Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for superblock creation [ver #9]

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