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

[James Bottomley]

On Wed, 2016-06-01 at 03:40 +0100, Al Viro wrote:
> 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.

Yes, sorry, after the new BUG_ON in the d_invalidate() code I didn't
have much choice but to run two sets of dentry hashes; I'm afraid I
just forgot to remove the comment (I did remove the blurb about the
single dentry cache from the changelog).  I'll remove it (the comment)
in the next go around.

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

But I did that especially for you, Al ...

James