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Date: Tue, 17 Jun 2014 14:13:29 +0200
From: Florian Weimer <fweimer@redhat.com>
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Subject: Re: [PATCH v3 0/7] File Sealing & memfd_create()
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To: David Herrmann <dh.herrmann@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Michael Kerrisk <mtk.manpages@gmail.com>, Ryan Lortie <desrt@desrt.ca>, Linus Torvalds <torvalds@linux-foundation.org>, Andrew Morton <akpm@linux-foundation.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, Linux FS Devel <linux-fsdevel@vger.kernel.org>, Linux API <linux-api@vger.kernel.org>, Greg Kroah-Hartman <greg@kroah.com>, John Stultz <john.stultz@linaro.org>, Lennart Poettering <lennart@poettering.net>, Daniel Mack <zonque@gmail.com>, Kay Sievers <kay@vrfy.org>, Hugh Dickins <hughd@google.com>, Tony Battersby <tonyb@cybernetics.com>

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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