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=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 BB806C433E1 for ; Tue, 2 Jun 2020 23:31:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A20FD2067B for ; Tue, 2 Jun 2020 23:31:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728404AbgFBXbC (ORCPT ); Tue, 2 Jun 2020 19:31:02 -0400 Received: from albireo.enyo.de ([37.24.231.21]:55286 "EHLO albireo.enyo.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726809AbgFBXbC (ORCPT ); Tue, 2 Jun 2020 19:31:02 -0400 Received: from [172.17.203.2] (helo=deneb.enyo.de) by albireo.enyo.de with esmtps (TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256) id 1jgGN0-0005GC-1l; Tue, 02 Jun 2020 23:30:58 +0000 Received: from fw by deneb.enyo.de with local (Exim 4.92) (envelope-from ) id 1jgGMz-0001Se-VB; Wed, 03 Jun 2020 01:30:57 +0200 From: Florian Weimer To: Christian Brauner Cc: torvalds@linux-foundation.org, linux-kernel@vger.kernel.org, Kyle Evans , Victor Stinner , viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, fweimer@redhat.com, jannh@google.com, oleg@redhat.com, arnd@arndb.de, shuah@kernel.org, dhowells@redhat.com, ldv@altlinux.org Subject: Re: [PATCH v5 1/3] open: add close_range() References: <20200602204219.186620-1-christian.brauner@ubuntu.com> <20200602204219.186620-2-christian.brauner@ubuntu.com> Date: Wed, 03 Jun 2020 01:30:57 +0200 In-Reply-To: <20200602204219.186620-2-christian.brauner@ubuntu.com> (Christian Brauner's message of "Tue, 2 Jun 2020 22:42:17 +0200") Message-ID: <87d06hdozy.fsf@mid.deneb.enyo.de> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Christian Brauner: > 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; > } > > [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. Just to be clear, this code is not compiled into glibc anymore in typical configurations. I have posted a patch to turn grantpt into a no-op: I'm not entirely convinced that it's safe to keep iterating over /proc/self/fd while also closing descriptors. Ideally, I think an application should call getdents64, process the file names for descriptors in the buffer, and if any have been closed, seek to zero before the next getdents64 call. Maybe procfs is different, but with other file systems, unlinking files can trigger directory reordering, and then you get strange effects. The d_ino behavior for /proc/self/fd is a bit strange as well (it's not consistently descriptor plus 3).