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 E505DC4332F for ; Thu, 19 May 2022 18:59:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243590AbiESS7F (ORCPT ); Thu, 19 May 2022 14:59:05 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54352 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236387AbiESS7E (ORCPT ); Thu, 19 May 2022 14:59:04 -0400 Received: from mail-il1-x133.google.com (mail-il1-x133.google.com [IPv6:2607:f8b0:4864:20::133]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A07EAA88BB for ; Thu, 19 May 2022 11:59:02 -0700 (PDT) Received: by mail-il1-x133.google.com with SMTP id l15so4280062ilh.3 for ; Thu, 19 May 2022 11:59:02 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=WtXD347NgGS/+YijWWiRuBafqv4C6u9VlK5CEu8TAEA=; b=eQti3jrkdY7scusFOxnuKhJ5Vf4Z+DDvEqC+qVkSzJirkfUkUo5yo7/PzJFKP1AnQd EOXByCNcv+7E3dhF7/eAZlUtc4Ds73cawnE2aCgg30dI7mHPxjhBBOoH1YSJQFqNtOr1 R6LZoXW729dcASQMsmmF3k/+wo+sYd4kz14Sa2qe+rZR083fhb8vmsk7nO+eI7I5+46r B1WqKIZ6hq+/H8JAv3J/ab4RVaL/KfqCAbC/76TQsnKpljZd0qDTXT5ChI6MnY24iwgb lnWLpnhJC+QTbOm2LnMQ7SPKSoLPf/ImHkqspMRV5ORcKQKsElyIaKIG7d4X6ohSIUeO KuBw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=WtXD347NgGS/+YijWWiRuBafqv4C6u9VlK5CEu8TAEA=; b=rhgvQ+KS7dF0VNIsMBiWUWCCnAxTzoG3fHdzj88R+HBnqca7OBak02hoK/U1VTjHs7 cxuSaWpqgPB+wIVLJg4/IsA9YDEO+2PWsAzVoXakIlb3hboVOiWMeoYJ1LT7oIbzv0Kp Me/APNKfriO/vzm+AL90yAJ4isfY6IBShkthXc8S02Ys6d6WxNhw6+qZCM0LtVQKy5Jo WVR25SyPKGPbmcRszFvHEuaAJsO9/478wB6VLrHVCbDDyGlxpvmgEGXHHM4UnSbghxR+ 7EMUx8OqgMmoMcFT0brPgEvrfvwp1FTlbTsu7aIfeqbbQRO9gp3RytpGcn8Z13iNWUX8 1/7A== X-Gm-Message-State: AOAM5304QBVrsq3ld00/Yt4um+fHZkBc5ZHU1WmutQxrtZdlPHwt44mk hXnC/DCTxHMcUbA1I2FQxDOlt7tqyjyHTzc2jxMH/Q== X-Google-Smtp-Source: ABdhPJxroZKPAtKdyjG/WvTdRj8QijwwaYZoKH+fVYQqNh/ZqMiSWs7lA9A9PN2uIxTVX6Z0QHBF2y1TIkugRbTRbhE= X-Received: by 2002:a92:da0c:0:b0:2cf:9e8e:d9d8 with SMTP id z12-20020a92da0c000000b002cf9e8ed9d8mr3662939ilm.192.1652986741747; Thu, 19 May 2022 11:59:01 -0700 (PDT) MIME-Version: 1.0 References: <20220422212945.2227722-1-axelrasmussen@google.com> <20220422212945.2227722-5-axelrasmussen@google.com> In-Reply-To: From: Axel Rasmussen Date: Thu, 19 May 2022 11:58:25 -0700 Message-ID: Subject: Re: [PATCH v2 4/6] userfaultfd: update documentation to describe /dev/userfaultfd To: Shuah Khan Cc: Alexander Viro , Andrew Morton , Charan Teja Reddy , Dave Hansen , "Dmitry V . Levin" , Gleb Fotengauer-Malinovskiy , Hugh Dickins , Jan Kara , Jonathan Corbet , Mel Gorman , Mike Kravetz , Mike Rapoport , Nadav Amit , Peter Xu , Shuah Khan , Suren Baghdasaryan , Vlastimil Babka , zhangyi , linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org, LKML , Linux MM , Linuxkselftest Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Tue, Apr 26, 2022 at 9:46 AM Shuah Khan wrote: > > On 4/22/22 3:29 PM, Axel Rasmussen wrote: > > Explain the different ways to create a new userfaultfd, and how access > > control works for each way. > > > > Signed-off-by: Axel Rasmussen > > --- > > Documentation/admin-guide/mm/userfaultfd.rst | 38 ++++++++++++++++++-- > > Documentation/admin-guide/sysctl/vm.rst | 3 ++ > > 2 files changed, 39 insertions(+), 2 deletions(-) > > > > diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/admin-guide/mm/userfaultfd.rst > > index 6528036093e1..4c079b5377d4 100644 > > --- a/Documentation/admin-guide/mm/userfaultfd.rst > > +++ b/Documentation/admin-guide/mm/userfaultfd.rst > > @@ -17,7 +17,10 @@ of the ``PROT_NONE+SIGSEGV`` trick. > > Design > > ====== > > > > -Userfaults are delivered and resolved through the ``userfaultfd`` syscall. > > Please keep this sentence in there and rephrase it to indicate how it was > done in the past. > > Also explain here why this new approach is better than the syscall approach > before getting into the below details. Hmm, so the old sentence I think was incorrect already. Notifications of *the faults* aren't delivered and resolved through the syscall. Rather, the syscall just gives you a file descriptor, and then notification / resolution of faults happens though the file descriptor, not through the syscall. So I think it needs to be reworded in any case. I think the overall structure of the doc as-is makes the most sense as well - first explain how this will be used at a very high level, and then go into the details (first how to create a userfaultfd, then how to use it). So, in the end I reworded the "Creating a userfaultfd" section, to cover the two things you mentioned: - Which is the "older" way and which is the "newer" way - What the benefit of the newer way is Hopefully this addresses the comment? I can tweak it more if needed. In any case, thanks for taking a look at this series! > > > +Userspace creates a new userfaultfd, initializes it, and registers one or more > > +regions of virtual memory with it. Then, any page faults which occur within the > > +region(s) result in a message being delivered to the userfaultfd, notifying > > +userspace of the fault. > > > > The ``userfaultfd`` (aside from registering and unregistering virtual > > memory ranges) provides two primary functionalities: > > @@ -39,7 +42,7 @@ Vmas are not suitable for page- (or hugepage) granular fault tracking > > when dealing with virtual address spaces that could span > > Terabytes. Too many vmas would be needed for that.