Message-ID: <CANq1E4QA53-L8vPOWJfjhzXaYVQnNvgvR6AcYev0EaP6SfOjQQ@mail.gmail.com>
Date:	Mon, 24 Nov 2014 21:16:30 +0100
From:	David Herrmann <dh.herrmann@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Tom Gundersen <teg@...m.no>, Jiri Kosina <jkosina@...e.cz>,
	Linux API <linux-api@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Daniel Mack <daniel@...que.org>,
	Djalal Harouni <tixxdz@...ndz.org>
Subject: Re: kdbus: add documentation

Hi Andy!

On Fri, Nov 21, 2014 at 6:12 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Thu, Nov 20, 2014 at 9:02 PM, Greg Kroah-Hartman
> <gregkh@...uxfoundation.org> wrote:
>> From: Daniel Mack <daniel@...que.org>
>>
>> kdbus is a system for low-latency, low-overhead, easy to use
>> interprocess communication (IPC).
>>
>> The interface to all functions in this driver is implemented through
>> ioctls on files exposed through the mount point of a kdbusfs.  This
>> patch adds detailed documentation about the kernel level API design.
>>
>> Signed-off-by: Daniel Mack <daniel@...que.org>
>> Signed-off-by: David Herrmann <dh.herrmann@...il.com>
>> Signed-off-by: Djalal Harouni <tixxdz@...ndz.org>
>> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>> ---
>
>> +  Pool:
>> +    Each connection allocates a piece of shmem-backed memory that is used
>> +    to receive messages and answers to ioctl command from the kernel. It is
>> +    never used to send anything to the kernel. In order to access that memory,
>> +    userspace must mmap() it into its task.
>> +    See section 12 for more details.
>
> At the risk of opening a can of worms, wouldn't this be much more
> useful if you could share a pool between multiple connections?

Within a process it could theoretically be possible to share the same
memory pool between multiple connections made by the process. However,
note that normally a process only has a single connection to the bus
open (possibly two, if it opens a connection to both the system and
the user bus). Now, sharing the receiver buffer could certainly be
considered an optimization, but it would have no effect on
"usefulness", though, as just allocating space from a single shared
per-process receiver won't give you any new possibilities...

We have thought about this, but decided to delay it for now. Shared
pools can easily be added as an extension later on.

[snip]
>> +KDBUS_CMD_BUS_MAKE, and KDBUS_CMD_ENDPOINT_MAKE take a
>> +struct kdbus_cmd_make argument.
>> +
>> +struct kdbus_cmd_make {
>> +  __u64 size;
>> +    The overall size of the struct, including its items.
>> +
>> +  __u64 flags;
>> +    The flags for creation.
>> +
>> +    KDBUS_MAKE_ACCESS_GROUP
>> +      Make the device file group-accessible
>> +
>> +    KDBUS_MAKE_ACCESS_WORLD
>> +      Make the device file world-accessible
>
> This thing is a file.  What's wrong with using a normal POSIX mode?
> (And what to the read, write, and exec modes do?)

Domains and buses are directories, endpoints are files. Domains also
create control-files implicitly.

For kdbus clients there is just access or no access, but not
distinction between read, write and execute access. Due to that we
just break this down to per-group and world access bits, since doing
more is pointless, and we shouldn't allow shoehorning more stuff into
the access mode.

[snip]
>> +      KDBUS_ITEM_CREDS
>> +      KDBUS_ITEM_SECLABEL
>> +        Privileged bus users may submit these types in order to create
>> +        connections with faked credentials. The only real use case for this
>> +        is a proxy service which acts on behalf of some other tasks. For a
>> +        connection that runs in that mode, the message's metadata items will
>> +        be limited to what's specified here. See section 13 for more
>> +        information.
>
> This is still confusing.  There are multiple places in which metadata
> is attached.  Which does this apply to?  And why are only creds and
> seclabel listed?

Yes, and there are multiple places where metadata is *gathered*. This
ioctl creates connections, so only the items that are actually
*gathered* by that ioctl are documented here. These items are not part
of any messages, but are used as identification of the connection
owner (and in this particular case, to allow privileged proxies to
overwrite the items so they can properly proxy a legacy-dbus peer
connection).

