linux-kernel - Re: [PATCH v3 0/7] File Sealing & memfd

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Date:	Tue, 17 Jun 2014 18:51:18 +0200
From:	David Herrmann <dh.herrmann@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Greg KH <greg@...ah.com>, Florian Weimer <fweimer@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Lennart Poettering <lennart@...ttering.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux API <linux-api@...r.kernel.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Kay Sievers <kay@...y.org>,
	John Stultz <john.stultz@...aro.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Daniel Mack <zonque@...il.com>, Ryan Lortie <desrt@...rt.ca>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Tony Battersby <tonyb@...ernetics.com>
Subject: Re: [PATCH v3 0/7] File Sealing & memfd_create()

Hi

On Tue, Jun 17, 2014 at 6:41 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Tue, Jun 17, 2014 at 9:36 AM, David Herrmann <dh.herrmann@...il.com> wrote:
>> Hi
>>
>> On Tue, Jun 17, 2014 at 6:20 PM, Andy Lutomirski <luto@...capital.net> 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?

I assumed Hugh was talking about read(). So no, this is not about
memory-reads on mmap()ed regions.

Looking at shmem_file_read_iter() I can see a ZERO_PAGE(0) call in
case shmem_getpage_gfp(SGP_READ) tells us there's a hole. I cannot see
anything like that in the mmap_region() and shmem_fault() paths.

Thanks
David
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