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Date:   Mon, 1 Apr 2019 01:09:37 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Christian Brauner <christian@brauner.io>,
        Andy Lutomirski <luto@amacapital.net>,
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Subject: Re: [PATCH v2 0/5] pid: add pidfd_open()
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On Sun, Mar 31, 2019 at 04:40:15PM -0700, Linus Torvalds wrote:

> The clever alternative, which might be the RightWay(tm) is to just
> create a completely unattached dentry, and basically tie it into the
> actual /proc filesystem hierarchy at lookup() time when somebody does
> the openat() using it for the first time. You'd get a very simple
> callback: since the dentry would be unattached, you'd be guaranteed to
> get a "lookup()" from the VFS layer, and that lookup would then do the
> "hook into the actual /proc filesystem".

Ugh...  Which vfsmount would you have to go with it?

> We already kind of do things like that in the VFS layer when we have
> unattached dentries (because of "look up by filehandle" etc). In many
> ways the "pidfd_open()" really is exactly a "look up by file handle"
> operation, it just so happens that the "file handle" is just the
> pid/namespace combination.

Except that we never let unattached _directory_ dentries out - if
we can't reattach them to the tree, open-by-handle will tell you to
take a hike.

> And if the splice alias (which is what the VFS layer calls that "tie
> aliased dentry to the parent" operation) fails, because the /proc
> filesystem isn't mounted or whatever, then trying to look up names off
> the thing will also fail.

> It's a tiny bit too clever for my taste, and it's not *exactly* the
> same thing as our normal inode alias handling, but it's pretty close
> conceptually (and even practically).

It's more than a tiny bit too clever for mine...

> So it would basically do something very similar to the ioctl, but it
> would do it implicitly and automatically at that first lookup.
> 
> That would also mean that you'd not actually pay the cost of doing any
> of this *unless* you also end up trying to open things in /proc using
> that pidfd.

Al, back to normal life and digging through several flamefests from
hell...