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=-11.6 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable 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 43C4AC433DF for ; Wed, 3 Jun 2020 10:24:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 157E120679 for ; Wed, 3 Jun 2020 10:24:54 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="JYh8geY1" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726955AbgFCKYx (ORCPT ); Wed, 3 Jun 2020 06:24:53 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46512 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726770AbgFCKYf (ORCPT ); Wed, 3 Jun 2020 06:24:35 -0400 Received: from mail-ej1-x643.google.com (mail-ej1-x643.google.com [IPv6:2a00:1450:4864:20::643]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 20CDBC08C5C0; Wed, 3 Jun 2020 03:24:35 -0700 (PDT) Received: by mail-ej1-x643.google.com with SMTP id l27so1606440ejc.1; Wed, 03 Jun 2020 03:24:35 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:reply-to:from:date:message-id :subject:to:cc; bh=ijpgz/H39QX77UCIJyYAxLfDtqADya0tOsEpKq3wQ6I=; b=JYh8geY1wQ0ZgJDu4N9TkLlJALYHfZIdjZLmNB7QaxY0k/oIh4MiSgR4mNToFAn/8i +PZsJXLzCOz2aeo0p4OXbPwsHCi6Uh3/a8JIorLa/luW8B3yVaxLWi/89KB8RcsZJaZK 4F4KV4b7mwfvONiMK5RVlYeoFODIUGIPbM/63LsKF/V4c4wnwLqNK6xw0HIUbThP7ZIl xlbcsjrId0JqWZ7PkbRdpiMjF8lEWlzfkJmlndloIW0LZwz754FpqRlXyKykB8EnFM4c 7X0A0qe7RHITIIMOJkHWvxT7wtMt+3I+MABcukqQvBeEyY/kLdgdmU20TEGaMq1tRN7j K2+A== 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:reply-to :from:date:message-id:subject:to:cc; bh=ijpgz/H39QX77UCIJyYAxLfDtqADya0tOsEpKq3wQ6I=; b=fTzBjWG3c5JoZxAqGAhcDeWl3tLXThJiksiBG8/mAQ8WtCowNIXdqX9kthJ5qPLOi5 45U1HhhNP1hhc/oxnCqIF90fz2a4REaPxv4rEAkrI9YMPN6JKCL8IWCQFbD3LWhc+92u uKYIhddWaFLFRwaQBr8OMUOUzpX0q4N1WdhD40Uys9ARLCqZ2GVcU7RATlVgDT0gMHR9 MKmXfal5mP/M8/k8jOkB2kZwyftJo6dtivXW4etBN7xTmsqMm/R/PIDiPRLBWvkdLDii /ecbKLfrvxmdjZuB1ZC5vmHok0bL/ySfGk1c0u5Xt2dFfbyAnPM1NYvaPK+/9bY1vKVz rE3g== X-Gm-Message-State: AOAM533aDQ7JM+XdErTV5IPNjwKnC1XLxF+v6z08vtRtOT0C59j6iVCX uzusAWHYcZkGmnSI0FpesM0kOIqYM66/5DCviGc= X-Google-Smtp-Source: ABdhPJyI2JagoGjNsl+fP1yfKx6yMIk9qDdtfqmxH9Fc3w3zOU7sysFdqsJkz4rzBeaFzAbfTJf0H6e/InhwmXhUE4g= X-Received: by 2002:a17:906:6d4b:: with SMTP id a11mr19511522ejt.108.1591179873683; Wed, 03 Jun 2020 03:24:33 -0700 (PDT) MIME-Version: 1.0 References: <20200602204219.186620-1-christian.brauner@ubuntu.com> <20200602204219.186620-2-christian.brauner@ubuntu.com> In-Reply-To: <20200602204219.186620-2-christian.brauner@ubuntu.com> Reply-To: mtk.manpages@gmail.com From: "Michael Kerrisk (man-pages)" Date: Wed, 3 Jun 2020 12:24:22 +0200 Message-ID: Subject: Re: [PATCH v5 1/3] open: add close_range() To: Christian Brauner Cc: Linus Torvalds , lkml , Kyle Evans , Victor Stinner , Alexander Viro , "linux-fsdevel@vger.kernel.org" , Linux API , Florian Weimer , Jann Horn , Oleg Nesterov , Arnd Bergmann , Shuah Khan , David Howells , "Dmitry V. Levin" Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Christian, Could we have a manual page for this API (best before it's merged)? Thanks, Michael On Tue, 2 Jun 2020 at 22:44, Christian Brauner wrote: > > This adds the close_range() syscall. It allows to efficiently close a range > of file descriptors up to all file descriptors of a calling task. > > I've also coordinated with some FreeBSD developers who got in touch with > me (Cced below). FreeBSD intends to add the same syscall once we merged it. > Quite a bunch of projects in userspace are waiting on this syscall > including Python and systemd. > > The syscall came up in a recent discussion around the new mount API and > making new file descriptor types cloexec by default. During this > discussion, Al suggested the close_range() syscall (cf. [1]). Note, a > syscall in this manner has been requested by various people over time. > > First, it helps to close