From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ie0-f180.google.com (mail-ie0-f180.google.com [209.85.223.180]) by kanga.kvack.org (Postfix) with ESMTP id 8A4196B0038 for ; Fri, 13 Jun 2014 11:15:42 -0400 (EDT) Received: by mail-ie0-f180.google.com with SMTP id rl12so2615604iec.11 for ; Fri, 13 Jun 2014 08:15:42 -0700 (PDT) Received: from mail-ie0-x22f.google.com (mail-ie0-x22f.google.com [2607:f8b0:4001:c03::22f]) by mx.google.com with ESMTPS id f9si2676267igi.28.2014.06.13.08.15.41 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 13 Jun 2014 08:15:42 -0700 (PDT) Received: by mail-ie0-f175.google.com with SMTP id tp5so2621399ieb.34 for ; Fri, 13 Jun 2014 08:15:41 -0700 (PDT) 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 Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: 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 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, 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