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

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Message-ID: <1487260590.2944.21.camel@HansenPartnership.com>
Date:   Thu, 16 Feb 2017 07:56:30 -0800
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Vivek Goyal <vgoyal@...hat.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>,
        Al Viro <viro@...IV.linux.org.uk>,
        "Serge E. Hallyn" <serge@...lyn.com>, Phil Estes <estesp@...il.com>
Subject: Re: [RFC 1/1] shiftfs: uid/gid shifting bind mount

On Wed, 2017-02-15 at 15:34 -0500, Vivek Goyal wrote:
> On Sat, Feb 04, 2017 at 11:19:32AM -0800, James Bottomley wrote:
> 
> [..]
> > +static struct dentry *shiftfs_lookup(struct inode *dir, struct
> > dentry *dentry,
> > +				     unsigned int flags)
> > +{
> > +	struct dentry *real = dir->i_private, *new;
> > +	struct inode *reali = real->d_inode, *newi;
> > +	const struct cred *oldcred, *newcred;
> > +
> > +	inode_lock(reali);
> > +	oldcred = shiftfs_new_creds(&newcred, dentry->d_sb);
> > +	new = lookup_one_len(dentry->d_name.name, real, dentry
> > ->d_name.len);
> > +	shiftfs_old_creds(oldcred, &newcred);
> > +	inode_unlock(reali);
> > +
> > +	if (IS_ERR(new))
> > +		return new;
> > +
> > +	dentry->d_fsdata = new;
> > +
> > +	if (!new->d_inode)
> > +		return NULL;
> > +
> > +	newi = shiftfs_new_inode(dentry->d_sb, new->d_inode
> > ->i_mode, new);
> > +	if (!newi) {
> > +		dput(new);
> > +		return ERR_PTR(-ENOMEM);
> > +	}
> > +
> > +	d_splice_alias(newi, dentry);
> 
> Hi James,
> 
> Should it be "return d_splice_alias()" so that if we find an alias it 
> is returned back to caller and passed in dentry can be freed. Though 
> I don't know in what cases alias can be found. And if alias is found 
> how do we make sure alias_dentry->d_fsdata is pointing to new (real
> dentry).

It probably should be for the sake of the pattern.  In our case I don't
think we can have any root aliases because the root dentry is always
pinned in the cache, so cache lookup should always find it.

James