[snip]
>> +6.5 Getting information about a connection's bus creator
>> +--------------------------------------------------------
>> +
>> +The KDBUS_CMD_BUS_CREATOR_INFO ioctl takes the same struct as
>> +KDBUS_CMD_CONN_INFO but is used to retrieve information about the creator of
>> +the bus the connection is attached to. The metadata returned by this call is
>> +collected during the creation of the bus and is never altered afterwards, so
>> +it provides pristine information on the task that created the bus, at the
>> +moment when it did so.
>
> What's this for?  I understand the need for the creator of busses to
> be authenticated, but doing it like this mean that anyone who will
> *fail* authentication can DoS the authentic creator.

This returns information on a bus owner, to determine whether a
connection is connected to a system, user or session bus. Note that
the bus-creator itself is not a valid peer on the bus, so you cannot
send messages to them. Which kind of DoS do you have in mind?

>> +
>> +7.3 Passing of Payload Data
>> +---------------------------
>> +
>> +When connecting to the bus, receivers request a memory pool of a given size,
>> +large enough to carry all backlog of data enqueued for the connection. The
>> +pool is internally backed by a shared memory file which can be mmap()ed by
>> +the receiver.
>> +
>> +KDBUS_MSG_PAYLOAD_VEC:
>> +  Messages are directly copied by the sending process into the receiver's pool,
>> +  that way two peers can exchange data by effectively doing a single-copy from
>> +  one process to another, the kernel will not buffer the data anywhere else.
>> +
>> +KDBUS_MSG_PAYLOAD_MEMFD:
>> +  Messages can reference memfd files which contain the data.
>> +  memfd files are tmpfs-backed files that allow sealing of the content of the
>> +  file, which prevents all writable access to the file content.
>> +  Only sealed memfd files are accepted as payload data, which enforces
>> +  reliable passing of data; the receiver can assume that neither the sender nor
>> +  anyone else can alter the content after the message is sent.
>
> This should specify *which* seals are checked.

True. Will be added to the documentation. For the record, it's the
full set of seals right now, so
F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_SEAL.

>> +
>> +Apart from the sender filling-in the content into memfd files, the data will
>> +be passed as zero-copy from one process to another, read-only, shared between
>> +the peers.
>> +
>> +
>> +7.4 Receiving messages
>> +----------------------
>> +
>> +Messages are received by the client with the KDBUS_CMD_MSG_RECV ioctl. The
>> +endpoint file of the bus supports poll() to wake up the receiving process when
>> +new messages are queued up to be received.
>> +
>> +With the KDBUS_CMD_MSG_RECV ioctl, a struct kdbus_cmd_recv is used.
>> +
>> +struct kdbus_cmd_recv {
>> +  __u64 flags;
>> +    Flags to control the receive command.
>> +
>> +    KDBUS_RECV_PEEK
>> +      Just return the location of the next message. Do not install file
>> +      descriptors or anything else. This is usually used to determine the
>> +      sender of the next queued message.
>> +
>> +    KDBUS_RECV_DROP
>> +      Drop the next message without doing anything else with it, and free the
>> +      pool slice. This a short-cut for KDBUS_RECV_PEEK and KDBUS_CMD_FREE.
>> +
>> +    KDBUS_RECV_USE_PRIORITY
>> +      Use the priority field (see below).
>> +
>> +  __u64 kernel_flags;
>> +    Valid flags for this command, returned by the kernel upon each call.
>> +
>> +  __s64 priority;
>> +      With KDBUS_RECV_USE_PRIORITY set in flags, receive the next message in
>> +      the queue with at least the given priority. If no such message is waiting
>> +      in the queue, -ENOMSG is returned.
>> +
>> +  __u64 offset;
>> +      Upon return of the ioctl, this field contains the offset in the
>> +      receiver's memory pool.
>> +};
>> +
>> +Unless KDBUS_RECV_DROP was passed, and given that the ioctl succeeded, the
>> +offset field contains the location of the new message inside the receiver's
>> +pool. The message is stored as struct kdbus_msg at this offset, and can be
>> +interpreted with the semantics described above.
>
> I'm confused here.  Is sent data written to the pool when send is
> called or when recv is called?

When send is called.

> If the former, what prevents DoS, especially DoS due to sending too many fds?

