Re: [RFC v2 PATCH 0/8] VFS:userns: support portable root filesystems

[Al Viro <viro@ZenIV.linux.org.uk>]

On Tue, May 10, 2016 at 04:36:56PM -0700, James Bottomley wrote:
> +static int shiftfs_rename2(struct inode *olddir, struct dentry *old,
> +			   struct inode *newdir, struct dentry *new,
> +			   unsigned int flags)
> +{
> +	struct dentry *rodd = olddir->i_private, *rndd = newdir->i_private,
> +		*realold = old->d_inode->i_private,
> +		*realnew = new->d_inode->i_private;
> +	struct inode *realolddir = rodd->d_inode, *realnewdir = rndd->d_inode;
> +	const struct inode_operations *iop = realolddir->i_op;
> +	int err;
> +	const struct cred *oldcred, *newcred;
> +
> +	oldcred = shiftfs_new_creds(&newcred, old->d_sb);
> +	err = iop->rename2(realolddir, realold, realnewdir, realnew, flags);
> +	shiftfs_old_creds(oldcred, &newcred);

... and you've just violated all locking rules for ->rename2().

> +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 */

Bloody wonderful.  So
	* we lose caching the negative lookups
	* we've got buggered hardlinks (different inodes for those)
	* it has never, ever been tried on -next (would do rather nasty
things on that d_instantiate())

> +
> +	kfree(sfc);
> +
> +	return err;
> +}

> +	file->f_op = &sfc->fop;

Lovely - now try that with underlying fs something built modular.

Or try to use it on top of something with non-trivial dentry_operations
(hell, on top of itself, for starters).