> > > -The ``userfaultfd`` once opened by invoking the syscall, can also be > > +The ``userfaultfd``, once created, can also be > > This is sentence is too short and would look odd. Combine the sentences > so it renders well in the generated doc. Not 100% sure I understood the concern, but I do think it makes sense to move "Vmas are not suitable ..." up into the same paragraph with the other sentence about scalability. I'll do this in v3 as it looks a bit nicer. This leaves the "The userfaultfd, once created, ..." part alone, though. I think s/once opened by invoking the syscall/once created/ is correct, since there are now various ways to create it. I also think that second comma technically should have been there even in the previous version. > > > passed using unix domain sockets to a manager process, so the same > > manager process could handle the userfaults of a multitude of > > different processes without them being aware about what is going on > > @@ -50,6 +53,37 @@ is a corner case that would currently return ``-EBUSY``). > > API > > === > > > > +Creating a userfaultfd > > +---------------------- > > + > > +There are two mechanisms to create a userfaultfd. There are various ways to > > +restrict this too, since userfaultfds which handle kernel page faults have > > +historically been a useful tool for exploiting the kernel. > > + > > +The first is the userfaultfd(2) syscall. Access to this is controlled in several > > +ways: > > + > > +- By default, the userfaultfd will be able to handle kernel page faults. This > > + can be disabled by passing in UFFD_USER_MODE_ONLY. > > + > > +- If vm.unprivileged_userfaultfd is 0, then the caller must *either* have > > + CAP_SYS_PTRACE, or pass in UFFD_USER_MODE_ONLY. > > + > > +- If vm.unprivileged_userfaultfd is 1, then no particular privilege is needed to > > + use this syscall, even if UFFD_USER_MODE_ONLY is *not* set. > > + > > +Alternatively, userfaultfds can be created by opening /dev/userfaultfd, and > > +issuing a USERFAULTFD_IOC_NEW ioctl to this device. Access to this device is > > New ioctl? I thought we are moving away from using ioctls? Hmm, looking at alternatives [1] am not sure I see a viable one: We could have defined a new "userfaultfdfs" filesystem, but it seems to me to be overkill for this feature. We could have used a syscall instead and supported fine-grained access control with a new capability, but this approach was rejected [2] generally because we prefer to avoid adding capabilities, and this new capability's scope (just userfaultfd) was considered too narrow. So, I'm not sure of another better way to do this. I suppose one could argue that the dislike of ioctls outweighs the usefulness of this feature, but to me at least the tradeoff seems worth it. :) [1]: https://www.kernel.org/doc/html/latest/driver-api/ioctl.html#alternatives-to-ioctl [2]: https://lkml.org/lkml/2022/2/24/1012 > > > +controlled via normal filesystem permissions (user/group/mode for example) - no > > +additional permission (capability/sysctl) is needed to be able to handle kernel > > +faults this way. This is useful because it allows e.g. a specific user or group > > +to be able to create kernel-fault-handling userfaultfds, without allowing it > > +more broadly, or granting more privileges in addition to that particular ability > > +(CAP_SYS_PTRACE). In other words, it allows permissions to be minimized. > > + > > +Initializing up a userfaultfd > > +------------------------ > > + > > This will generate doc warn very likley - extend the dashes to the > entire length of the subtitle. I'll fix this in v3. > > > When first opened the ``userfaultfd`` must be enabled invoking the > > ``UFFDIO_API`` ioctl specifying a ``uffdio_api.api`` value set to ``UFFD_API`` (or > > a later API version) which will specify the ``read/POLLIN`` protocol > > diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst > > index f4804ce37c58..8682d5fbc8ea 100644 > > --- a/Documentation/admin-guide/sysctl/vm.rst > > +++ b/Documentation/admin-guide/sysctl/vm.rst > > @@ -880,6 +880,9 @@ calls without any restrictions. > > > > The default value is 0. > > > > +An alternative to this sysctl / the userfaultfd(2) syscall is to create > > +userfaultfds via /dev/userfaultfd. See > > +Documentation/admin-guide/mm/userfaultfd.rst. > > > > user_reserve_kbytes > > =================== > > > > thanks, > -- Shuah