linux-kernel - Re: [PATCH v2 0/2] implement OA2_INHERIT

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Date: Thu, 25 Apr 2024 02:43:58 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Andy Lutomirski <luto@...capital.net>
Cc: stsp <stsp2@...dex.ru>, linux-kernel@...r.kernel.org,
	Stefan Metzmacher <metze@...ba.org>,
	Eric Biederman <ebiederm@...ssion.com>,
	Andy Lutomirski <luto@...nel.org>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Jeff Layton <jlayton@...nel.org>,
	Chuck Lever <chuck.lever@...cle.com>,
	Alexander Aring <alex.aring@...il.com>,
	linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
	Paolo Bonzini <pbonzini@...hat.com>,
	Christian Göttsche <cgzones@...glemail.com>,
	Aleksa Sarai <cyphar@...har.com>
Subject: Re: [PATCH v2 0/2] implement OA2_INHERIT_CRED flag for openat2()

On Wed, Apr 24, 2024 at 05:43:02PM -0700, Andy Lutomirski wrote:

> I like that, but you're blocking it the wrong way.  My concern is that
> someone does dfd = open("/proc/PID/fd/3") and then openat(dfd, ...,
> OA2_INHERIT_CRED);  IIRC open("/proc/PID/fd/3") is extremely magical
> and returns the _same open file description_ (struct file) as PID's fd
> 3.

No, it doesn't.  We could implement that, but if we do that'll be
*not* a part of procfs and it's going to be limited to current task
only.

There are two different variants of /dev/fd/* semantics - one is
"opening /dev/fd/42 is an equivalent of dup(42)", another is
"opening /dev/fd/42 is an equivalent of opening the same fs object
that is currently accessed via descriptor 42".  Linux is doing the
latter, and we can't switch - that would break a lot of userland
software, including a lot of scripts.

I'm not saying I like the series, but this particular objection is bogus -
open via procfs symlinks is *not* an equivalent of dup() and that is not
going to change.