From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl1-f198.google.com (mail-pl1-f198.google.com [209.85.214.198]) by kanga.kvack.org (Postfix) with ESMTP id 0887E6B0005 for ; Wed, 17 Oct 2018 13:45:32 -0400 (EDT) Received: by mail-pl1-f198.google.com with SMTP id j9-v6so18060304plt.3 for ; Wed, 17 Oct 2018 10:45:32 -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 i69-v6sor8203468pge.17.2018.10.17.10.45.30 for (Google Transport Security); Wed, 17 Oct 2018 10:45:30 -0700 (PDT) Date: Wed, 17 Oct 2018 10:45:27 -0700 From: Joel Fernandes Subject: Re: [PATCH v2 1/2] mm: Add an F_SEAL_FS_WRITE seal to memfd Message-ID: <20181017174527.GA107185@joelaf.mtv.corp.google.com> References: <20181009222042.9781-1-joel@joelfernandes.org> <20181017095155.GA354@infradead.org> <20181017103958.GB230639@joelaf.mtv.corp.google.com> <20181017120829.GA19731@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181017120829.GA19731@infradead.org> Sender: owner-linux-mm@kvack.org List-ID: To: Christoph Hellwig Cc: linux-kernel@vger.kernel.org, kernel-team@android.com, jreck@google.com, john.stultz@linaro.org, tkjos@google.com, gregkh@linuxfoundation.org, Andrew Morton , dancol@google.com, "J. Bruce Fields" , Jeff Layton , Khalid Aziz , linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, Mike Kravetz , minchan@google.com, Shuah Khan On Wed, Oct 17, 2018 at 05:08:29AM -0700, Christoph Hellwig wrote: > On Wed, Oct 17, 2018 at 03:39:58AM -0700, Joel Fernandes wrote: > > > > This usecase cannot be implemented with the existing F_SEAL_WRITE seal. > > > > To support the usecase, this patch adds a new F_SEAL_FS_WRITE seal which > > > > prevents any future mmap and write syscalls from succeeding while > > > > keeping the existing mmap active. The following program shows the seal > > > > working in action: > > > > > > Where does the FS come from? I'd rather expect this to be implemented > > > as a 'force' style flag that applies the seal even if the otherwise > > > required precondition is not met. > > > > The "FS" was meant to convey that the seal is preventing writes at the VFS > > layer itself, for example vfs_write checks FMODE_WRITE and does not proceed, > > it instead returns an error if the flag is not set. I could not find a better > > name for it, I could call it F_SEAL_VFS_WRITE if you prefer? > > I don't think there is anything VFS or FS about that - at best that > is an implementation detail. > > Either do something like the force flag I suggested in the last mail, > or give it a name that matches the intention, e.g F_SEAL_FUTURE_WRITE. > Ok, I agree. I like the name F_SEAL_FUTURE_WRITE you are proposing so I will use that. > > I could make it such that this seal would not be allowed unless F_SEAL_SHRINK > > and F_SEAL_GROW are either previously set, or they are passed along with this > > seal. Would that make more sense to you? > > Yes. Cool. > > > > static int memfd_add_seals(struct file *file, unsigned int seals) > > > > { > > > > @@ -219,6 +220,9 @@ static int memfd_add_seals(struct file *file, unsigned int seals) > > > > } > > > > } > > > > > > > > + if ((seals & F_SEAL_FS_WRITE) && !(*file_seals & F_SEAL_FS_WRITE)) > > > > + file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE); > > > > + > > > > > > This seems to lack any synchronization for f_mode. > > > > The f_mode is set when the struct file is first created and then memfd sets > > additional flags in memfd_create. Then later we are changing it here at the > > time of setting the seal. I donot see any possiblity of a race since it is > > impossible to set the seal before memfd_create returns. Could you provide > > more details about what kind of synchronization is needed and what is the > > race condition scenario you were thinking off? > > Even if no one changes these specific flags we still need a lock due > to rmw cycles on the field. For example fadvise can set or clear > FMODE_RANDOM. It seems to use file->f_lock for synchronization. Ok, I will acquire the f_lock before setting these, thanks for the explanation. Will post updated patches today. - Joel