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=-15.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 CB7CFC43461 for ; Mon, 19 Apr 2021 13:56:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A72E461246 for ; Mon, 19 Apr 2021 13:56:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241047AbhDSN4f (ORCPT ); Mon, 19 Apr 2021 09:56:35 -0400 Received: from mx2.suse.de ([195.135.220.15]:53596 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S241094AbhDSN4W (ORCPT ); Mon, 19 Apr 2021 09:56:22 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 09EEFAFC9; Mon, 19 Apr 2021 13:55:51 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id C29471F2C6A; Mon, 19 Apr 2021 15:55:50 +0200 (CEST) Date: Mon, 19 Apr 2021 15:55:50 +0200 From: Jan Kara To: Christian Brauner Cc: Matthew Bobrowski , jack@suse.cz, amir73il@gmail.com, linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH 2/2] fanotify: Add pidfd support to the fanotify API Message-ID: <20210419135550.GH8706@quack2.suse.cz> References: <20210419132020.ydyb2ly6e3clhe2j@wittgenstein> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210419132020.ydyb2ly6e3clhe2j@wittgenstein> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org On Mon 19-04-21 15:20:20, Christian Brauner wrote: > On Fri, Apr 16, 2021 at 09:22:25AM +1000, Matthew Bobrowski wrote: > > Introduce a new flag FAN_REPORT_PIDFD for fanotify_init(2) which > > allows userspace applications to control whether a pidfd is to be > > returned instead of a pid for `struct fanotify_event_metadata.pid`. > > > > FAN_REPORT_PIDFD is mutually exclusive with FAN_REPORT_TID as the > > pidfd API is currently restricted to only support pidfd generation for > > thread-group leaders. Attempting to set them both when calling > > fanotify_init(2) will result in -EINVAL being returned to the > > caller. As the pidfd API evolves and support is added for tids, this > > is something that could be relaxed in the future. > > > > If pidfd creation fails, the pid in struct fanotify_event_metadata is > > set to FAN_NOPIDFD(-1). Falling back and providing a pid instead of a > > pidfd on pidfd creation failures was considered, although this could > > possibly lead to confusion and unpredictability within userspace > > applications as distinguishing between whether an actual pidfd or pid > > was returned could be difficult, so it's best to be explicit. > > > > Signed-off-by: Matthew Bobrowski > > --- > > fs/notify/fanotify/fanotify_user.c | 33 +++++++++++++++++++++++++++--- > > include/linux/fanotify.h | 2 +- > > include/uapi/linux/fanotify.h | 2 ++ > > 3 files changed, 33 insertions(+), 4 deletions(-) > > > > diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c > > index 9e0c1afac8bd..fd8ae88796a8 100644 > > --- a/fs/notify/fanotify/fanotify_user.c > > +++ b/fs/notify/fanotify/fanotify_user.c > > @@ -329,7 +329,7 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group, > > struct fanotify_info *info = fanotify_event_info(event); > > unsigned int fid_mode = FAN_GROUP_FLAG(group, FANOTIFY_FID_BITS); > > struct file *f = NULL; > > - int ret, fd = FAN_NOFD; > > + int ret, pidfd, fd = FAN_NOFD; > > int info_type = 0; > > > > pr_debug("%s: group=%p event=%p\n", __func__, group, event); > > @@ -340,7 +340,25 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group, > > metadata.vers = FANOTIFY_METADATA_VERSION; > > metadata.reserved = 0; > > metadata.mask = event->mask & FANOTIFY_OUTGOING_EVENTS; > > - metadata.pid = pid_vnr(event->pid); > > + > > + if (FAN_GROUP_FLAG(group, FAN_REPORT_PIDFD) && > > + pid_has_task(event->pid, PIDTYPE_TGID)) { > > + /* > > + * Given FAN_REPORT_PIDFD is to be mutually exclusive with > > + * FAN_REPORT_TID, panic here if the mutual exclusion is ever > > + * blindly lifted without pidfds for threads actually being > > + * supported. > > + */ > > + WARN_ON(FAN_GROUP_FLAG(group, FAN_REPORT_TID)); > > + > > + pidfd = pidfd_create(event->pid, 0); > > + if (unlikely(pidfd < 0)) > > + metadata.pid = FAN_NOPIDFD; > > + else > > + metadata.pid = pidfd; > > I'm not a fan of overloading fields (Yes, we did this for the _legacy_ > clone() syscall for CLONE_PIDFD/CLONE_PARENT_SETTID but in general it's > never a good idea if there are other options, imho.). > Could/should we consider the possibility of adding a new pidfd field to > struct fanotify_event_metadata? I'm not a huge fan of overloading fields either but in this particular case I'm fine with that because: a) storage size & type matches b) it describes exactly the same information, just in a different way It is not possible to store the pidfd elsewhere in fanotify_event_metadata. But it is certainly possible to use extended event info to return pidfd instead - similarly to how we return e.g. handle + fsid for some notification groups. It just means somewhat longer events and more complicated parsing of structured events in userspace. But as I write above, in this case I don't think it is worth it - only if we think that returning both pid and pidfd could ever be useful. Honza -- Jan Kara SUSE Labs, CR