FDs are installed into the task only when the receiver issues
CMD_RECV, so a task won't explode unless it issues that ioctl. Plus,
receiving FDs is already a per-connection opt-in. Furthermore, you can
CMD_PEEK messages which allows you to look at the full message
_without_ installing FDs. If you don't want the FDs, you can CMD_DROP
the message.

> If the latter, where is the data buffered in the mean time?
>
>> +
>> +Also, if the connection allowed for file descriptor to be passed
>> +(KDBUS_HELLO_ACCEPT_FD), and if the message contained any, they will be
>> +installed into the receiving process after the KDBUS_CMD_MSG_RECV ioctl
>> +returns. The receiving task is obliged to close all of them appropriately.
>
> This makes it sound like fds are installed at receive time.  What
> prevents resource exhaustion due to having excessive numbers of fds in
> transit (that are presumably not accounted to anyone)?

We have a per-user message accounting for undelivered messages, as
well as a maximum number of pending messages per connection on the
receiving end. These limits are accounted on a "user<->user" basis, so
the limit of a user A will not affect two other users (B and C)
talking.

>> +
>> +7.5 Canceling messages synchronously waiting for replies
>> +--------------------------------------------------------
>> +
>> +When a connection sends a message with KDBUS_MSG_FLAGS_SYNC_REPLY and
>> +blocks while waiting for the reply, the KDBUS_CMD_MSG_CANCEL ioctl can be
>> +used on the same file descriptor to cancel the message, based on its cookie.
>> +If there are multiple messages with the same cookie that are all synchronously
>> +waiting for a reply, all of them will be canceled. Obviously, this is only
>> +possible in multi-threaded applications.
>
> What does "cancel the message" mean?  Does it just mean that the wait
> for the reply is cancelled?

Yes, exactly. That will be made clearer in the docs. Thanks.

>> +11. Policy
>> +===============================================================================
>> +
>> +A policy databases restrict the possibilities of connections to own, see and
>> +talk to well-known names. It can be associated with a bus (through a policy
>> +holder connection) or a custom endpoint.
>
> ISTM metadata items on bus names should be replaced with policy that
> applies to the domain as a whole and governs bus creation.

No, well-known names are bound to buses, so a bus is really the right
place to hold policy about which process is allowed to claim them.
Every user is allowed to create a bus of its own, there's no policy
for that, and there shouldn't be.

It has nothing to do with metadata items.

>> +A set of policy rules is described by a name and multiple access rules, defined
>> +by the following struct.
>> +
>> +struct kdbus_policy_access {
>> +  __u64 type;  /* USER, GROUP, WORLD */
>> +    One of the following.
>> +
>> +    KDBUS_POLICY_ACCESS_USER
>> +      Grant access to a user with the uid stored in the 'id' field.
>> +
>> +    KDBUS_POLICY_ACCESS_GROUP
>> +      Grant access to a user with the gid stored in the 'id' field.
>> +
>> +    KDBUS_POLICY_ACCESS_WORLD
>> +      Grant access to everyone. The 'id' field is ignored.
>> +
>> +  __u64 access;        /* OWN, TALK, SEE */
>> +    The access to grant.
>> +
>> +    KDBUS_POLICY_SEE
>> +      Allow the name to be seen.
>> +
>> +    KDBUS_POLICY_TALK
>> +      Allow the name to be talked to.
>> +
>> +    KDBUS_POLICY_OWN
>> +      Allow the name to be owned.
>> +
>> +  __u64 id;
>> +    For KDBUS_POLICY_ACCESS_USER, stores the uid.
>> +    For KDBUS_POLICY_ACCESS_GROUP, stores the gid.
>> +};
>
>
> What happens if there are multiple matches?

We only have _granting_ policy entries. We search through the
policy-db until we find an entry that grants access. We do _not_ stop
on the first item that matches.

>
>> +
>> +11.4 TALK access and multiple well-known names per connection
>> +-------------------------------------------------------------
>> +
>> +Note that TALK access is checked against all names of a connection.
>> +For example, if a connection owns both 'org.foo.bar' and 'org.blah.baz', and
>> +the policy database allows 'org.blah.baz' to be talked to by WORLD, then this
>> +permission is also granted to 'org.foo.bar'. That might sound illogical, but
>> +after all, we allow messages to be directed to either the name or a well-known
>> +name, and policy is applied to the connection, not the name. In other words,
>> +the effective TALK policy for a connection is the most permissive of all names
>> +the connection owns.
>
> This does seem illogical.  Does the recipient at least know which
> well-known name was addressed?

