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Date: Wed, 1 Jun 2016 03:40:00 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Djalal Harouni <tixxdz@gmail.com>, Chris Mason <clm@fb.com>,
	tytso@mit.edu, Serge Hallyn <serge.hallyn@canonical.com>,
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Subject: Re: [PATCH 1/1] shiftfs: uid/gid shifting bind mount
Message-ID: <20160601023959.GF14480@ZenIV.linux.org.uk>
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On Tue, May 31, 2016 at 08:31:33PM -0400, 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;
> +
> +	/* note: violation of usual fs rules here: dentries are never
> +	 * added with d_add.  This is because we want no dentry cache
> +	 * for shiftfs.  All lookups proceed through the dentry cache
> +	 * of the underlying filesystem, meaning we always see any
> +	 * changes in the underlying */
> +
> +	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);
> +
> +	return NULL;

This is utter crap.  First of all, d_splice_alias() *WILL* hash them, so you
get all the coherency problems, in spades.  Moreover, if you did manage to
avoid hashing, you would get something absolutely unusable.
	* no mounting of anything on top of that thing
	* performance shot to hell
and that's just for starters.  I hadn't looked into the locking and semantics
issues - those would really depends upon how you would achieve that "no dentry
cache" thing; again, right now that's *not* what your code is doing.

PS: and then there's the choice of name.  I mean, just try to say it over the
phone several times in a row...