it says there are not known users (I don't trust
> it, but I suspect at least there are no prominent users, which makes sense ...).
>
>
> Getting shared memory when requesting MAP_SHARED does sound like the right thing
> to do ...
>
> So let's try this:
>
> Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

Thanks!

Yeah let's revisit this one later more broadly.

>
> --
> Cheers,
>
> David

--
Cheers, Lorenzo