From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753987AbXITUAL (ORCPT ); Thu, 20 Sep 2007 16:00:11 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752969AbXITTx3 (ORCPT ); Thu, 20 Sep 2007 15:53:29 -0400 Received: from e36.co.us.ibm.com ([32.97.110.154]:51421 "EHLO e36.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752453AbXITTxT (ORCPT ); Thu, 20 Sep 2007 15:53:19 -0400 Subject: [PATCH 06/25] elevate write count open()'d files To: akpm@osdl.org Cc: linux-kernel@vger.kernel.org, hch@infradead.org, Dave Hansen From: Dave Hansen Date: Thu, 20 Sep 2007 12:52:56 -0700 References: <20070920195249.852667D5@kernel> In-Reply-To: <20070920195249.852667D5@kernel> Message-Id: <20070920195256.EDA48D0F@kernel> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org This is the first really tricky patch in the series. It elevates the writer count on a mount each time a non-special file is opened for write. This is not completely apparent in the patch because the two if() conditions in may_open() above the mnt_want_write() call are, combined, equivalent to special_file(). There is also an elevated count around the vfs_create() call in open_namei(). The count needs to be kept elevated all the way into the may_open() call. Otherwise, when the write is dropped, a ro->rw transisition could occur. This would lead to having rw access on the newly created file, while the vfsmount is ro. That is bad. Some filesystems forego the use of normal vfs calls to create struct files. Make sure that these users elevate the mnt writer count because they will get __fput(), and we need to make sure they're balanced. Signed-off-by: Dave Hansen Acked-by: Christoph Hellwig --- lxc-dave/fs/file_table.c | 9 ++++++++- lxc-dave/fs/namei.c | 20 ++++++++++++++++---- lxc-dave/ipc/mqueue.c | 3 +++ 3 files changed, 27 insertions(+), 5 deletions(-) diff -puN fs/file_table.c~tricky-elevate-write-count-files-are-open-ed fs/file_table.c --- lxc/fs/file_table.c~tricky-elevate-write-count-files-are-open-ed 2007-09-20 12:16:11.000000000 -0700 +++ lxc-dave/fs/file_table.c 2007-09-20 12:16:11.000000000 -0700 @@ -194,6 +194,10 @@ int init_file(struct file *file, struct file->f_mapping = dentry->d_inode->i_mapping; file->f_mode = mode; file->f_op = fop; + if (mode & FMODE_WRITE) { + error = mnt_want_write(mnt); + WARN_ON(error); + } return error; } EXPORT_SYMBOL(init_file); @@ -231,8 +235,11 @@ void fastcall __fput(struct file *file) if (unlikely(S_ISCHR(inode->i_mode) && inode->i_cdev != NULL)) cdev_put(inode->i_cdev); fops_put(file->f_op); - if (file->f_mode & FMODE_WRITE) + if (file->f_mode & FMODE_WRITE) { put_write_access(inode); + if (!special_file(inode->i_mode)) + mnt_drop_write(mnt); + } put_pid(file->f_owner.pid); file_kill(file); file->f_path.dentry = NULL; diff -puN fs/namei.c~tricky-elevate-write-count-files-are-open-ed fs/namei.c --- lxc/fs/namei.c~tricky-elevate-write-count-files-are-open-ed 2007-09-20 12:16:11.000000000 -0700 +++ lxc-dave/fs/namei.c 2007-09-20 12:16:11.000000000 -0700 @@ -1595,8 +1595,15 @@ int may_open(struct nameidata *nd, int a return -EACCES; flag &= ~O_TRUNC; - } else if (IS_RDONLY(inode) && (flag & FMODE_WRITE)) - return -EROFS; + } else if (flag & FMODE_WRITE) { + /* + * effectively: !special_file() + * balanced by __fput() + */ + error = mnt_want_write(nd->mnt); + if (error) + return error; + } error = vfs_permission(nd, acc_mode); if (error) @@ -1739,14 +1746,17 @@ do_last: } if (IS_ERR(nd->intent.open.file)) { - mutex_unlock(&dir->d_inode->i_mutex); error = PTR_ERR(nd->intent.open.file); - goto exit_dput; + goto exit_mutex_unlock; } /* Negative dentry, just create the file */ if (!path.dentry->d_inode) { + error = mnt_want_write(nd->mnt); + if (error) + goto exit_mutex_unlock; error = open_namei_create(nd, &path, flag, mode); + mnt_drop_write(nd->mnt); if (error) goto exit; return 0; @@ -1784,6 +1794,8 @@ ok: goto exit; return 0; +exit_mutex_unlock: + mutex_unlock(&dir->d_inode->i_mutex); exit_dput: dput_path(&path, nd); exit: diff -puN ipc/mqueue.c~tricky-elevate-write-count-files-are-open-ed ipc/mqueue.c --- lxc/ipc/mqueue.c~tricky-elevate-write-count-files-are-open-ed 2007-09-20 12:16:11.000000000 -0700 +++ lxc-dave/ipc/mqueue.c 2007-09-20 12:16:11.000000000 -0700 @@ -686,6 +686,9 @@ asmlinkage long sys_mq_open(const char _ goto out; filp = do_open(dentry, oflag); } else { + error = mnt_want_write(mqueue_mnt); + if (error) + goto out; filp = do_create(mqueue_mnt->mnt_root, dentry, oflag, mode, u_attr); } _