Re: [PATCH 14/39] ovl: stack file ops

From: Al Viro
Date: Mon Jun 11 2018 - 22:40:38 EST

On Tue, Jun 12, 2018 at 03:29:26AM +0100, Al Viro wrote:

> It might (or might not) work for the filesystems you'd been testing
> on, but it's a lot of trouble waiting to happen. Hell, try and use
> ecryptfs as lower layer, see how fast it'll blow up. Sure, it's
> a dumb testcase, but I don't see how to check if something more
> realistic is trouble-free.
> I'd been trying to come up with some way to salvage that kludge of yours,
> but I don't see any solutions. We don't have good proxies for "this
> filesystem might be unsafe as lower layer" ;-/

Note that anything that uses file_dentry() anywhere near ->open(),
->read_iter() or ->write_iter() is an instant trouble with your scheme.
Such as
int nfs_open(struct inode *inode, struct file *filp)
struct nfs_open_context *ctx;

ctx = alloc_nfs_open_context(file_dentry(filp), filp->f_mode, filp);
if (IS_ERR(ctx))
return PTR_ERR(ctx);
nfs_file_set_open_context(filp, ctx);
nfs_fscache_open_file(inode, filp);
return 0;

You do want to support NFS for lower layers, right?