Re: [RFC PATCH] ext4: increase the protection of drop nlink and ext4 inode destroy

[Theodore Ts'o]

On Fri, Jan 13, 2017 at 03:42:19AM +0000, Al Viro wrote:
> On Thu, Jan 12, 2017 at 12:03:28PM -0500, Theodore Ts'o wrote:
> > On Thu, Jan 12, 2017 at 04:00:16PM +0800, zhangyi (F) wrote:
> > > 
> > > At the same time, I think other file systems may have the same problem, do
> > > you think we should put these detections on the VFS layer? Thus other file
> > > systems no need to do the same things, but the disadvantage is that we can
> > > not call ext4_error to report ext4 inconsistency.
> > 
> > There are file systems which don't have inodes per-se where the
> > i_nlinks could be a something which is simulated by the file system.
> > So it's not *necessarily* an on-disk inconsistency.
> > 
> > We'll have to see if Al and other file system developers are
> > agreeable, but one thing that we could do is to do the detection in
> > the VFS layer (which it is actually easier to do), and if they find an
> > issue, they can just pass a report via a callback function found in
> > the struct_operations structure.  If there isn't such a function
> > defined, or the function returns 0, the VFS could just do nothing; if
> > it returns an error code, then that would get reflected back up to
> > userspace, plus whatever other action the file system sees fit to do.
> 
> 	Detection of what?  Zero ->i_nlink on inode of dentry that passes e.g.
> may_delete()?

Or other impossible cases where there is a valid dentry pointing at an
inode with zero i_nlink.  I am fairly sure this should **never**
happen in the case of unlink(2), rmdir(2), or by the time we call
file_ops->open(), and if it does, it indicates that the underlying
on-disk file system (at least for ext4) is corrupt.

Am I missing a case?  And do you have an opinion about whether we
should be trying to do this check at the VFS layer versus inside ext4?

Thanks,

					- Ted