If the sender addressed it to a well-known name, yes. If the sender
addressed the message to a unique-ID, there will be no such name, of
course. Still, the policy applies to such transactions either way
(standard D-Bus behavior).

Note however that dbus1 will not pass along the destination well-known
name, hence most userspace libraries will ignore this information too,
even if they run on kdbus which might pass this information around.
The right way for services that carry multiple service names to
discern which actual service is being talked to is having separate
object paths for the different functionality to hide between the
services.

>> +11.5 Implicit policies
>> +----------------------
>> +
>> +Depending on the type of the endpoint, a set of implicit rules might be
>> +enforced. On default endpoints, the following set is enforced:
>> +
>
> How do these rules interact with installed policy?

As said before, all policy entries _grant_ access. We look through all
entries until we find one that grants access.

>> +  * Privileged connections always override any installed policy. Those
>> +    connections could easily install their own policies, so there is no
>> +    reason to enforce installed policies.
>> +  * Connections can always talk to connections of the same user. This
>> +    includes broadcast messages.
>
> Why?

All limits on buses are enforced on a user<->user basis. We don't want
to provide policies that are more fine-grained than our accounting.

> If anyone ever strengthens the concept of identity to include
> things other than users (hmm -- there are already groups), this could
> be very limiting.

If user-based accounting is not suitable, you can create custom
endpoints. Future extensions to that are always welcome. So far, the
default user-based accounting was enough. And I think it's suitable as
default.

>> +  * Connections that own names might send broadcast messages to other
>> +    connections that belong to a different user, but only if that
>> +    destination connection does not own any name.
>> +
>
> This is weird.  It is also differently illogical than the "illogical"
> thing above.

Actually it follows the same model described above. If two connections
are running under the same user then broadcasts are allowed, but if
they are running under different users *and* if the destination owns a
well-known name, then broadcasts are subject to TALK policy checks
since that destination may own a restricted well-known name that is
not interested in broadcasts. So this implicit policy is just
fast-path for the common case where the target is subscribed to a
broadcast and does not own any name.

>> +12. Pool
>> +===============================================================================
>> +
>> +A pool for data received from the kernel is installed for every connection of
>> +the bus, and is sized according to kdbus_cmd_hello.pool_size. It is accessed
>> +when one of the following ioctls is issued:
>> +
>> +  * KDBUS_CMD_MSG_RECV, to receive a message
>> +  * KDBUS_CMD_NAME_LIST, to dump the name registry
>> +  * KDBUS_CMD_CONN_INFO, to retrieve information on a connection
>> +
>> +Internally, the pool is organized in slices, stored in an rb-tree. The offsets
>> +returned by either one of the aforementioned ioctls describe offsets inside the
>> +pool. In order to make the slice available for subsequent calls, KDBUS_CMD_FREE
>> +has to be called on the offset.
>
> Why are you documenting that the slices are stored in an rb-tree?
> That's just an implementation details, right?

Dropped, thanks.

>> +
>> +To access the memory, the caller is expected to mmap() it to its task, like
>> +this:
>> +
>> +  /*
>> +   * POOL_SIZE has to be a multiple of PAGE_SIZE, and it must match the
>> +   * value that was previously passed in the .pool_size field of struct
>> +   * kdbus_cmd_hello.
>> +   */
>> +
>> +  buf = mmap(NULL, POOL_SIZE, PROT_READ, MAP_PRIVATE, conn_fd, 0);
>> +
>
> Will mapping with PROT_WRITE fail?  What about MAP_SHARED?

PROT_WRITE will fail and VM_MAYWRITE is cleared.

> And why are you suggesting MAP_PRIVATE?  That's just strange.

This was a leftover from pre-3.17. memfds require you to use
MAP_PRIVATE if SEAL_WRITE is set (which is a linux-specific behavior,
where MAP_SHARED is always accounted as writable mapping as
mprotect(VM_WRITE) can be called any time). I fixed this up.

