From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Theodore Y. Ts'o" <tytso@mit.edu>
Subject: Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for
 superblock creation [ver #9]
Date: Thu, 12 Jul 2018 18:32:23 -0400
Message-ID: <20180712223223.GA28610@thunk.org>
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To: David Howells <dhowells@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, Andrew Lutomirski <luto@kernel.org>, Al Viro <viro@zeniv.linux.org.uk>, Linux API <linux-api@vger.kernel.org>, linux-fsdevel <linux-fsdevel@vger.kernel.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Jann Horn <jannh@google.com>
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On Thu, Jul 12, 2018 at 10:26:37PM +0100, David Howells wrote:
> The problem is that there's more than one actual "open" involved.
> 
> 	fd = fsopen("ext4");				<--- #1
> 	whatever_interface(fd, "s /dev/sda1");
> 	whatever_interface(fd, "o journal_path=/dev/sda2");
> 	do_the_create_thing(fd);			<--- #2 and #3
> 
> The initial check to see whether you can mount or not is done at #1.
> 
> But later there are two nested file opens.  Internally, deep down inside the
> block layer, /dev/sda1 and /dev/sda2 are opened and further permissions checks
> are done, whether you like it or not.  But these have no access to the creds
> attached to fd as things currently stand.

So maybe the answer is that you open /dev/sda1 and /dev/sda2 and then
pass the file descriptors to the fsopen object?  We can require that
the fd's be opened with O_RDWR and O_EXCL, which has the benefit where
if you have multiple block devices, you know *which* block device had
a problem with being grabbed for an exclusive open.

Just a thought.

						- Ted