Message-ID: <87r38ek0xp.fsf@x220.int.ebiederm.org>
Date:   Tue, 20 Sep 2016 11:09:38 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Ian Kent <raven@...maw.net>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        autofs mailing list <autofs@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...IV.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Omar Sandoval <osandov@...ndov.com>
Subject: Re: [PATCH 3/4] autofs - make mountpoint checks namespace aware

Ian Kent <raven@...maw.net> writes:

> On Fri, 2016-09-16 at 10:58 +0800, Ian Kent wrote:
>> On Thu, 2016-09-15 at 19:47 -0500, Eric W. Biederman wrote:
>> > Ian Kent <raven@...maw.net> writes:
>> > 
>> > > On Wed, 2016-09-14 at 21:08 -0500, Eric W. Biederman wrote:
>> > > > Ian Kent <raven@...maw.net> writes:
>> > > > 
>> > > > > On Wed, 2016-09-14 at 12:28 -0500, Eric W. Biederman wrote:
>> > > > > > Ian Kent <raven@...maw.net> writes:
>> > > > > > 
>> > > > > > > If an automount mount is clone(2)ed into a file system that is
>> > > > > > > propagation private, when it later expires in the originating
>> > > > > > > namespace subsequent calls to autofs ->d_automount() for that
>> > > > > > > dentry in the original namespace will return ELOOP until the
>> > > > > > > mount is manually umounted in the cloned namespace.
>> > > > > > > 
>> > > > > > > In the same way, if an autofs mount is triggered by automount(8)
>> > > > > > > running within a container the dentry will be seen as mounted in
>> > > > > > > the root init namespace and calls to ->d_automount() in that
>> > > > > > > namespace
>> > > > > > > will return ELOOP until the mount is umounted within the
>> > > > > > > container.
>> > > > > > > 
>> > > > > > > Also, have_submounts() can return an incorect result when a mount
>> > > > > > > exists in a namespace other than the one being checked.
>> > > > > > 
>> > > > > > Overall this appears to be a fairly reasonable set of changes.  It
>> > > > > > does
>> > > > > > increase the expense when an actual mount point is encountered, but
>> > > > > > if
>> > > > > > these are the desired some increase in cost when a dentry is a
>> > > > > > mountpoint is unavoidable.
>> > > > > > 
>> > > > > > May I ask the motiviation for this set of changes?  Reading through
>> > > > > > the
>> > > > > > changes I don't grasp why we want to change the behavior of autofs.
>> > > > > > What problem is being solved?  What are the benefits?
>> > > > > 
>> > > > > LOL, it's all too easy for me to give a patch description that I think
>> > > > > explains
>> > > > > a problem I need to solve without realizing it isn't clear to others
>> > > > > what
>> > > > > the
>> > > > > problem is, sorry about that.
>> > > > > 
>> > > > > For quite a while now, and not that frequently but consistently, I've
>> > > > > been
>> > > > > getting reports of people using autofs getting ELOOP errors and not
>> > > > > being
>> > > > > able
>> > > > > to mount automounts.
>> > > > > 
>> > > > > This has been due to the cloning of autofs file systems (that have
>> > > > > active
>> > > > > automounts at the time of the clone) by other systems.
>> > > > > 
>> > > > > An unshare, as one example, can easily result in the cloning of an
>> > > > > autofs
>> > > > > file
>> > > > > system that has active mounts which shows this problem.
>> > > > > 
>> > > > > Once an active mount that has been cloned is expired in the namespace
>> > > > > that
>> > > > > performed the unshare it can't be (auto)mounted again in the the
>> > > > > originating
>> > > > > namespace because the mounted check in the autofs module will think it
>> > > > > is
>> > > > > already mounted.
>> > > > > 
>> > > > > I'm not sure this is a clear description either, hopefully it is
>> > > > > enough
>> > > > > to
>> > > > > demonstrate the type of problem I'm typing to solve.
>> > > > 
>> > > > So to rephrase the problem is that an autofs instance can stop working
>> > > > properly from the perspective of the mount namespace it is mounted in
>> > > > if the autofs instance is shared between multiple mount namespaces.  The
>> > > > problem is that mounts and unmounts do not always propogate between
>> > > > mount namespaces.  This lack of symmetric mount/unmount behavior
>> > > > leads to mountpoints that become unusable.
>> > > 
>> > > That's right.
>> > > 
>> > > It's also worth considering that symmetric mount propagation is usually
>> > > not
>> > > the
>> > > behaviour needed either and things like LXC and Docker are set propagation
>> > > slave
>> > > because of problems caused by propagation back to the parent namespace.
>> > > 
>> > > So a mount can be triggered within a container, mounted by the automount
>> > > daemon
