linux-kernel - Re: [PATCH v2 0/5] pid: add pidfd

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Message-ID: <20190401002147.vv23md5spi34uyjz@brauner.io>
Date:   Mon, 1 Apr 2019 02:21:49 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...capital.net>,
        Daniel Colascione <dancol@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Andrew Lutomirski <luto@...nel.org>,
        David Howells <dhowells@...hat.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Linux API <linux-api@...r.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Kees Cook <keescook@...omium.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Michael Kerrisk-manpages <mtk.manpages@...il.com>,
        Jonathan Kowalski <bl0pbl33p@...il.com>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Nagarathnam Muthusamy <nagarathnam.muthusamy@...cle.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH v2 0/5] pid: add pidfd_open()

On Sun, Mar 31, 2019 at 05:18:10PM -0700, Linus Torvalds wrote:
> On Sun, Mar 31, 2019 at 5:09 PM Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > Ugh...  Which vfsmount would you have to go with it?
> 
> I'd literally just do a lookup of "/proc" in the current root
> directory in the lookup() function for that special pseudo-dentry.
> 
> If it's not mounted, or not a /proc filesystem, screw it.
> 
> > 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.
> 
> Absolutely. Which is why I said it's _conceptually_ similar to the alias lookup.
> 
> And I suspect we can even use some of the same practical logic, but
> it's definitely not _exactly_ the same. This thing very much involves
> magic hooking into the lookup() function (but we then have to look up
> the alias not for the path we're looking up, but for the _parent_
> we're looking that path up in, which is very different from the normal
> case).
> 
> > It's more than a tiny bit too clever for mine...
> 
> Fair enough. The whole "just do the whole lookup at pidfd creation
> time" is certainly a whole lot simpler.

Even just from a pure maintenance perspective this sounds a better to me.