Re: [PATCH 3/4] autofs4 - track uid and gid of last mount requestor

Quoting Ian Kent (raven@xxxxxxxxxx):

On Thu, 2008-02-28 at 13:51 -0600, Serge E. Hallyn wrote:
Quoting Jeff Moyer (jmoyer@xxxxxxxxxx):
Ian Kent <raven@xxxxxxxxxx> writes:

On Wed, 2008-02-27 at 23:23 -0800, Andrew Morton wrote:
On Thu, 28 Feb 2008 16:08:20 +0900 Ian Kent <raven@xxxxxxxxxx> wrote:
which includes the process uid and gid, and as part of
the lookup we set macros for several mount map substitution variables,
derived from the uid and gid of the process requesting the mount and
they can be used within autofs maps.

yeah, could be a problem. Hopefully the namespace people can advise.
Perhaps we need a concept of an exportable-to-userspace namespace-id+uid,
namespace-id+gid, namespace-id+pid, etc for this sort of thing. It has
come up before. Recently, but I forget what the context was.

I'm all ears to any feedback from others on this, please.

I think there is some confusion surrounding what the UID and GID are
used for in this context. I'll try to explain it as best I can.

When the automount daemon parses a map entry, it will do some amount of
variable substitution. So, let's say you're running on an i386 box, and
you want to mount a library directory from a server. You might have a
map entry that looks like this:

lib server:/export/$ARCH/lib

In this case, the automount daemon will replace $ARCH with i386, and
will try the following mount command:

mount -t nfs server:/export/i386/lib /automountdir/lib

There are cases where it would be helpful to use the requesting
process's UID in such a variable substitution. Consider the case of a
CIFS share, where the automount daemon runs as user root, but we want to
mount the share using the credentials of the requesting user. In this
case, the UID and GID can be helpful in formatting the mount options for
mounting the share.

So, the UID and GID are used only for map substitutions. Now, having
said all of that, I'll have to look more closely at why we even need to
keep track of it, given that it only needs to be used when performing
the lookup, and at that time we have information on the requesting UID
and GID.

Thanks Jeff. If that's the case then user namespaces don't affect this
at all.

Yep, that's precisely the way this is used, by autofs anyway.

I guess the problem we face is that since this is a public interface
other applications could use this in a different way and we can't
control that. I think I need more information so I can document the
defined usage in my revised patch set.

In version 5 I set $UID, $GID, $USER, $GROUP and $HOME in addition to
the standard autofs macros, $ARCH, $CPU, $HOST, $OSNAME, $OSREL and
$OSVERS, and then expand the map entry.

The question that Jeff is asking himself is, why do we need this
information when we re-connect at startup, since the mounts are already
present.

The answer isn't easy to explain and will be lengthy, sorry, but let me
try anyway.

There are two types on maps, direct (in the module source you will see a
third type called an offset, which is just a direct mount in disguise)
and indirect.

For example, here is master map with direct and indirect map entries:

/- /etc/auto.direct
/test /etc/auto.indirect

/etc/auto.direct:

/automount/dparse/g6 budgie:/autofs/export1
/automount/dparse/g1 shark:/autofs/export1
and so on.

/etc/auto.indirect:

g1 shark:/autofs/export1
g6 budgie:/autofs/export1
and so on.

For the above indirect map an autofs file system is mounted on /test and
mounts are triggered by the inode lookup operation. So we see a mount of
shark:/autofs/export1 on /test/g1, for example.

The way that direct mounts are handled is by makeing an autofs mount on
each full path, such as /automount/dparse/g1, and using it as a mount
trigger. So when we walk on the path we mount shark:/autofs/export1 on
top of this mount point, for example. Since these are always a
directories we can use the follow_link inode operation to trigger the
mount.

But, each entry in direct and indirect maps can have offsets (often
called multi-mount map entries).

For example,

a direct mount map entry could also be:

/automount/dparse/g1 \
/ shark:/autofs/export5/testing/test \
/s1 shark:/autofs/export/testing/test/s1 \
/s2 shark:/autofs/export5/testing/test/s2 \
/s1/ss1 shark:/autofs/export2 \
/s2/ss2 shark:/autofs/export2

and a similar indirect mount:

g1 \
/ shark:/autofs/export5/testing/test \
/s1 shark:/autofs/export/testing/test/s1 \
/s2 shark:/autofs/export5/testing/test/s2 \
/s1/ss1 shark:/autofs/export1 \
/s2/ss2 shark:/autofs/export2

One of the issues with version 4 of autofs was that, when mounting an
entry with a large number of offsets, possibly with nesting, we needed
to mount and umount all of them as a single unit. Not really a problem,
except for people with a large number of offsets in map entries. This
mechanism is used for the well known "hosts" map and we have seen cases
(in 2.4) where the available number of mounts are exhausted or where the
number of privileged ports available is exhausted.

In version 5 we mount only as we go down the tree of offsets and
similarly for expiring them which resolves the above problem. There is
somewhat more detail to the implementation but it isn't needed for the
sake of the explanation. The one important detail is that these offsets
are implemented using the same mechanism as the direct mounts above and
so can also be covered by another mount.

To be able to do this I need to maintain context. In the daemon I use a
list to represent these offsets and use it to manage the mounting and
expiration of the mount tree, something which can only be discovered
from the original map entry. So, to rebuild this context at startup for
existing mounts I need to do the lookup to get the map entry as part of
the process of re-connecting to the mounts. Hence the need to get the
uid and gid of the original requester.

The way the user namespaces work right now is similar to say the IPC
namespace - a task belongs to one user, that user belongs to precisely
one user namespace.

Even in my additional userns patches, I was changing uid to store the
(uid, userns) so a struct user still belonged to just one user
namespace.

In contrast, with pid namespaces a task is associated with a 'struct
pid' which links it to multiple process ids, one in each pid namespace
to which it belongs.

Perhaps we should be treating user namespaces like pid namespaces?

For autofs this would mean that when autofs wants a uid for some task,
it would be given the uid in the user namespace which autofs 'knows'.

It would also help me fix the siginfo problems I haven't solved yet -
rather than having to worry about user namespace lifetimes with siginfos
(which last a little while but have no clearly defined lifespan) we
could send the uid in an init user namespace or the uid in the target
uid namespace, or just a lightweight user struct proxy akin to 'struct
pid'.

And it also obviates the need for any sort of delegation.

So if I'm user 500 in what I think is the initial user namespace, I can
create a container with a new user namespace, the init task of which is
both uid 0 in the child userns, and uid 500 in the higher level,
automatically giving the container access to any files I own.

Eric, when you get a chance (I know you're overloaded atm) I'd love to
hear your thoughts on this...

Few, that was hard work, just for those last couple of sentences.

Please, lets not go into the issue that the maps can change during a
restart, that's an issue for another time and isn't a kernel issue
anyway.


(Still trying to follow the rest of the thread bc i definately feel like
I'm missing something. I swear I understood autofs 10+ years ago :)

Me too, but now I've change it so much even I'm confused most of the
time, ;)

Hopefully the above explanation is useful in some small way.

Ian


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