On Wed 12-05-21 17:26:25, Christian Brauner wrote:
> On Mon, May 10, 2021 at 02:37:59PM +0300, Amir Goldstein wrote:
> > On Mon, May 10, 2021 at 1:13 PM Jan Kara <jack@xxxxxxx> wrote:
> > > OK, so this feature would effectively allow sb-wide watching of events that
> > > are generated from within the container (or its descendants). That sounds
> > > useful. Just one question: If there's some part of a filesystem, that is
> > > accesible by multiple containers (and thus multiple namespaces), or if
> > > there's some change done to the filesystem say by container management SW,
> > > then event for this change won't be visible inside the container (despite
> > > that the fs change itself will be visible).
> >
> > That is correct.
> > FYI, a privileged user can already mount an overlayfs in order to indirectly
> > open and write to a file.
> >
> > Because overlayfs opens the underlying file FMODE_NONOTIFY this will
> > hide OPEN/ACCESS/MODIFY/CLOSE events also for inode/sb marks.
> > Since 459c7c565ac3 ("ovl: unprivieged mounts"), so can unprivileged users.
> >
> > I wonder if that is a problem that we need to fix...
> >
> > > This is kind of a similar
> > > problem to the one we had with mount marks and why sb marks were created.
> > > So aren't we just repeating the mistake with mount marks? Because it seems
> > > to me that more often than not, applications are interested in getting
> > > notification when what they can actually access within the fs has changed
> > > (and this is what they actually get with the inode marks) and they don't
> > > care that much where the change came from... Do you have some idea how
> > > frequent are such cross-ns filesystem changes?
> >
> > The use case surely exist, the question is whether this use case will be
> > handled by a single idmapped userns or multiple userns.
> >
> > You see, we simplified the discussion to an idmapped mount that uses
> > the same userns and the userns the container processes are associated
> > with, but in fact, container A can use userns A container B userns B and they
> > can both access a shared idmapped mount mapped with userns AB.
> >
> > I think at this point in time, there are only ideas about how the shared data
> > case would be managed, but Christian should know better than me.
>
> I think there are two major immediate container use-cases right now that
> are already actively used:
> 1. idmapped rootfs
> A container manager wants to avoid recursively chowning the rootfs or
> image for a container. To this end an idmapped mount is created. The
> idmapped mount can either share the same userns as the container itself
> or a separate userns can be used. What people use depends on their
> concept of a container.
> For example, systemd has merged support for idmapping a containers
> rootfs in [1]. The systemd approach to containers never puts the
> container itself in control of most things including most of its mounts.
> That is very much the approach of having it be a rather tightly managed
> system. Specifically, this means that systemd currently uses a separate
> userns to idmap.
> In contrast other container managers usually treat the container as a
> mostly separate system and put it in charge of all its mounts. This
> means the userns used for the idmapped mount will be the same as the
> container runs in (see [2]).
OK, thanks for explanation. So to make fanotify idmap-filtered marks work
for systemd-style containers we would indeed need what Amir proposed -
i.e., the container manager intercepts fanotify_mark calls and decides
which namespace to setup the mark in as there's no sufficient priviledge
within the container to do that AFAIU.
> 2. data sharing among containers or among the host and containers etc.
> The most common use-case is to share data from the host with the
> container such as a download folder or the Linux folder on ChromeOS.
> Most container managers will simly re-use the container's userns for
> that too. More complex cases arise where data is shared between
> containers with different idmappings then often a separate userns will
> have to be used.
OK, but if say on ChromeOS you copy something to the Linux folder by app A
(say file manager) and containerized app B (say browser) watches that mount
for changes with idmap-filtered mark, then it won't see notification for
those changes because A presumably runs in a different namespace than B, am
I imagining this right? So mark which filters events based on namespace of
the originating process won't be usable for such usecase AFAICT.
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