all file descriptors of an exec()ing task. This > can be done safely via (quoting Al's example from [1] verbatim): > > /* that exec is sensitive */ > unshare(CLONE_FILES); > /* we don't want anything past stderr here */ > close_range(3, ~0U); > execve(....); > > The code snippet above is one way of working around the problem that file > descriptors are not cloexec by default. This is aggravated by the fact that > we can't just switch them over without massively regressing userspace. For > a whole class of programs having an in-kernel method of closing all file > descriptors is very helpful (e.g. demons, service managers, programming > language standard libraries, container managers etc.). > (Please note, unshare(CLONE_FILES) should only be needed if the calling > task is multi-threaded and shares the file descriptor table with another > thread in which case two threads could race with one thread allocating file > descriptors and the other one closing them via close_range(). For the > general case close_range() before the execve() is sufficient.) > > Second, it allows userspace to avoid implementing closing all file > descriptors by parsing through /proc//fd/* and calling close() on each > file descriptor. From looking at various large(ish) userspace code bases > this or similar patterns are very common in: > - service managers (cf. [4]) > - libcs (cf. [6]) > - container runtimes (cf. [5]) > - programming language runtimes/standard libraries > - Python (cf. [2]) > - Rust (cf. [7], [8]) > As Dmitry pointed out there's even a long-standing glibc bug about missing > kernel support for this task (cf. [3]). > In addition, the syscall will also work for tasks that do not have procfs > mounted and on kernels that do not have procfs support compiled in. In such > situations the only way to make sure that all file descriptors are closed > is to call close() on each file descriptor up to UINT_MAX or RLIMIT_NOFILE, > OPEN_MAX trickery (cf. comment [8] on Rust). > > The performance is striking. For good measure, comparing the following > simple close_all_fds() userspace implementation that is essentially just > glibc's version in [6]: > > static int close_all_fds(void) > { > int dir_fd; > DIR *dir; > struct dirent *direntp; > > dir = opendir("/proc/self/fd"); > if (!dir) > return -1; > dir_fd = dirfd(dir); > while ((direntp = readdir(dir))) { > int fd; > if (strcmp(direntp->d_name, ".") == 0) > continue; > if (strcmp(direntp->d_name, "..") == 0) > continue; > fd = atoi(direntp->d_name); > if (fd == dir_fd || fd == 0 || fd == 1 || fd == 2) > continue; > close(fd); > } > closedir(dir); > return 0; > } > > to close_range() yields: > 1. closing 4 open files: > - close_all_fds(): ~280 us > - close_range(): ~24 us > > 2. closing 1000 open files: > - close_all_fds(): ~5000 us > - close_range(): ~800 us > > close_range() is designed to allow for some flexibility. Specifically, it > does not simply always close all open file descriptors of a task. Instead, > callers can specify an upper bound. > This is e.g. useful for scenarios where specific file descriptors are > created with well-known numbers that are supposed to be excluded from > getting closed. > For extra paranoia close_range() comes with a flags argument. This can e.g. > be used to implement extension. Once can imagine userspace wanting to stop > at the first error instead of ignoring errors under certain circumstances. > There might be other valid ideas in the future. In any case, a flag > argument doesn't hurt and keeps us on the safe side. > > From an implementation side this is kept rather dumb. It saw some input > from David and Jann but all nonsense is obviously my own! > - Errors to close file descriptors are currently ignored. (Could be changed > by setting a flag in the future if needed.) > - __close_range() is a rather