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=-6.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT 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 5D2B7ECE599 for ; Wed, 16 Oct 2019 22:12:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2AEBE21925 for ; Wed, 16 Oct 2019 22:12:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1571263931; bh=G+ewoKReLo8kaLUZAu05EM3mfMfbkjAxb+wWkYXUIVc=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=fpB+3oodDvqgUYj9IPp6gM2g1L+Xfyr3m9HK2xLOPc8CDWEczi+wHK+LeyGRFnjpf rd7/E2+3dr48DFZIkB9503/PiAsc20NImUDEi0WElF2Xty6i8q2KHQjRA/IfrsxJNV qZxJWdok3S3VXqprZ5slH4npVxbQ+FGIgeE3Tn7c= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2393151AbfJPWMJ (ORCPT ); Wed, 16 Oct 2019 18:12:09 -0400 Received: from mail.kernel.org ([198.145.29.99]:48214 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2406659AbfJPV4c (ORCPT ); Wed, 16 Oct 2019 17:56:32 -0400 Received: from localhost (unknown [192.55.54.58]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id C9EA021D7A; Wed, 16 Oct 2019 21:56:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1571262991; bh=G+ewoKReLo8kaLUZAu05EM3mfMfbkjAxb+wWkYXUIVc=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=F1l2K/Mx9gYyA04odaaxpIAPiuy415YwiN5xjnjZLVanUrvLka2O1UUjmoJOC1Fjs CfUWvPNIekB5WwbOOItt7dh67DVWAOxWZPgmVWlUHPz1VqveDHcuBhQIv7O9JvK2Nr q6+dBiJcQfRXM28NPaz7pA78U+zmElFSo3jdyFr0= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, "zhengbin (A)" , Al Viro Subject: [PATCH 4.14 58/65] Fix the locking in dcache_readdir() and friends Date: Wed, 16 Oct 2019 14:51:12 -0700 Message-Id: <20191016214839.027507782@linuxfoundation.org> X-Mailer: git-send-email 2.23.0 In-Reply-To: <20191016214756.457746573@linuxfoundation.org> References: <20191016214756.457746573@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Al Viro commit d4f4de5e5ef8efde85febb6876cd3c8ab1631999 upstream. There are two problems in dcache_readdir() - one is that lockless traversal of the list needs non-trivial cooperation of d_alloc() (at least a switch to list_add_rcu(), and probably more than just that) and another is that it assumes that no removal will happen without the directory locked exclusive. Said assumption had always been there, never had been stated explicitly and is violated by several places in the kernel (devpts and selinuxfs). * replacement of next_positive() with different calling conventions: it returns struct list_head * instead of struct dentry *; the latter is passed in and out by reference, grabbing the result and dropping the original value. * scan is under ->d_lock. If we run out of timeslice, cursor is moved after the last position we'd reached and we reschedule; then the scan continues from that place. To avoid livelocks between multiple lseek() (with cursors getting moved past each other, never reaching the real entries) we always skip the cursors, need_resched() or not. * returned list_head * is either ->d_child of dentry we'd found or ->d_subdirs of parent (if we got to the end of the list). * dcache_readdir() and dcache_dir_lseek() switched to new helper. dcache_readdir() always holds a reference to dentry passed to dir_emit() now. Cursor is moved to just before the entry where dir_emit() has failed or into the very end of the list, if we'd run out. * move_cursor() eliminated - it had sucky calling conventions and after fixing that it became simply list_move() (in lseek and scan_positives) or list_move_tail() (in readdir). All operations with the list are under ->d_lock now, and we do not depend upon having all file removals done with parent locked exclusive anymore. Cc: stable@vger.kernel.org Reported-by: "zhengbin (A)" Signed-off-by: Al Viro Signed-off-by: Greg Kroah-Hartman --- fs/libfs.c | 134 +++++++++++++++++++++++++++++++------------------------------ 1 file changed, 69 insertions(+), 65 deletions(-) --- a/fs/libfs.c +++ b/fs/libfs.c @@ -86,58 +86,47 @@ int dcache_dir_close(struct inode *inode EXPORT_SYMBOL(dcache_dir_close); /* parent is locked at least shared */ -static struct dentry *next_positive(struct dentry *parent, - struct list_head *from, - int count) +/* + * Returns an element of siblings' list. + * We are looking for th positive after

