From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-14.7 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED,USER_IN_DEF_DKIM_WL autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 15884C43441 for ; Fri, 9 Nov 2018 21:07:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C9DF820892 for ; Fri, 9 Nov 2018 21:07:02 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="UcSfkIkn" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C9DF820892 Authentication-Results: mail.kernel.org; dmarc=fail (p=reject dis=none) header.from=google.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728223AbeKJGtT (ORCPT ); Sat, 10 Nov 2018 01:49:19 -0500 Received: from mail-oi1-f193.google.com ([209.85.167.193]:35706 "EHLO mail-oi1-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725799AbeKJGtT (ORCPT ); Sat, 10 Nov 2018 01:49:19 -0500 Received: by mail-oi1-f193.google.com with SMTP id v198-v6so2657637oif.2 for ; Fri, 09 Nov 2018 13:06:59 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=uuNsoCL6TY9N8reORgaap3+lQJ+Lj5HeljDywqjigV8=; b=UcSfkIknGPT26FPfW5mofZrobvMAm+toKikUBNEnJZ/uoVb19qVjIwZZHnGBJ6skCf AJ+gW5lAsfsWT/aMojx6QvqWUxgO9mvJL2KpWDqZUyfgtL7qwi2bJusHQxg/ca0/6Z+z rCkZ1sINqR1W1uvIr7epvTSIGC6MS3OCrCAVQZ7GtzB36MQFuHBcsM2wmM2oETy2cS76 4RlDs65OJb9H7MKajpHJaFKuhk+iVXSIbX9RlflLs6UIr/bcba/r62jywrUzjOI4ijSq H+rfk1zjLMdQ/BMLwE1own1wN+Y9halERGTj2gAwio80guTZ6oXiNszfwWNrj6lEPA/J WUPw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=uuNsoCL6TY9N8reORgaap3+lQJ+Lj5HeljDywqjigV8=; b=Yi0xSQxbA/Ea9TJ5psctGvm2EO5vtTQDPmamexsF42rOk5feQZPP7PC5JsRetN5Zfh o/YIVX9EkC0xoWPcVqXnxIx8BRG6qARle31TkUEU+DtNbRqoDDn/SrcQj8ATXAd3QZyr CwZROFR9e18A4A2+VykK1Mr/3LFV3/aXnIe9DSoSASLs+ZS+n7ibeptuOha3InDS00pr 0kmLrTTEJiCerAadMqyW8CrpXeCBKgkAMO2XwRq7kexjktKqEMQdpj45V1DEFwK1iEIa 6PDov0OS79S2LTQIeSpzitJQeJMLt6MRrABken5Fej9S02lG8H9E+00r4uQW2Gpy5/Pj P4mw== X-Gm-Message-State: AGRZ1gL1Nol+bFnEFrHd62os52w9nw+1n0MsKHgQQ2i+JNw2zSZBNrc+ e4h0f4Yn/n76VHrA3aEGc34tiERHlra4wBhGwSKwJQ== X-Google-Smtp-Source: AJdET5evI5RfRqJGlKQ7UsU2lwyhoz3I0XwFn+3Zssez0Ir2BUDAGKtC/m1B23k/og9y7D2xtf32G5Z9D48YZVzQ4Jk= X-Received: by 2002:aca:fd12:: with SMTP id b18-v6mr6173719oii.337.1541797619210; 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 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 Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org +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.