From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Wed, 17 Oct 2018 02:51:55 -0700 From: Christoph Hellwig To: "Joel Fernandes (Google)" 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 Subject: Re: [PATCH v2 1/2] mm: Add an F_SEAL_FS_WRITE seal to memfd Message-ID: <20181017095155.GA354@infradead.org> References: <20181009222042.9781-1-joel@joelfernandes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181009222042.9781-1-joel@joelfernandes.org> Sender: owner-linux-mm@kvack.org List-ID: On Tue, Oct 09, 2018 at 03:20:41PM -0700, Joel Fernandes (Google) wrote: > One of the main usecases Android has is the ability to create a region > and mmap it as writeable, then drop its protection for "future" writes > while keeping the existing already mmap'ed writeable-region active. s/drop/add/ ? Otherwise this doesn't make much sense to me. > 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. > Note: This seal will also prevent growing and shrinking of the memfd. > This is not something we do in Android so it does not affect us, however > I have mentioned this behavior of the seal in the manpage. This seems odd, as that is otherwise split into the F_SEAL_SHRINK / F_SEAL_GROW flags. > 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.