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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id AB239C4321E for ; Thu, 24 Nov 2022 12:49:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229737AbiKXMtg (ORCPT ); Thu, 24 Nov 2022 07:49:36 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34694 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229506AbiKXMtg (ORCPT ); Thu, 24 Nov 2022 07:49:36 -0500 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CE8022A420; Thu, 24 Nov 2022 04:49:34 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 6B59D62111; Thu, 24 Nov 2022 12:49:34 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id E033AC433C1; Thu, 24 Nov 2022 12:49:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1669294173; bh=cY0zoRDBBqP3t4FgqnJqWRlOpTVPEPD40LQxnSFLXD4=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=AAyWKZIRRxmldjMLNe4g1TIt/dn3auYo4I2AlIXGwLzFkcsgexSJ3DuPsyn1zzhM9 1nVGQ3VIiLxtrLwHWqAqXPPB1bGEIPtSJg23ODXIVfh7qPGVufYazyyJnVWn3iYgub EXvh9r7+eqC2Glz2aJWPzNbhxb2dD54/8YAvtLFH1T7QEe8MnDTKDuq/6E0fgHohlu Wo0bdNR+0CPShkYNEnQtxKVec9ym46Gev1N+vQ3e9SjUvYVDviLyTIpGETnd7679in 2mKMUDknZihL4Vd3XAw/pHlDlE0c4CaXXKlQw92O9Ozt+r9cV4FL4wBwUOL4PrfwHj M4du5bKF6EgrA== Date: Thu, 24 Nov 2022 13:49:27 +0100 From: Christian Brauner To: "Jason A. Donenfeld" Cc: Florian Weimer , linux-kernel@vger.kernel.org, patches@lists.linux.dev, tglx@linutronix.de, linux-crypto@vger.kernel.org, x86@kernel.org, Greg Kroah-Hartman , Adhemerval Zanella Netto , Carlos O'Donell , linux-api@vger.kernel.org, Arnd Bergmann Subject: Re: [PATCH v6 1/3] random: add vgetrandom_alloc() syscall Message-ID: <20221124124927.argohuob2bslolbt@wittgenstein> References: <20221121152909.3414096-1-Jason@zx2c4.com> <20221121152909.3414096-2-Jason@zx2c4.com> <87v8n6lzh9.fsf@oldenburg.str.redhat.com> <87a64g7wks.fsf@oldenburg.str.redhat.com> <87cz9c5z1f.fsf@oldenburg.str.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org On Thu, Nov 24, 2022 at 01:24:42PM +0100, Jason A. Donenfeld wrote: > Hi Florian, > > On Thu, Nov 24, 2022 at 01:15:24PM +0100, Florian Weimer wrote: > > * Jason A. Donenfeld: > > > > > Hi Florian, > > > > > > On Thu, Nov 24, 2022 at 06:25:39AM +0100, Florian Weimer wrote: > > >> * Jason A. Donenfeld: > > >> > > >> > Hi Florian, > > >> > > > >> > On Wed, Nov 23, 2022 at 11:46:58AM +0100, Florian Weimer wrote: > > >> >> * Jason A. Donenfeld: > > >> >> > > >> >> > + * The vgetrandom() function in userspace requires an opaque state, which this > > >> >> > + * function provides to userspace, by mapping a certain number of special pages > > >> >> > + * into the calling process. It takes a hint as to the number of opaque states > > >> >> > + * desired, and returns the number of opaque states actually allocated, the > > >> >> > + * size of each one in bytes, and the address of the first state. > > >> >> > + */ > > >> >> > +SYSCALL_DEFINE3(vgetrandom_alloc, unsigned long __user *, num, > > >> >> > + unsigned long __user *, size_per_each, unsigned int, flags) > > >> >> > > >> >> I think you should make this __u64, so that you get a consistent > > >> >> userspace interface on all architectures, without the need for compat > > >> >> system calls. > > >> > > > >> > That would be quite unconventional. Most syscalls that take lengths do > > >> > so with the native register size (`unsigned long`, `size_t`), rather > > >> > than u64. If you can point to a recent trend away from this by > > >> > indicating some commits that added new syscalls with u64, I'd be happy > > >> > to be shown otherwise. But AFAIK, that's not the way it's done. > > >> > > >> See clone3 and struct clone_args. For system calls that take structs as arguments we use u64 in the struct for proper alignment so we can extend structs without regressing old kernels. We have a few of those extensible struct system calls. But we don't really have a lot system calls that pass u64 as a pointer outside of a structure so far. Neither as register and nor as pointer iirc. Passing them as a register arg is problematic because of 32bit arches. But passing as pointer should be fine but it is indeed uncommon. > > > > > > The struct is one thing. But actually, clone3 takes a `size_t`: > > > > > > SYSCALL_DEFINE2(clone3, struct clone_args __user *, uargs, size_t, size) > > > > > > I take from this that I too should use `size_t` rather than `unsigned > > > long.` And it doesn't seem like there's any compat clone3. > > > > But vgetrandom_alloc does not use unsigned long, but unsigned long *. > > You need to look at the contents for struct clone_args for comparison. > > Ah! I see what you mean; that's a good point. The usual register > clearing thing isn't going to happen because these are addresses. > > I still am somewhat hesitant, though, because `size_t` is really the > "proper" type to be used. Maybe the compat syscall thing is just a > necessary evil? We try to avoid adding new compat-requiring syscalls like the plague usually. (At least for new syscalls that don't need to inherit behavior from earlier syscalls they are a revisions of.) > > The other direction would be making this a u32, since 640k ought to be > enough for anybody and such, but maybe that'd be a mistake too. I think making this a size_t is fine. We haven't traditionally used u32 for sizes. All syscalls that pass structs versioned by size use size_t. So I would recommend to stick with that. Alternatively, you could also introduce a simple struct versioned by size for this system call similar to mount_setatt() and clone3() and so on. This way you don't need to worry about future extensibilty. Just a thought.