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

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Date:	Tue, 17 Jun 2014 12:01:55 +0200
From:	David Herrmann <dh.herrmann@...il.com>
To:	Florian Weimer <fweimer@...hat.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Ryan Lortie <desrt@...rt.ca>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Greg Kroah-Hartman <greg@...ah.com>,
	John Stultz <john.stultz@...aro.org>,
	Lennart Poettering <lennart@...ttering.net>,
	Daniel Mack <zonque@...il.com>, Kay Sievers <kay@...y.org>,
	Hugh Dickins <hughd@...gle.com>,
	Tony Battersby <tonyb@...ernetics.com>
Subject: Re: [PATCH v3 0/7] File Sealing & memfd_create()

Hi

On Tue, Jun 17, 2014 at 11:54 AM, Florian Weimer <fweimer@...hat.com> wrote:
> On 06/13/2014 05:33 PM, David Herrmann wrote:
>>
>> On Fri, Jun 13, 2014 at 5:17 PM, Andy Lutomirski <luto@...capital.net>
>> wrote:
>>>
>>> Isn't the point of SEAL_SHRINK to allow servers to mmap and read
>>> safely without worrying about SIGBUS?
>>
>>
>> No, I don't think so.
>> The point of SEAL_SHRINK is to prevent a file from shrinking. SIGBUS
>> is an effect, not a cause. It's only a coincidence that "OOM during
>> reads" and "reading beyond file-boundaries" has the same effect:
>> SIGBUS.
>> We only protect against reading beyond file-boundaries due to
>> shrinking. Therefore, OOM-SIGBUS is unrelated to SEAL_SHRINK.
>>
>> Anyone dealing with mmap() _has_ to use mlock() to protect against
>> OOM-SIGBUS. Making SEAL_SHRINK protect against OOM-SIGBUS would be
>> redundant, because you can achieve the same with SEAL_SHRINK+mlock().
>
>
> I don't think this is what potential users expect because mlock requires
> capabilities which are not available to them.
>
> A couple of weeks ago, sealing was to be applied to anonymous shared memory.
> Has this changed?  Why should *reading* it trigger OOM?

The file might have holes, therefore, you'd have to allocate backing
pages. This might hit a soft-limit and fail. To avoid this, use
fallocate() to allocate pages prior to mmap() or mlock() to make the
kernel lock them in memory.

Thanks
David
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