Re: [PATCH] fs: show locked and lock_ro options in mountinfo

From: Andrey Wagin <avagin@gmail.com>
To: Richard Weinberger <richard.weinberger@gmail.com>
Cc: linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Cyrill Gorcunov <gorcunov@openvz.org>,
	Pavel Emelyanov <xemul@parallels.com>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH] fs: show locked and lock_ro options in mountinfo
Date: Sat, 28 Mar 2015 01:35:32 +0300	[thread overview]
Message-ID: <CANaxB-xbG80-YbRqypYVXbnOMxBPfYep4y0suNOH7tGYs2ao2A@mail.gmail.com> (raw)
In-Reply-To: <CAFLxGvxaJdRV7c9jRhjQibyk+55GWSKyn+vuWbgXuvNSsPdQ5w@mail.gmail.com>

2015-03-28 0:42 GMT+03:00 Richard Weinberger <richard.weinberger@gmail.com>:
> On Fri, Mar 27, 2015 at 9:39 PM, Andrey Vagin <avagin@openvz.org> wrote:
>> I don't see any reasons to hide them. This information can help to
>> understand errors.
>
> Because these flags are set/read only internally by the VFS. In contrast
> to the other flags shown by mountinfo MNT_LOCKED is not a mount option.

But this flag is set as a result of the specified user action, when he
unshares userns and mntns. This flag affects visiable behaviour.

>
> Why does it help to debug errors?
> How would a user know that mount() with MS_BIND returns EINVAL because
> the mount source is MNT_LOCKED? This information is useless for her.

If I see lock_ro, I can be sure that mount -o remount,bind,rw /XXX will fail.
If I see locked, I  know that this mount can't be umounted or moved
and can be bind-mounted only recursively.

If a user see these flags, he can check that a mount namespace is
configured correctly without security issues.

Sorry but I don't understand why you think that this information is
useless for users.

> If you argue like that you'd have to expose the whole VFS state to userland.

I have not noticed other MNT_LOCK_* flags. I should think more about
what information are a really required for dumping mount namespaces.

>
>> And this information is required for correct checkpoint/restore of mount
>> namespaces.
>
> Why especially MNT_LOCKED and not all the other flags used by VFS?

My goal is to dump enough information about a mount namespace to be
able to restore it back later. I don't know how to do this without
knowledge about locked mounts. I will think.

> Say MNT_DOOMED?

Mounts with MNT_DOOMED are never shown in mountinfo, are they?

Thank you for looking at this patch.

>
> --
> Thanks,
> //richard