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=-0.8 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, 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 11A3CC3A5A1 for ; Wed, 28 Aug 2019 16:10:25 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 875F420828 for ; Wed, 28 Aug 2019 16:10:24 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="pqTccn5J" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 875F420828 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Received: from bilbo.ozlabs.org (lists.ozlabs.org [IPv6:2401:3900:2:1::3]) by lists.ozlabs.org (Postfix) with ESMTP id 46JW1B40lpzDrGG for ; Thu, 29 Aug 2019 02:10:22 +1000 (AEST) Received: from ozlabs.org (bilbo.ozlabs.org [203.11.71.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 46JVhX2PlDzDr81 for ; Thu, 29 Aug 2019 01:55:56 +1000 (AEST) Authentication-Results: lists.ozlabs.org; dmarc=pass (p=none dis=none) header.from=kernel.org Authentication-Results: lists.ozlabs.org; dkim=pass (1024-bit key; unprotected) header.d=kernel.org header.i=@kernel.org header.b="pqTccn5J"; dkim-atps=neutral Received: from ozlabs.org (bilbo.ozlabs.org [203.11.71.1]) by bilbo.ozlabs.org (Postfix) with ESMTP id 46JVhS6wXXz8x6w for ; Thu, 29 Aug 2019 01:55:52 +1000 (AEST) Received: by ozlabs.org (Postfix) id 46JVhS4WGBz9sDB; Thu, 29 Aug 2019 01:55:52 +1000 (AEST) Authentication-Results: ozlabs.org; spf=pass (mailfrom) smtp.mailfrom=kernel.org (client-ip=198.145.29.99; helo=mail.kernel.org; envelope-from=jlayton@kernel.org; receiver=) Authentication-Results: ozlabs.org; dmarc=pass (p=none dis=none) header.from=kernel.org Authentication-Results: ozlabs.org; dkim=pass (1024-bit key; unprotected) header.d=kernel.org header.i=@kernel.org header.b="pqTccn5J"; dkim-atps=neutral Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ozlabs.org (Postfix) with ESMTPS id 46JVhR5FpQz9sBF for ; Thu, 29 Aug 2019 01:55:51 +1000 (AEST) Received: from tleilax.poochiereds.net (68-20-15-154.lightspeed.rlghnc.sbcglobal.net [68.20.15.154]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 765322064A; Wed, 28 Aug 2019 15:55:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1567007749; bh=tm23iPOHz5qNiVmbKBRthv6o0H14qpr8dY7bd4MTQKs=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=pqTccn5JcnucNCAhPvukewbhK7LnqfZiA1JhrrT1FDFY4mCrVB8AZyQ8GN7mLjP8y 7s9EDdOPkKuIVFWDEaVrPbihjrm8teyPYUCsz+WciUJXkEpW4kr0ySk55f/ASUGrn8 zpEFoJeA5Hy5XDMtaE0vrkdWUVKHa2wWQhJO2xxg= Message-ID: <4da231cd52880991d8a038adb8fbb2ef3d724db9.camel@kernel.org> Subject: Re: [PATCH RESEND v11 7/8] open: openat2(2) syscall From: Jeff Layton To: sbaugh@catern.com, linux-fsdevel@vger.kernel.org Date: Wed, 28 Aug 2019 11:55:47 -0400 In-Reply-To: <854l2366zp.fsf@catern.com> References: <20190820033406.29796-1-cyphar@cyphar.com> <20190820033406.29796-8-cyphar@cyphar.com> <854l2366zp.fsf@catern.com> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.32.4 (3.32.4-1.fc30) MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arch@vger.kernel.org, linux-ia64@vger.kernel.org, linux-parisc@vger.kernel.org, linux-sh@vger.kernel.org, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mips@vger.kernel.org, linuxppc-dev@ozlabs.org, linux-alpha@vger.kernel.org, sparclinux@vger.kernel.org, linux-arm-kernel@lists.infradead.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Mon, 2019-08-26 at 19:50 +0000, sbaugh@catern.com wrote: > Aleksa Sarai writes: > > To this end, we introduce the openat2(2) syscall. It provides all of the > > features of openat(2) through the @how->flags argument, but also > > also provides a new @how->resolve argument which exposes RESOLVE_* flags > > that map to our new LOOKUP_* flags. It also eliminates the long-standing > > ugliness of variadic-open(2) by embedding it in a struct. > > I don't like this usage of a structure in memory to pass arguments that > would fit in registers. This would be quite inconvenient for me as a > userspace developer. > > Others have brought up issues with this: the issue of seccomp, and the > issue of mismatch between the userspace interface and the kernel > interface, are the most important for me. I want to add another, > admittedly somewhat niche, concern. > > This interfaces requires a program to allocate memory (even on the > stack) just to pass arguments to the kernel which could be passed > without allocating that memory. That makes it more difficult and less > efficient to use this syscall in any case where memory is not so easily > allocatable: such as early program startup or assembly, where the stack > may be limited in size or not even available yet, or when injecting a > syscall while ptracing. > > A struct-passing interface was needed for clone, since we ran out of > registers; but we have not run out of registers yet for openat, so it > would be nice to avoid this if we can. We can always expand later... > We can't really expand later like you suggest. Suppose in a couple of years that we need to add some new argument to openat2 that isn't just a new flag. If all these values are passed by individual arguments, you can't add one later without adding yet another syscall. Using a struct for this allows this to be extended later, OTOH. You can extend it, and add a flag that tells the kernel that it can access the new field. No new syscall required. -- Jeff Layton