Re: [PATCH v3 0/7] File Sealing & memfd_create()

[Andy Lutomirski]

On Tue, Jun 17, 2014 at 9:36 AM, David Herrmann <dh.herrmann@xxxxxxxxx> wrote:
> Hi
>
> On Tue, Jun 17, 2014 at 6:20 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> Can you summarize why holes can't be reliably backed by the zero page?
>
> To answer this, I will quote Hugh from "PATCH v2 1/3":
>
>> We do already use the ZERO_PAGE instead of allocating when it's a
>> simple read; and on the face of it, we could extend that to mmap
>> once the file is sealed.  But I am rather afraid to do so - for
>> many years there was an mmap /dev/zero case which did that, but
>> it was an easily forgotten case which caught us out at least
>> once, so I'm reluctant to reintroduce it now for sealing.
>>
>> Anyway, I don't expect you to resolve the issue of sealed holes:
>> that's very much my territory, to give you support on.
>
> Holes can be avoided with a simple fallocate(). I don't understand why
> I should make SEAL_WRITE do the fallocate for the caller. During the
> discussion of memfd_create() I was told to drop the "size" parameter,
> because it is redundant. I don't see how this implicit fallocate()
> does not fall into the same category?
>

I'm really confused now.

If I SEAL_WRITE a file, and then I mmap it PROT_READ, and then I read
it, is that a "simple read"?  If so, doesn't that mean that there's no
problem?

--Andy
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