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

Al Viro
2016-06-01T02:40:17Z

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...