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=-0.6 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 03CD2CA9EAE for ; Wed, 23 Oct 2019 06:06:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CECF52084C for ; Wed, 23 Oct 2019 06:06:39 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="LwkqCuhw" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1733273AbfJWGGh (ORCPT ); Wed, 23 Oct 2019 02:06:37 -0400 Received: from mail-yw1-f67.google.com ([209.85.161.67]:40983 "EHLO mail-yw1-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725852AbfJWGGh (ORCPT ); Wed, 23 Oct 2019 02:06:37 -0400 Received: by mail-yw1-f67.google.com with SMTP id o195so2272218ywd.8; Tue, 22 Oct 2019 23:06:36 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=Kv+6IoQR6bEB2iX/xqg9xXvx4UX3aidmtCoBvt739tg=; b=LwkqCuhwjrZ+s2dT41pwHds7Laitwn8zRsM4BRLBieEW3LRdNfgO0pyGQhChE/9XZo FFzsPqUrJX93YsVOu89BexM0esXwfeCBR1SpacmgZDQq6Ev1tw3TGGsHr/qpeP6u3eYW vKUmaXm53lCNy4hshKSkjN6VrKUQVlR6jVd9OUc5i4kAQLycuVUnwPpR5kufnzEBDgOf p/+VRrQ8KSo9t8yZBZbNsTnzzbwZZks3xLIi+NWVJqELZqZDJ3ymOB917eWMbygb/lAE T86SHOTYC0+GePfAegXYIfHILkN/5jF1qNwM1FEdHkwW4DR/RQpF0YZF0Jx/WvVNUl06 GR2g== 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=Kv+6IoQR6bEB2iX/xqg9xXvx4UX3aidmtCoBvt739tg=; b=QysdAxntw5Fn0qaVOzEy8oM3y2BkyFyrvlD30j/aaYA2HofH7q23aX3ZsVgOJrTu7B Rpj2Pck9RqVWy3Q6y1uoWtBheJdqz6CMm/oJI6PVqlwfB+kJ8gjc3prpams7HtxDsd/f 2G2jUlgVUeDUHPSvYr1gX9kCD/lVrkK363sw+bYUxSouUF6hPRl7v5jm+0a/Hnt59RxD zFkMPCh2MMhl9fzhGT7BZIQ/LJZwg1tnBGB/lhmnic39TMAJARyuETqrr8WHKUZH5vEu jtEkPW6myZuqLp/4IA2iI+MORwgNfsw5uQgrlpCfH+kC1FQpOsbumfIDIPGUEoJ6QpdD Scpw== X-Gm-Message-State: APjAAAV+Z8wVz71QE8yjC3Mh0m++8oWyLm4i580VLH/LPRaM5DyoYAVz soPPPecuvkp450bpFEWSi8OjWCKX1W5kvqtaR8w= X-Google-Smtp-Source: APXvYqyO/UpMuAPduTSsdjCSuogzNS7cxq3uHwOvGAm5Zm4mlxa1F5AFrdhhyVClN9rWcZjE2ekn4dtvmNntGOUnLP0= X-Received: by 2002:a81:4a02:: with SMTP id x2mr1537190ywa.31.1571810796157; Tue, 22 Oct 2019 23:06:36 -0700 (PDT) MIME-Version: 1.0 References: <20191021185356.GB81648@vader> <20191023044430.alow65tnodgnu5um@yavin.dot.cyphar.com> In-Reply-To: <20191023044430.alow65tnodgnu5um@yavin.dot.cyphar.com> From: Amir Goldstein Date: Wed, 23 Oct 2019 09:06:24 +0300 Message-ID: Subject: Re: [PATCH man-pages] Document encoded I/O To: Aleksa Sarai Cc: Omar Sandoval , linux-fsdevel , Linux Btrfs , Dave Chinner , Jann Horn , Linux API , kernel-team@fb.com, Theodore Tso Content-Type: text/plain; charset="UTF-8" Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org > > > > No, I see why you choose to add the flag to open(2). > > I have no objection. > > > > I once had a crazy thought how to add new open flags > > in a non racy manner without adding a new syscall, > > but as you wrote, this is not relevant for O_ALLOW_ENCODED. > > > > Something like: > > > > /* > > * Old kernels silently ignore unsupported open flags. > > * New kernels that gets __O_CHECK_NEWFLAGS do > > * the proper checking for unsupported flags AND set the > > * flag __O_HAVE_NEWFLAGS. > > */ > > #define O_FLAG1 __O_CHECK_NEWFLAGS|__O_FLAG1 > > #define O_HAVE_FLAG1 __O_HAVE_NEWFLAGS|__O_FLAG1 > > > > fd = open(path, O_FLAG1); > > if (fd < 0) > > return -errno; > > flags = fcntl(fd, F_GETFL, 0); > > if (flags < 0) > > return flags; > > if ((flags & O_HAVE_FLAG1) != O_HAVE_FLAG1) { > > close(fd); > > return -EINVAL; > > } > > You don't need to add __O_HAVE_NEWFLAGS to do this -- this already works > today for userspace to check whether a flag works properly > (specifically, __O_FLAG1 will only be set if __O_FLAG1 is supported -- > otherwise it gets cleared during build_open_flags). That's a behavior of quite recent kernels since 629e014bb834 fs: completely ignore unknown open flags and maybe some stable kernels. Real old kernels don't have that luxury. > > The problem with adding new flags is that an *old* program running on a > *new* kernel could pass a garbage flag (__O_CHECK_NEWFLAGS for instance) > that causes an error only on the new kernel. > That's a theoretic problem. Same as O_PATH|O_TMPFILE. Show me a real life program that passes garbage files to open. > The only real solution to this (and several other problems) is > openat2(). No argue about that. Come on, let's get it merged ;-) > As for O_ALLOW_ENCODED -- the current semantics (-EPERM if it > is set without CAP_SYS_ADMIN) *will* cause backwards compatibility > issues for programs that have garbage flags set... > Again, that's theoretical. In practice, O_ALLOW_ENCODED can work with open()/openat(). In fact, even if O_ALLOW_ENCODED gets merged after openat2(), I don't think it should be forbidden by open()/openat(), right? Do in that sense, O_ALLOW_ENCODED does not depend on openat2(). Thanks, Amir.