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

[Jan Kara]

On Tue 19-02-13 19:47:30, Li Zefan wrote:
> 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.
  But returning error and advancing f_pos would be a bug - specification
says write() returns the number of bytes written or -1 and f_pos should be
advanced by the number of bytes written.

> >   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.
  Yes, that might be a useful test to add.

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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