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Date:   Sat, 29 Aug 2020 20:25:22 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Miklos Szeredi <miklos@szeredi.hu>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Dave Chinner <david@fromorbit.com>,
        Christian Schoenebeck <qemu_oss@crudebyte.com>,
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        Greg Kurz <groug@kaod.org>, linux-fsdevel@vger.kernel.org,
        Stefan Hajnoczi <stefanha@redhat.com>,
        Miklos Szeredi <mszeredi@redhat.com>,
        Vivek Goyal <vgoyal@redhat.com>,
        Giuseppe Scrivano <gscrivan@redhat.com>,
        Daniel J Walsh <dwalsh@redhat.com>,
        Chirantan Ekbote <chirantan@chromium.org>
Subject: Re: xattr names for unprivileged stacking?
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On Sat, Aug 29, 2020 at 09:13:24PM +0200, Miklos Szeredi wrote:

> > d_path() is the least of the problems, actually.  Directory tree structure on
> > those, OTOH, is a serious problem.  If you want to have getdents(2) on that
> > shite, you want an opened descriptor that looks like a directory.  And _that_
> > opens a large can of worms.  Because now you have fchdir(2) to cope with,
> > lookups going through /proc/self/fd/<n>/..., etc., etc.
> 
> Seriously, nobody wants fchdir().  And getdents() does not imply fchdir().

Yes, it does.  If it's a directory, fchdir(2) gets to deal with it.
If it's not, no getdents(2).  Unless you special-case the damn thing in
said fchdir(2).

> As for whether we'd need foobarat() on such a beast or let
> /proc/self/fd/<n> be dereferenced, I think no.  So comes the argument:
>  but then we'll break all those libraries and whatnot relying on these
> constructs.  Well, sorry, so would we if we didn't introduce this in
> the first place.  That's not really breaking anything, it's just
> setting expectations.

Translation: we'll special-case that in procfs, etc., etc. and handwave
the problems away.  Lovely...