Re: [PATCH 4/9] Implement fsopen() to prepare for a mount

From: Karel Zak
Date: Thu May 04 2017 - 06:41:01 EST

On Wed, May 03, 2017 at 05:05:08PM +0100, David Howells wrote:
> mfd = fsopen("ext4", -1, O_CLOEXEC);
> write(mfd, "d /dev/sdb1"); // note I'm ignoring write's length arg

Not sure about 'd', in many cases it is not device, for mount(2)
syscall we call it "source".

> write(mfd, "o noatime");
> write(mfd, "o acl");
> write(mfd, "o user_attr");
> write(mfd, "o iversion");
> write(mfd, "o ");
> write(mfd, "r /my/container"); // root inside the fs
> fsmount(mfd, container_fd, "/mnt", AT_NO_FOLLOW);
> mfd = fsopen("afs", -1);
> write(mfd, "d");
> write(mfd, "o");
> write(mfd, "r /");
> fsmount(mfd, AT_FDCWD, "/mnt", 0);
> If an error is reported at any step, an error message may be available to be
> read() back (ENODATA will be reported if there isn't an error available) in
> the form:
> "e <subsys>:<problem>"
> "e SELinux:Mount on mountpoint not permitted"
> Once fsmount() has been called, further write() calls will incur EBUSY,
> even if the fsmount() fails. read() is still possible to retrieve error
> information.

The very basic mount(2) problem is that you have to parse
/proc/self/mountinfo to get information about the mounted filesystem.
It seems that your read() is also one way communication.

What we really need is to have a way how to specify *what* you want to
read. The error message is not enough, I want to know the finally used
mount options, mount ID, etc. It would be nice to have something like

fsmount(mfd, AT_FDCWD, "/mnt", 0);

write(mfd, "o");
read(mfd, ....); // read mount options

write(mdf, "i");
read(mfd, ....); // read mount ID

but it seems ugly. Maybe introduce another function like

fsinfo(mdf, "o", buf, bufsz)

to get mount options (etc.) and to avoid separate write & read.

> Netlink is not used because it is optional.


Karel Zak <kzak@xxxxxxxxxx>