Re: [RFC][PATCH] vfs: always protect diretory file->fpos withinode mutex

[Jan Kara]

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 <jack@xxxxxxx>
SUSE Labs, CR
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