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

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Message-ID: <20170217175118.GF29622@ZenIV.linux.org.uk>
Date:   Fri, 17 Feb 2017 17:51:18 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     James Bottomley <James.Bottomley@...senPartnership.com>
Cc:     Djalal Harouni <tixxdz@...il.com>, Chris Mason <clm@...com>,
        Theodore Tso <tytso@....edu>,
        Josh Triplett <josh@...htriplett.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        Seth Forshee <seth.forshee@...onical.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Dongsu Park <dongsu@...ocode.com>,
        David Herrmann <dh.herrmann@...glemail.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Alban Crequy <alban.crequy@...il.com>,
        "Serge E. Hallyn" <serge@...lyn.com>, Phil Estes <estesp@...il.com>
Subject: Re: [RFC 1/1] shiftfs: uid/gid shifting bind mount

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.