Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753170AbaFMPPo (ORCPT ); Fri, 13 Jun 2014 11:15:44 -0400 Received: from mail-ie0-f169.google.com ([209.85.223.169]:61750 "EHLO mail-ie0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752577AbaFMPPm (ORCPT ); Fri, 13 Jun 2014 11:15:42 -0400 MIME-Version: 1.0 In-Reply-To: References: <1402655819-14325-1-git-send-email-dh.herrmann@gmail.com> Date: Fri, 13 Jun 2014 17:15:41 +0200 Message-ID: Subject: Re: [PATCH v3 0/7] File Sealing & memfd_create() From: David Herrmann To: Andy Lutomirski Cc: "linux-kernel@vger.kernel.org" , Michael Kerrisk , Ryan Lortie , Linus Torvalds , Andrew Morton , "linux-mm@kvack.org" , Linux FS Devel , Linux API , Greg Kroah-Hartman , John Stultz , Lennart Poettering , Daniel Mack , Kay Sievers , Hugh Dickins , Tony Battersby Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi On Fri, Jun 13, 2014 at 5:10 PM, Andy Lutomirski wrote: > On Fri, Jun 13, 2014 at 3:36 AM, David Herrmann 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.) No, this is not guaranteed. See the previous discussion in v2 on Patch 2/4 between Hugh and me. Summary is: If you want mmap-reads to not fail, use mlock(). There are many situations where a fault might fail (think: OOM) and sealing is not meant to protect against that. Btw., holes are automatically filled with fresh pages by shmem. So a read only fails in OOM situations (or memcg limits, etc.). Thanks David -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/