From: Jan Kara Subject: Re: [RFC][PATCH] vfs: always protect diretory file->fpos with inode mutex Date: Tue, 19 Feb 2013 10:19:31 +0100 Message-ID: <20130219091931.GB21945@quack.suse.cz> References: <5122D3E0.6070800@huawei.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-fsdevel@vger.kernel.org, LKML , Ext4 Developers List , Jan Kara , Theodore Ts'o , Andrew Morton , andi@firstfloor.org, Wuqixuan , Al Viro , gregkh@linuxfoundation.org To: Li Zefan Return-path: Content-Disposition: inline In-Reply-To: <5122D3E0.6070800@huawei.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Tue 19-02-13 09:22:40, Li Zefan wrote: > There's a long long-standing bug...As long as I don't know when it dates > from. > > I've written and attached a simple program to reproduce this bug, and it can > immediately trigger the bug in my box. It uses two threads, one keeps calling > read(), and the other calling readdir(), both on the same directory fd. So the fact that read() or even write() to fd opened O_RDONLY has *any* effect on f_pos looks really unexpected to me. I think we really should have there: if (ret >= 0) file_pos_write(...); That would solve problems with read() and write() on directories for pretty much every filesystem since the first usually returns -EISDIR and the second -EBADF. > When I ran it on ext3 (can be replaced with ext2/ext4) which has _dir_index_ > feature disabled, I got this: > > EXT3-fs error (device loop1): ext3_readdir: bad entry in directory #34817: rec_len is smaller than minimal - offset=993, inode=0, rec_len=0, name_len=0 > EXT3-fs error (device loop1): ext3_readdir: bad entry in directory #34817: rec_len is smaller than minimal - offset=1009, inode=0, rec_len=0, name_len=0 > EXT3-fs error (device loop1): ext3_readdir: bad entry in directory #34817: rec_len is smaller than minimal - offset=993, inode=0, rec_len=0, name_len=0 > EXT3-fs error (device loop1): ext3_readdir: bad entry in directory #34817: rec_len is smaller than minimal - offset=1009, inode=0, rec_len=0, name_len=0 > ... > > If we configured errors=remount-ro, the filesystem will become read-only. > > SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, size_t, count) > { > ... > loff_t pos = file_pos_read(file); > ret = vfs_read(file, buf, count, &pos); > file_pos_write(file, pos); > fput_light(file, fput_needed); > ... > } > > While readdir() is protected with i_mutex, f_pos can be changed without > any locking in various read()/write() syscalls, which leads to this bug. > > What makes things worse is Andi removed i_mutex from generic_file_llseek, > so you can trigger the same bug by replacing read() with lseek() in the > test program. Yes, and here I'd say it's a filesystem issue. If filesystem needs f_pos changed only under i_mutex, it should use default_llseek() or get the mutex itself. That's what the callback is for. We shouldn't unnecessarily impose the i_mutex restriction on llseek on a directory for every filesystem. Honza -- Jan Kara SUSE Labs, CR