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

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Date:	Tue, 17 Jun 2014 14:13:29 +0200
From:	Florian Weimer <fweimer@...hat.com>
To:	David Herrmann <dh.herrmann@...il.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()

On 06/17/2014 12:10 PM, David Herrmann wrote:

>>> 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()
>>
>> This does not work because the consuming side does not know how the
>> descriptor was set up if sealing does not imply that.
>
> The consuming side has to very seals via F_GET_SEALS. After that, it
> shall do a simple fallocate() on the whole file if it wants to go sure
> that all pages are allocated. Why shouldn't that be possible? Please
> elaborate.

Hmm.  You permit general fallocate even for WRITE seals.  That's really 
unexpected.

The inode_newsize_ok check in shmem_fallocate can result in SIGXFSZ, 
which doesn't seem to be what's intended here.

Will the new pages attributed to the process calling fallocate, or to 
the process calling memfd_create?

-- 
Florian Weimer / Red Hat Product Security Team
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