From: Li Zefan Subject: Re: [RFC][PATCH] vfs: always protect diretory file->fpos with inode mutex Date: Tue, 19 Feb 2013 19:47:30 +0800 Message-ID: <51236652.1050608@huawei.com> References: <5122D3E0.6070800@huawei.com> <20130219091931.GB21945@quack.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit Cc: , LKML , Ext4 Developers List , "Theodore Ts'o" , Andrew Morton , , Wuqixuan , Al Viro , To: Jan Kara Return-path: In-Reply-To: <20130219091931.GB21945@quack.suse.cz> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On 2013/2/19 17:19, Jan Kara wrote: > 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(...); I thought about this. The problem is then we have to check every fop->write() to see if any of them can return -errno with file->f_pos changed and fix them, though it's do-able. > 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. Yeah, seems ceph is the only filesystem that allows read() on directories. > >> 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. > One of my concern is, concurrent lseek() and readdir() doesn't seem to be well tested. I'll add a test case in xfstests.