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

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Message-ID: <CALCETrVoE+JO2rLsBUHAOJdvescEEjxikj8iQ339Nxfopfc7pw@mail.gmail.com>
Date:	Fri, 13 Jun 2014 08:10:57 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	David Herrmann <dh.herrmann@...il.com>
Cc:	"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 Fri, Jun 13, 2014 at 3:36 AM, David Herrmann <dh.herrmann@...il.com> wrote:
> Hi
>
> This is v3 of the File-Sealing and memfd_create() patches. You can find v1 with
> a longer introduction at gmane:
>   http://thread.gmane.org/gmane.comp.video.dri.devel/102241
> An LWN article about memfd+sealing is available, too:
>   https://lwn.net/Articles/593918/
> v2 with some more discussions can be found here:
>   http://thread.gmane.org/gmane.linux.kernel.mm/115713
>
> This series introduces two new APIs:
>   memfd_create(): Think of this syscall as malloc() but it returns a
>                   file-descriptor instead of a pointer. That file-descriptor is
>                   backed by anon-memory and can be memory-mapped for access.
>   sealing: The sealing API can be used to prevent a specific set of operations
>            on a file-descriptor. You 'seal' the file and give thus the
>            guarantee, that it cannot be modified in the specific ways.
>
> A short high-level introduction is also available here:
>   http://dvdhrm.wordpress.com/2014/06/10/memfd_create2/

Potentially silly question: is it guaranteed that mmapping and reading
a SEAL_SHRINKed fd within size bounds will not SIGBUS?  If so, should
this be documented?  (The particular issue here would be reading
holes.  It should work by using the zero page, but, if so, we should
probably make it a real documented guarantee.)

--Andy
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