>> +
>> +13. Metadata
>> +===============================================================================
[snip]
>> +13.1 Known item types
>> +---------------------
>> +
>> +The following attach flags are currently supported.
>> +
>> +  KDBUS_ATTACH_TIMESTAMP
>> +    Attaches an item of type KDBUS_ITEM_TIMESTAMP which contains both the
>> +    monotonic and the realtime timestamp, taken when the message was
>> +    processed on the kernel side.
>> +
>> +  KDBUS_ATTACH_CREDS
>> +    Attaches an item of type KDBUS_ITEM_CREDS, containing credentials as
>> +    described in kdbus_creds: the uid, gid, pid, tid and starttime of the task.
>> +
>
> As mentioned last time, please remove or justify starttime.

starttime allows detecting PID overflows. Exposing the process
starttime is useful to detect when a PID is getting reused.
Unfortunately, we don't have 64bit pids, so we need the pid+time
combination to avoid ambiguity.

>> +  KDBUS_ATTACH_AUXGROUPS
>> +    Attaches an item of type KDBUS_ITEM_AUXGROUPS, containing a dynamic
>> +    number of auxiliary groups the sending task was a member of.
>> +
>> +  KDBUS_ATTACH_NAMES
>> +    Attaches items of type KDBUS_ITEM_OWNED_NAME, one for each name the sending
>> +    connection currently owns. The name and flags are stored in kdbus_item.name
>> +    for each of them.
>> +
>
> That's interesting.  What's it for?

It a valuable piece of information for receivers to know which bus
names a sender has claimed. For instance, we need this information for
the D-Bus proxy service, because we have to apply D-Bus1 policy in
that case, and we need to get a list of owned names in a race-free
manner to check the policy against.

>> +  KDBUS_ATTACH_TID_COMM
>> +    Attaches an items of type KDBUS_ITEM_TID_COMM, transporting the sending
>> +    task's 'comm', for the tid.  The string is stored in kdbus_item.str.
>> +
>> +  KDBUS_ATTACH_PID_COMM
>> +    Attaches an items of type KDBUS_ITEM_PID_COMM, transporting the sending
>> +    task's 'comm', for the pid.  The string is stored in kdbus_item.str.
>> +
>> +  KDBUS_ATTACH_EXE
>> +    Attaches an item of type KDBUS_ITEM_EXE, containing the path to the
>> +    executable of the sending task, stored in kdbus_item.str.
>> +
>> +  KDBUS_ATTACH_CMDLINE
>> +    Attaches an item of type KDBUS_ITEM_CMDLINE, containing the command line
>> +    arguments of the sending task, as an array of strings, stored in
>> +    kdbus_item.str.
>
> Please remove these four items.  They are genuinely useless.  Anything
> that uses them for anything is either buggy or should have asked the
> sender to put the value in the payload (and immediately wondered why
> it was doing that).

We use them for logging, debugging and monitoring. With our wireshark
extension it's pretty useful to know the comm-name of a process when
monitoring a bus. As we explained last time, this is not about
security. We're aware that a process can modify them. We use them only
as additional meta-data for logging and debugging.

If we put those items into the payload, we have to transmit this data
even if the destination process is not interested in this.
Furthermore, each caller has to run multiple syscalls on each message
to retrieve those values.

We use these items heavily for filtering and debugging, regardless of
the payload protocol that is transmitted on the bus.

To give another specific use-case here: dbus supports bus activation,
where a message sent to a non-running service causes it to be spawned
implicitly without losing the message. Now, with such a scheme it is
incredibly useful to be able to log which client caused a service to
be triggered, hence we want to know the cmdline/exe/comm of the
client. Not knowing this is a major pita when trying to trace the boot
process and figuring out why a specific service got activated.

Also note that since v2 of the patch there's actually a per-sender
mask for meta-data like this, hence a peer which doesn't want to pass
its exec/cmdline/comm along can do that. Of course, this will
seriously hamper debuggability and transparency...

