From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-vc0-f179.google.com (mail-vc0-f179.google.com [209.85.220.179]) by kanga.kvack.org (Postfix) with ESMTP id 8CD4A6B0031 for ; Fri, 13 Jun 2014 11:18:15 -0400 (EDT) Received: by mail-vc0-f179.google.com with SMTP id id10so2412309vcb.38 for ; Fri, 13 Jun 2014 08:18:15 -0700 (PDT) Received: from mail-vc0-f175.google.com (mail-vc0-f175.google.com [209.85.220.175]) by mx.google.com with ESMTPS id xa4si1514411vcb.12.2014.06.13.08.18.14 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 13 Jun 2014 08:18:14 -0700 (PDT) Received: by mail-vc0-f175.google.com with SMTP id hy4so2429124vcb.6 for ; Fri, 13 Jun 2014 08:18:14 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: References: <1402655819-14325-1-git-send-email-dh.herrmann@gmail.com> From: Andy Lutomirski Date: Fri, 13 Jun 2014 08:17:54 -0700 Message-ID: Subject: Re: [PATCH v3 0/7] File Sealing & memfd_create() Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: To: David Herrmann 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 On Fri, Jun 13, 2014 at 8:15 AM, David Herrmann wrote: > 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.). > Isn't the point of SEAL_SHRINK to allow servers to mmap and read safely without worrying about SIGBUS? --Andy > Thanks > David -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org