Re: [RFC 1/1] shiftfs: uid/gid shifting bind mount

From: Vivek Goyal
Date: Fri Feb 17 2017 - 15:28:03 EST

On Fri, Feb 17, 2017 at 05:51:18PM +0000, Al Viro wrote:
> On Fri, Feb 17, 2017 at 09:24:40AM -0800, James Bottomley wrote:
> > > What happens when somebody comes along and creates the damn thing on
> > > the underlying fs? _Not_ via your code, that is - using the
> > > underlying fs mounted elsewhere.
> >
> > Point taken. This, I think fixes the dcache revalidation issue.
> No, it doesn't. Consider a local filesystem. Those do not have any
> ->d_revalidate() - the kernel bloody well knows what happens to
> directories. If e.g. a previously absent file gets created, it's
> been done by the kernel itself and dentry has been made positive; if
> a previously existing file has been removed, dentry has either become
> negative or, if it had been pinned (e.g. file was opened at the time,
> or your code had been holding a reference to it, etc.) it will be unhashed
> so that new lookups won't find it, etc. No need to revalidate anything.
> Now, consider your code. You've done a lookup in the underlying fs.
> It has, at the time, come negative, so you have your (negative) dentry
> pointing to that on the underlying fs. If somebody comes and does
> e.g. mkdir() via your fs, it will call vfs_mkdir() on the underlying
> sucker, hopefully turning it positive and associate a new in-core inode
> with your previously negative dentry. But what happens if mkdir is done
> via underlying fs, or via another instance of yours over the same tree?
> Underlying dentry goes positive; yours is still negative. The underlying
> fs either doesn't have ->d_revalidate() or, if there is one it says that
> the underlying dentry is valid, thank you very much, no need to invalidate
> anything.
> In other words, your patch does nothing for object getting created.

I thought assumption here is that underlying subtree is not changed
outside of shiftfs. IIUC, overlayfs has the same assumption.

Two shiftfs instances writing to same dir will be a problem though.