>> +
>> +  KDBUS_ATTACH_CGROUP
>> +    Attaches an item of type KDBUS_ITEM_CGROUP with the task's cgroup path.
>> +
>> +  KDBUS_ATTACH_CAPS
>> +    Attaches an item of type KDBUS_ITEM_CAPS, carrying sets of capabilities
>> +    that should be accessed via kdbus_item.caps.caps. Also, userspace should
>> +    be written in a way that it takes kdbus_item.caps.last_cap into account,
>> +    and derive the number of sets and rows from the item size and the reported
>> +    number of valid capability bits.
>> +
>
> Please remove this, too, or justify its use.

cgroup information tells us which service is doing a bus request. This
is useful for a variety of things. For example, the bus activation
logging item above benefits from it. In general, if some message shall
be logged about any client it is useful to know its service name.

Capabilities are useful to authenticate specific method calls. For
example, when a client asks systemd to reboot, without this concept we
can only check for UID==0 to decide whether to allow this. Breaking
this down to capabilities in a race-free way has the benefit of
allowing systemd to bind this to CAP_SYS_BOOT instead. There is no
reason to deny a process with CAP_SYS_BOOT to reboot via bus-APIs, as
they could just enforce it via syscalls, anyway.

We think it's a useful and reliable authentication method. Why should
we remove it?

Anyway, these items are  just optional. The sender can refuse the
reveal them, and the item is only transmitted if the receiver opted in
for it, too. So there's no need to drop any item type from the
protocol.

>> +  KDBUS_ATTACH_SECLABEL
>> +    Attaches an item of type KDBUS_ITEM_SECLABEL, which contains the SELinux
>> +    security label of the sending task. Access via kdbus_item->str.
>> +
>
> This one, too, and please justify why code that uses it will work in
> containers an on non-selinux systems.

This maps to SCM_PEERSEC on AF_UNIX sockets. This is actually heavily
used already on dbus1. For example, systemd actually uses this
information about a bus peer to check whether certain operations are
allowed, on top of the normal policy. To give an explicit example:
when starting a service systemd gets the client's peer label from
AF_UNIX via SCM_PEERSEC, reads the security label of the service file
in question, and then checks the selinux database (via one of
libselinux APIs) whether to allow this change.

Note that this is really nothing we came up with, it's code from the
SELinux folks, it's simple enough, and has been exposed and used that
way since ages in dbus1. libselinux offers all the right APIs to make
use of this, and kdbus really needs to provide the same functionality
as dbus1 in this regard here.

Let met stress that checking the selinux database here is alway *on
top* of the normal UID/caps based security checks services do. This is
exactly how selinux enforces checks on files on top of UID/caps
checks, or on process or anything else that is selinux-managed.

>> +  KDBUS_ATTACH_AUDIT
>> +    Attaches an item of type KDBUS_ITEM_AUDIT, which contains the audio label
>> +    of the sending taskj. Access via kdbus_item->str.
>> +
>
> I will NAK the hell out of this until, at the very least, someone
> documents what this means, how to parse it, what its stability rules
> are, who is allowed to see the value it contains, why that value will
> never evolve to become *more* security sensitive than it is now, etc.
>
> Audit gets to do crazy sh*t because it's restricted to privileged
> receivers.  This isn't restricted like that, so it doesn't deserve the
> same dispensation.  (And, honestly, I'm not sure that audit really
> deserves its free pass on making sense.)

This was based on a misunderstanding, so I will ignore it here. Lets
discuss this on the metadata-patch, in case it's still unclear.

>> +  KDBUS_ATTACH_CONN_DESCRIPTION
>> +    Attaches an item of type KDBUS_ITEM_CONN_DESCRIPTION that contains the
>> +    sender connection's current name in kdbus_item.str.
>> +
>
> Which name?  Can't there be several?

No. There is only one connection description string, copied verbatim
from what the connection supplied during HELLO.

Note that this is really just about debugging, since in some cases
processes might end up with multiple kdbus fds, and it is useful in
tools like "busctl" to know which one is which.

>> +
>> +13.1 Metadata and namespaces
>> +----------------------------
>> +
>> +Metadata such as PIDs, UIDs or GIDs are automatically translated to the
>> +namespaces of the domain that is used to send a message over. The namespaces
>> +of a domain are pinned at creation time, which is when the filesystem has been
>> +mounted.
>> +
>> +Metadata items that cannot be translated are dropped.
>
> What if the receiver said that the item was mandatory?

It is still dropped. It's the responsibility of the receiver to reject
messages that lack required metadata.

Thanks for the review, Andy. Documentation fixes coming up!
David
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