simplistic wrapper around __close_fd(). > My reasoning behind this is based on the nature of how __close_fd() needs > to release an fd. But maybe I misunderstood specifics: > We take the files_lock and rcu-dereference the fdtable of the calling > task, we find the entry in the fdtable, get the file and need to release > files_lock before calling filp_close(). > In the meantime the fdtable might have been altered so we can't just > retake the spinlock and keep the old rcu-reference of the fdtable > around. Instead we need to grab a fresh reference to the fdtable. > If my reasoning is correct then there's really no point in fancyfying > __close_range(): We just need to rcu-dereference the fdtable of the > calling task once to cap the max_fd value correctly and then go on > calling __close_fd() in a loop. > > /* References */ > [1]: https://lore.kernel.org/lkml/20190516165021.GD17978@ZenIV.linux.org.uk/ > [2]: https://github.com/python/cpython/blob/9e4f2f3a6b8ee995c365e86d976937c141d867f8/Modules/_posixsubprocess.c#L220 > [3]: https://sourceware.org/bugzilla/show_bug.cgi?id=10353#c7 > [4]: https://github.com/systemd/systemd/blob/5238e9575906297608ff802a27e2ff9effa3b338/src/basic/fd-util.c#L217 > [5]: https://github.com/lxc/lxc/blob/ddf4b77e11a4d08f09b7b9cd13e593f8c047edc5/src/lxc/start.c#L236 > [6]: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/grantpt.c;h=2030e07fa6e652aac32c775b8c6e005844c3c4eb;hb=HEAD#l17 > Note that this is an internal implementation that is not exported. > Currently, libc seems to not provide an exported version of this > because of missing kernel support to do this. > [7]: https://github.com/rust-lang/rust/issues/12148 > [8]: https://github.com/rust-lang/rust/blob/5f47c0613ed4eb46fca3633c1297364c09e5e451/src/libstd/sys/unix/process2.rs#L303-L308 > Rust's solution is slightly different but is equally unperformant. > Rust calls getdtablesize() which is a glibc library function that > simply returns the current RLIMIT_NOFILE or OPEN_MAX values. Rust then > goes on to call close() on each fd. That's obviously overkill for most > tasks. Rarely, tasks - especially non-demons - hit RLIMIT_NOFILE or > OPEN_MAX. > Let's be nice and assume an unprivileged user with RLIMIT_NOFILE set > to 1024. Even in this case, there's a very high chance that in the > common case Rust is calling the close() syscall 1021 times pointlessly > if the task just has 0, 1, and 2 open. > > Suggested-by: Al Viro > Signed-off-by: Christian Brauner > Cc: Arnd Bergmann > Cc: Kyle Evans > Cc: Jann Horn > Cc: David Howells > Cc: Dmitry V. Levin > Cc: Oleg Nesterov > Cc: Linus Torvalds > Cc: Florian Weimer > Cc: linux-api@vger.kernel.org > --- > /* v2 */ > - Linus Torvalds : > - add cond_resched() to yield cpu when closing a lot of file descriptors > - Al Viro : > - add cond_resched() to yield cpu when closing a lot of file descriptors > > /* v3 */ > unchanged > > /* v4 */ > - Oleg Nesterov : > - fix braino: s/max()/min()/ > > /* v5 */ > unchanged > --- > fs/file.c | 62 ++++++++++++++++++++++++++++++++++------ > fs/open.c | 20 +++++++++++++ > include/linux/fdtable.h | 2 ++ > include/linux/syscalls.h | 2 ++ > 4 files changed, 78 insertions(+), 8 deletions(-) > > diff --git a/fs/file.c b/fs/file.c > index abb8b7081d7a..e260bfe687d1 100644 > --- a/fs/file.c > +++ b/fs/file.c > @@ -10,6 +10,7 @@ > #include > #include > #include > +#include > #include > #include > #include > @@ -620,12 +621,9 @@ void fd_install(unsigned int fd, struct file *file) > > EXPORT_SYMBOL(fd_install); > > -/* > - * The same warnings as for __alloc_fd()/__fd_install() apply here... > - */ > -int __close_fd(struct files_struct *files, unsigned fd) > +static struct file *pick_file(struct files_struct *files, unsigned fd) > { > - struct file *file; > + struct file *file = NULL; > struct fdtable *fdt; > > spin_lock(&files->file_lock); > @@ -637,15 +635,63 @@ int __close_fd(struct files_struct *files, unsigned fd) > goto out_unlock; > rcu_assign_pointer(fdt->fd[fd], NULL); > __put_unused_fd(files, fd); > - spin_unlock(&files->file_lock); > - return filp_close(file, files); > > out_unlock: > spin_unlock(&files->file_lock); > - return -EBADF; > + return file; > +} > + > +/* > + * The same warnings as for __alloc_fd()/__fd_install() apply here... > + */ > +int __close_fd(struct files_struct *files, unsigned fd) > +{ > + struct file *file; > + > + file = pick_file(files, fd); > + if (!file) > + return -EBADF; > + > + return filp_close(file, files); > } > EXPORT_SYMBOL(__close_fd); /* for ksys_close() */ > > +/** > + * __close_range() - Close all file descriptors in a given range. > + * > + * @fd: starting file descriptor to close > + * @max_fd: last file descriptor to close > + * > + * This closes a range of file descriptors. All file descriptors > + * from @fd up to and including @max_fd are closed. > + */ > +int __close_range(struct files_struct *files, unsigned fd, unsigned max_fd) > +{ > + unsigned int cur_max; > + > + if (fd > max_fd) > + return -EINVAL; > + > + rcu_read_lock(); > + cur_max = files_fdtable(files)->max_fds; > + rcu_read_unlock(); > + > + /* cap to last valid index into fdtable */ > + max_fd = min(max_fd, (cur_max - 1)); > + while (fd <= max_fd) { > + struct file *file; > + > + file = pick_file(files, fd++); > + if (!file) > + continue; > + > + filp_close(file, files); > + cond_resched(); > + } > + > + return 0; > +} > + > /* > * variant of __close_fd that gets a ref on the file for later fput. > * The caller must ensure that filp_close() called on the file, and then > diff --git a/fs/open.c b/fs/open.c > index 719b320ede52..87e076e9e127 100644 > --- a/fs/open.c > +++ b/fs/open.c > @@ -1279,6 +1279,26 @@ SYSCALL_DEFINE1(close, unsigned int, fd) > return retval; > } > > +/** > + * close_range() - Close all file descriptors in a given range. > + * > + * @fd: starting file descriptor to close > + * @max_fd: last file descriptor to close > + * @flags: reserved for future extensions > + * > + * This closes a range of file descriptors. All file descriptors > + * from @fd up to and including @max_fd are closed. > + * Currently, errors to close a given file descriptor are ignored. > + */ > +SYSCALL_DEFINE3(close_range, unsigned int, fd, unsigned int, max_fd, > + unsigned int, flags) > +{ > + if (flags) > + return -EINVAL; > + > + return __close_range(current->files, fd, max_fd); > +} > + > /* > * This routine simulates a hangup on the tty, to arrange that users > * are given clean terminals at login time. > diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h > index f07c55ea0c22..fcd07181a365 100644 > --- a/include/linux/fdtable.h > +++ b/include/linux/fdtable.h > @@ -121,6 +121,8 @@ extern void __fd_install(struct files_struct *files, > unsigned int fd, struct file *file); > extern int __close_fd(struct files_struct *files, > unsigned int fd); > +extern int __close_range(struct files_struct *files, unsigned int fd, > + unsigned int max_fd); > extern int __close_fd_get_file(unsigned int fd, struct file **res); > > extern struct kmem_cache *files_cachep; > diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h > index 1815065d52f3..18fea399329b 100644 > --- a/include/linux/syscalls.h > +++ b/include/linux/syscalls.h > @@ -442,6 +442,8 @@ asmlinkage long sys_openat(int dfd, const char __user *filename, int flags, > asmlinkage long sys_openat2(int dfd, const char __user *filename, > struct open_how *how, size_t size); > asmlinkage long sys_close(unsigned int fd); > +asmlinkage long sys_close_range(unsigned int fd, unsigned int max_fd, > + unsigned int flags); > asmlinkage long sys_vhangup(void); > > /* fs/pipe.c */ > -- > 2.26.2 > -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/