From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-io0-f200.google.com (mail-io0-f200.google.com [209.85.223.200]) by kanga.kvack.org (Postfix) with ESMTP id 66EE46B0033 for ; Fri, 3 Nov 2017 13:41:52 -0400 (EDT) Received: by mail-io0-f200.google.com with SMTP id n137so10030130iod.20 for ; Fri, 03 Nov 2017 10:41:52 -0700 (PDT) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id f67sor3288904iod.27.2017.11.03.10.41.50 for (Google Transport Security); Fri, 03 Nov 2017 10:41:50 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: References: <20171031184052.25253-1-marcandre.lureau@redhat.com> <20171031184052.25253-5-marcandre.lureau@redhat.com> From: David Herrmann Date: Fri, 3 Nov 2017 18:41:50 +0100 Message-ID: Subject: Re: [PATCH 4/6] hugetlbfs: implement memfd sealing Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org List-ID: To: Mike Kravetz Cc: =?UTF-8?B?TWFyYy1BbmRyw6kgTHVyZWF1?= , linux-mm , linux-kernel , aarcange@redhat.com, Hugh Dickins , nyc@holomorphy.com Hi On Fri, Nov 3, 2017 at 6:12 PM, Mike Kravetz wrot= e: > On 11/03/2017 10:03 AM, David Herrmann wrote: >> Hi >> >> On Tue, Oct 31, 2017 at 7:40 PM, Marc-Andr=C3=A9 Lureau >> wrote: >>> Implements memfd sealing, similar to shmem: >>> - WRITE: deny fallocate(PUNCH_HOLE). mmap() write is denied in >>> memfd_add_seals(). write() doesn't exist for hugetlbfs. >>> - SHRINK: added similar check as shmem_setattr() >>> - GROW: added similar check as shmem_setattr() & shmem_fallocate() >>> >>> Except write() operation that doesn't exist with hugetlbfs, that >>> should make sealing as close as it can be to shmem support. >> >> SEAL, SHRINK, and GROW look fine to me. >> >> Regarding WRITE > > The commit message may not be clear. However, hugetlbfs does not support > the write system call (or aio). The only way to modify contents of a > hugetlbfs file is via mmap or hole punch/truncate. So, we do not really > need to worry about those special (a)io cases for hugetlbfs. This is not about the write(2) syscall. Please consider this scenario about shmem: You create a memfd via memfd_create() and map it writable. You now call another kernel syscall that takes as input _any mapped page range_. You pass your mapped memfd-addresses to it. Those syscalls tend to use get_user_pages() to pin arbitrary user-mapped pages, as such this also affects shmem. In this case, those pages might stay mapped even if you munmap() your memfd! One example of this is using AIO-read() on any other file that supports it, passing your mapped memfd as buffer to _read into_. The operations supported on the memfd are irrelevant here. The selftests contain a FUSE-based test for this, since FUSE allows user-space to GUP pages for an arbitrary amount of time. The original fix for this is: commit 05f65b5c70909ef686f865f0a85406d74d75f70f Author: David Herrmann Date: Fri Aug 8 14:25:36 2014 -0700 shm: wait for pins to be released when sealing Please have a look at this. Your patches use shmem_add_seals() almost unchanged, and as such you call into shmem_wait_for_pins() on hugetlbfs. I would really like to see an explicit ACK that this works on hugetlbfs. 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