From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ot1-f72.google.com (mail-ot1-f72.google.com [209.85.210.72]) by kanga.kvack.org (Postfix) with ESMTP id 0DCBF6B0720 for ; Fri, 9 Nov 2018 16:07:01 -0500 (EST) Received: by mail-ot1-f72.google.com with SMTP id c43so1834517otd.20 for ; Fri, 09 Nov 2018 13:07:01 -0800 (PST) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id c125-v6sor4597505oia.151.2018.11.09.13.06.59 for (Google Transport Security); Fri, 09 Nov 2018 13:06:59 -0800 (PST) MIME-Version: 1.0 References: <20181108041537.39694-1-joel@joelfernandes.org> In-Reply-To: <20181108041537.39694-1-joel@joelfernandes.org> From: Jann Horn Date: Fri, 9 Nov 2018 22:06:31 +0100 Message-ID: Subject: Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org List-ID: To: joel@joelfernandes.org Cc: kernel list , jreck@google.com, John Stultz , Todd Kjos , Greg Kroah-Hartman , Christoph Hellwig , Al Viro , Andrew Morton , Daniel Colascione , Bruce Fields , jlayton@kernel.org, Khalid Aziz , Lei.Yang@windriver.com, linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org, Linux-MM , marcandre.lureau@redhat.com, Mike Kravetz , minchan@kernel.org, shuah@kernel.org, valdis.kletnieks@vt.edu, Hugh Dickins , Linux API +linux-api for API addition +hughd as FYI since this is somewhat related to mm/shmem On Fri, Nov 9, 2018 at 9:46 PM Joel Fernandes (Google) wrote: > Android uses ashmem for sharing memory regions. We are looking forward > to migrating all usecases of ashmem to memfd so that we can possibly > remove the ashmem driver in the future from staging while also > benefiting from using memfd and contributing to it. Note staging drivers > are also not ABI and generally can be removed at anytime. > > One of the main usecases Android has is the ability to create a region > and mmap it as writeable, then add protection against making any > "future" writes while keeping the existing already mmap'ed > writeable-region active. This allows us to implement a usecase where > receivers of the shared memory buffer can get a read-only view, while > the sender continues to write to the buffer. > See CursorWindow documentation in Android for more details: > https://developer.android.com/reference/android/database/CursorWindow > > This usecase cannot be implemented with the existing F_SEAL_WRITE seal. > To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal > which prevents any future mmap and write syscalls from succeeding while > keeping the existing mmap active. Please CC linux-api@ on patches like this. If you had done that, I might have criticized your v1 patch instead of your v3 patch... > The following program shows the seal > working in action: [...] > Cc: jreck@google.com > Cc: john.stultz@linaro.org > Cc: tkjos@google.com > Cc: gregkh@linuxfoundation.org > Cc: hch@infradead.org > Reviewed-by: John Stultz > Signed-off-by: Joel Fernandes (Google) > --- [...] > diff --git a/mm/memfd.c b/mm/memfd.c > index 2bb5e257080e..5ba9804e9515 100644 > --- a/mm/memfd.c > +++ b/mm/memfd.c [...] > @@ -219,6 +220,25 @@ static int memfd_add_seals(struct file *file, unsigned int seals) > } > } > > + if ((seals & F_SEAL_FUTURE_WRITE) && > + !(*file_seals & F_SEAL_FUTURE_WRITE)) { > + /* > + * The FUTURE_WRITE seal also prevents growing and shrinking > + * so we need them to be already set, or requested now. > + */ > + int test_seals = (seals | *file_seals) & > + (F_SEAL_GROW | F_SEAL_SHRINK); > + > + if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) { > + error = -EINVAL; > + goto unlock; > + } > + > + spin_lock(&file->f_lock); > + file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE); > + spin_unlock(&file->f_lock); > + } So you're fiddling around with the file, but not the inode? How are you preventing code like the following from re-opening the file as writable? $ cat memfd.c #define _GNU_SOURCE #include #include #include #include #include #include int main(void) { int fd = syscall(__NR_memfd_create, "testfd", 0); if (fd == -1) err(1, "memfd"); char path[100]; sprintf(path, "/proc/self/fd/%d", fd); int fd2 = open(path, O_RDWR); if (fd2 == -1) err(1, "reopen"); printf("reopen successful: %d\n", fd2); } $ gcc -o memfd memfd.c $ ./memfd reopen successful: 4 $ That aside: I wonder whether a better API would be something that allows you to create a new readonly file descriptor, instead of fiddling with the writability of an existing fd.