; if + * found, dentry is grabbed and passed to caller via *. + * If no such element exists, the anchor of list is returned + * and * is set to NULL. + */ +static struct list_head *scan_positives(struct dentry *cursor, + struct list_head *p, + loff_t count, + struct dentry **res) { - unsigned *seq = &parent->d_inode->i_dir_seq, n; - struct dentry *res; - struct list_head *p; - bool skipped; - int i; + struct dentry *dentry = cursor->d_parent, *found = NULL; -retry: - i = count; - skipped = false; - n = smp_load_acquire(seq) & ~1; - res = NULL; - rcu_read_lock(); - for (p = from->next; p != &parent->d_subdirs; p = p->next) { + spin_lock(&dentry->d_lock); + while ((p = p->next) != &dentry->d_subdirs) { struct dentry *d = list_entry(p, struct dentry, d_child); - if (!simple_positive(d)) { - skipped = true; - } else if (!--i) { - res = d; - break; + // we must at least skip cursors, to avoid livelocks + if (d->d_flags & DCACHE_DENTRY_CURSOR) + continue; + if (simple_positive(d) && !--count) { + spin_lock_nested(&d->d_lock, DENTRY_D_LOCK_NESTED); + if (simple_positive(d)) + found = dget_dlock(d); + spin_unlock(&d->d_lock); + if (likely(found)) + break; + count = 1; + } + if (need_resched()) { + list_move(&cursor->d_child, p); + p = &cursor->d_child; + spin_unlock(&dentry->d_lock); + cond_resched(); + spin_lock(&dentry->d_lock); } } - rcu_read_unlock(); - if (skipped) { - smp_rmb(); - if (unlikely(*seq != n)) - goto retry; - } - return res; -} - -static void move_cursor(struct dentry *cursor, struct list_head *after) -{ - struct dentry *parent = cursor->d_parent; - unsigned n, *seq = &parent->d_inode->i_dir_seq; - spin_lock(&parent->d_lock); - for (;;) { - n = *seq; - if (!(n & 1) && cmpxchg(seq, n, n + 1) == n) - break; - cpu_relax(); - } - __list_del(cursor->d_child.prev, cursor->d_child.next); - if (after) - list_add(&cursor->d_child, after); - else - list_add_tail(&cursor->d_child, &parent->d_subdirs); - smp_store_release(seq, n + 2); - spin_unlock(&parent->d_lock); + spin_unlock(&dentry->d_lock); + dput(*res); + *res = found; + return p; } loff_t dcache_dir_lseek(struct file *file, loff_t offset, int whence) @@ -153,17 +142,28 @@ loff_t dcache_dir_lseek(struct file *fil return -EINVAL; } if (offset != file->f_pos) { + struct dentry *cursor = file->private_data; + struct dentry *to = NULL; + struct list_head *p; + file->f_pos = offset; - if (file->f_pos >= 2) { - struct dentry *cursor = file->private_data; - struct dentry *to; - loff_t n = file->f_pos - 2; - - inode_lock_shared(dentry->d_inode); - to = next_positive(dentry, &dentry->d_subdirs, n); - move_cursor(cursor, to ? &to->d_child : NULL); - inode_unlock_shared(dentry->d_inode); + inode_lock_shared(dentry->d_inode); + + if (file->f_pos > 2) { + p = scan_positives(cursor, &dentry->d_subdirs, + file->f_pos - 2, &to); + spin_lock(&dentry->d_lock); + list_move(&cursor->d_child, p); + spin_unlock(&dentry->d_lock); + } else { + spin_lock(&dentry->d_lock); + list_del_init(&cursor->d_child); + spin_unlock(&dentry->d_lock); } + + dput(to); + + inode_unlock_shared(dentry->d_inode); } return offset; } @@ -185,25 +185,29 @@ int dcache_readdir(struct file *file, st { struct dentry *dentry = file->f_path.dentry; struct dentry *cursor = file->private_data; - struct list_head *p = &cursor->d_child; - struct dentry *next; - bool moved = false; + struct list_head *anchor = &dentry->d_subdirs; + struct dentry *next = NULL; + struct list_head *p; if (!dir_emit_dots(file, ctx)) return 0; if (ctx->pos == 2) - p = &dentry->d_subdirs; - while ((next = next_positive(dentry, p, 1)) != NULL) { + p = anchor; + else + p = &cursor->d_child; + + while ((p = scan_positives(cursor, p, 1, &next)) != anchor) { if (!dir_emit(ctx, next->d_name.name, next->d_name.len, d_inode(next)->i_ino, dt_type(d_inode(next)))) break; - moved = true; - p = &next->d_child; ctx->pos++; } - if (moved) - move_cursor(cursor, p); + spin_lock(&dentry->d_lock); + list_move_tail(&cursor->d_child, p); + spin_unlock(&dentry->d_lock); + dput(next); + return 0; } EXPORT_SYMBOL(dcache_readdir);