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

[Al Viro]

On Tue, Jun 12, 2018 at 07:24:23PM +0100, Al Viro wrote:
> On Tue, Jun 12, 2018 at 11:24:39AM +0200, Miklos Szeredi wrote:
> 
> > > 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);
> > >         put_nfs_open_context(ctx);
> > >         nfs_fscache_open_file(inode, filp);
> > >         return 0;
> > > }
> > >
> > > You do want to support NFS for lower layers, right?
> > 
> > There's no change regarding how file_dentry() works.  We've just
> > pushed these weird files (f_path points to overlay, f_inode points to
> > underlay) down into the guts of overlayfs and are not directly
> > referenced from the file table anymore.  That shouldn't make *any*
> > difference from the lower fs's pov.
> 
> *owwww*
> I'd managed to push that particular nest of horrors out of mind ;-/
> Having dug out my notes from back then and grepped around...  The real
> mess is not even /proc/*/maps - it's /proc/*/map_files/* and yes, the
> reasons for that kludge are still valid ;-/
> 
> Fuck.  OK, so we want to get rid of ->f_path.dentry accesses and see
> that they don't come back.  Leaving them around due to "it won't come
> anywhere near overlayfs" was a mistake of the same kind as leaving
> d_add() in ->lookup() instances where we'd been certain that filesystem
> would never get exported over NFS.  Just as we'd got open-by-handle for
> e.g. NFS, we'd got nothing to prevent ecryptfs as lower layer in
> overlayfs...
> 
> I hate it, but... consider path_open() objections withdrawn for now.
> Uses of ->vm_file (and rules for those) are too convoluted to untangle
> at the moment.  I still would love to get that straightened out, but
> it's not this cycle fodder, more's the pity...

PS: conversion of ->f_path.dentry is easy and that can probably go this
cycle - it's a fairly trivial change, with no functional changes unless
overlayfs is used with <filesystem>, fixing really bad shit if it ever
gets used thus.  I'm not asking to put that into overlayfs pull *and*
it's independent from the "want to kill that fucking kludge" stuff.
The latter is too hard for this cycle, unfortunately.