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Date: Thu, 25 Apr 2024 02:43:58 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
To: Andy Lutomirski <luto@amacapital.net>
Cc: stsp <stsp2@yandex.ru>, linux-kernel@vger.kernel.org,
	Stefan Metzmacher <metze@samba.org>,
	Eric Biederman <ebiederm@xmission.com>,
	Andy Lutomirski <luto@kernel.org>,
	Christian Brauner <brauner@kernel.org>, Jan Kara <jack@suse.cz>,
	Jeff Layton <jlayton@kernel.org>,
	Chuck Lever <chuck.lever@oracle.com>,
	Alexander Aring <alex.aring@gmail.com>,
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	Paolo Bonzini <pbonzini@redhat.com>,
	Christian =?iso-8859-1?Q?G=F6ttsche?= <cgzones@googlemail.com>,
	Aleksa Sarai <cyphar@cyphar.com>
Subject: Re: [PATCH v2 0/2] implement OA2_INHERIT_CRED flag for openat2()
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Sender: Al Viro <viro@ftp.linux.org.uk>

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.