>> > > in the parent namespace, and propagated to the child and similarly for
>> > > expires,
>> > > which is the common use case now.
>> > > 
>> > > > 
>> > > > Which leads to the question what is the expected new behavior with your
>> > > > patchset applied.  New mounts can be added in the parent mount namespace
>> > > > (because the test is local).  Does your change also allow the
>> > > > autofs mountpoints to be used in the other mount namespaces that share
>> > > > the autofs instance if everything becomes unmounted?
>> > > 
>> > > The problem occurs when the subordinate namespace doesn't deal with these
>> > > propagated mounts properly, although they can obviously be used by the
>> > > subordinate namespace.
>> > > 
>> > > > 
>> > > > Or is it expected that other mount namespaces that share an autofs
>> > > > instance will get changes in their mounts via mount propagation and if
>> > > > mount propagation is insufficient they are on their own.
>> > > 
>> > > Namespaces that receive updates via mount propagation from a parent will
>> > > continue to function as they do now.
>> > > 
>> > > Mounts that don't get updates via mount propagation will retain the mount
>> > > to
>> > > use
>> > > if they need to, as they would without this change, but the originating
>> > > namespace will also continue to function as expected.
>> > > 
>> > > The child namespace needs cleanup its mounts on exit, which it had to do
>> > > prior
>> > > to this change also.
>> > > 
>> > > > 
>> > > > I believe this is a question of how do notifications of the desire for
>> > > > an automount work after your change, and are those notifications
>> > > > consistent with your desired and/or expected behavior.
>> > > 
>> > > It sounds like you might be assuming the service receiving these cloned
>> > > mounts
>> > > actually wants to use them or is expecting them to behave like automount
>> > > mounts.
>> > > But that's not what I've seen and is not the way these cloned mounts
>> > > behave
>> > > without the change.
>> > > 
>> > > However, as has probably occurred to you by now, there is a semantic
>> > > change
>> > > with
>> > > this for namespaces that don't receive mount propogation.
>> > > 
>> > > If a mount request is triggered by an access in the subordinate namespace
>> > > for a
>> > > dentry that is already mounted in the parent namespace it will silently
>> > > fail
>> > > (in
>> > > that a mount won't appear in the subordinate namespace) rather than
>> > > getting
>> > > an
>> > > ELOOP error as it would now.
>> > > 
>> > > It's also the case that, if such a mount isn't already mounted, it will
>> > > cause a
>> > > mount to occur in the parent namespace. But that is also the way it is
>> > > without
>> > > the change.
>> > > 
>> > > TBH I don't know yet how to resolve that, ideally the cloned mounts would
>> > > not
>> > > appear in the subordinate namespace upon creation but that's also not
>> > > currently
>> > > possible to do and even if it was it would mean quite a change in to the
>> > > way
>> > > things behave now.
>> > > 
>> > > All in all I believe the change here solves a problem that needs to be
>> > > solved
>> > > without affecting normal usage at the expense of a small behaviour change
>> > > to
>> > > cases where automount isn't providing a mounting service.
>> > 
>> > That sounds like a reasonable semantic change.  Limiting the responses
>> > of the autofs mount path to what is present in the mount namespace
>> > of the program that actually performs the autofs mounts seems needed.
>> 
>> Indeed, yes.
>> 
>> > 
>> > In fact the entire local mount concept exists because I was solving a
>> > very similar problem for rename, unlink and rmdir.  Where a cloned mount
>> > namespace could cause a denial of service attack on the original
>> > mount namespace.
>> > 
>> > I don't know if this change makes sense for mount expiry.
>> 
>> Originally I thought it did but now I think your right, it won't actually make
>> a
>> difference.
>> 
>> Let me think a little more about it, I thought there was a reason I included
>> the
>> expire in the changes but I can't remember now.
>> 
>> It may be that originally I thought individual automount(8) instances within
>> containers could be affected by an instance of automount(8) in the root
>> namespace (and visa versa) but now I think these will all be isolated.
>
> I also thought that the autofs expire would continue to see the umounted mount
> and continue calling back to the daemon in an attempt to umount it.
>
> That isn't the case so I can drop the changes to the expire expire code as you
> recommend.

Sounds good.

Eric

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